Hospital Corpsman 3 & 2: June 1989

Chapter 3: Hospital Corpsman - Anatomy and Physiology

Naval Education and Training Command

Introduction

Knowledge of how the human body is constructed and how it works is an important part of the training of everyone concerned with healing the sick or managing emergency conditions following injury. This chapter will provide you with a general knowledge of the structures and functions of the body.

The human body is a combination of organ systems with a supporting framework of muscles and bones and an external covering of skin. The study of the body is divided into the three following sciences:

Anatomy - the study of body structures and the relation of one part to another

Physiology - the study of the processes and functions of the body tissue and organs. Basically, it is the study of how the body works-how the various parts function individually and in relation to each other.

Embryology - the study of the development of the body from a fertilized egg, or ovum.

Terms of Position and Direction

The planes of the body are imaginary lines dividing it into sections. They are used as reference points in locating anatomical structures. As shown in figure 3-1, the MEDIAN, or MIDSAGITTAL, PLANE divides the body into right and left halves on its vertical axis. This plane passes through the sagittal suture of the cranium; therefore, any plane parallel to it is called a SAGITTAL PLANE. FRONTAL PLANES are drawn perpendicular to the sagittal lines and divide the body into anterior and posterior sections. Since this line passes through the coronal suture of the cranium, frontal planes are also called CORONAL PLANES. The HORIZONTAL, or TRANSVERSE, PLANE, which is drawn at right angles to both sagittal and frontal planes, divides the body into superior and inferior sections.

To avoid misunderstanding in describing the location of anatomical structures, a standard body position, called the ANATOMICAL POSITION, is used as a point of reference. This anatomical position is assumed when the body stands erect, with arms hanging at the sides, and palms of the hands turned forward (fig. 3-2).

Other commonly used anatomical terms include the following:

Anterior or ventral-toward the front, or ventral (pertaining to the belly; abdomen), side of the body.
Posterior or dorsal-toward the back, or dorsal, side of the body.
Medial-near or toward the midline of the body.
Lateral-farther away from the midline of the body.
Internal-inside.
External-outside.
Proximal-nearer the point of origin or closer to the body.
Distal-away from the point of origin or away from the body.
Superior-higher than or above.
Cranial-toward the head.
Caudal-toward the lower end of the body.
Inferior-lower than or below.
Erect-normal standing position of the body.
Supine-lying position of the body, face up.
Prone-lying position of the body, face down.
Lateral recumbent-lying position of the body on either side.
Peripheral-The outward part or surface of a structure.

Characteristics of Living Matter

All living things, animals and plants, are ORGANISMS that undergo chemical processes by which they sustain life and regenerate cells. The difference between them is that animals have sensations and the power of voluntary movement and require oxygen and organic food. Plants require only carbon dioxide and inorganic matter for food and have neither voluntary movement nor special sensory organs.

In man, some of the characteristic functions necessary for survival include digestion, metabolism, and homeostasis. DIGESTION involves the physical and chemical breakdown of the food we eat into its simplest forms. METABOLISM is the process of absorption, storage, and use of these foods for body growth, maintenance, and repair. HOMEOSTASIS is the body's self-regulated control of its internal environment. It allows the organism to maintain a state of constancy or equilibrium, in spite of vast changes in the external environment.

The Cell

The smallest unit of life, the cell, is the basic structural unit of all living things and a functional unit all by itself. It is composed of a viscid, jellylike substance, called PROTOPLASM, upon which depend all the vital functions of nutrition, secretion, growth, circulation, reproduction, excitability, and movement. As such, protoplasm has been called "the secret of life."

A typical cell is made up of the plasma membrane, a nucleus, and the cytoplasm.

The PLASMA MEMBRANE is a selectively permeable membrane surrounding the cell. In addition to holding the cell together, the membrane selectively controls the exchange of materials between the cell and its environment by physical and chemical means. Solids and gases, such as oxygen, proteins, carbohydrates, and mineral salts, pass through the plasma membrane by a process known as DIFFUSION.

The NUCLEUS is a small, dense, usually spherical body that controls the chemical reactions occurring in the cell. The substance contained in the nucleus is called NUCLEOPLASM. It is also important in the cell's reproduction, since genetic information for the cell is stored there. Every human cell contains 46 chromosomes, and each chromosome has thousands of genes that determine the cell's function.

The CYTOPLASM is a water-to-gelatinous substance surrounding the nucleus and is contained by the plasma membrane. The cytoplasm is all of the cell protoplasm except the nucleus.

The simplest living organism consists of a single cell. The amoeba is a unicellular animal. The single cell of such a one-celled organism must be able to carry on all processes necessary for life. This cell is called a SIMPLE or UNDIFFERENTIATED CELL.

In multicellular organisms, cells vary in size, shape, and number of nuclei. When stained, the various cell structures can be more readily recognized under a microscope. Other differences such as the number and type of cells can be seen with the aid of a microscope. Many cells are highly specialized. SPECIALIZED CELLS perform special functions, such as muscle, which contracts, or epithelial cells of the skin, which protect.

Tissues

Tissues are groups of specialized cells similar in structure and function. They are classified into five main groups: epithelial, connective, muscular, liquid, and nervous.

EPITHELIAL. The lining tissue of the body is called epithelium. It forms the outer covering of the body known as the free surface of the skin. It also forms the lining of the digestive, respiratory, and urinary tracts; blood and lymph vessels; serous cavities; and tubules of certain secretory glands, such as the liver and kidneys. This tissue has little intercellular fluid and may be further subdivided into three types:

1. Columnar. The chief functions of this tissue are to secrete digestive fluids and absorb digested foods and fluids. It consists of long narrow cells set close together, resembling a palisade-type fence (fig. 3-3). In certain areas, such as the nostrils, bronchial tubes, and trachea, this tissue has a crown of microscopic hairlike processes known as cilia. These cilia provide motion to move secretions and other matter along the surfaces from which they extend. They also act as a barrier, preventing foreign matter from entering these cavities.

2. Squamous. This is the main protective tissue of the body. It is composed of thin platelike or scalelike cells forming a mosaic pattern (fig. 3-4). This tissue is found in the tympanic membrane (eardrum) as a single layer of cells or in the free skin surface in multiple layers.

3. Cuboidal. This is both a secretory and protective tissue whose cells are cubical (fig. 3-5). It is found in the more highly specialized organs of the body, such as the ovary and the kidney.

CONNECTIVE. This is the supporting tissue of the various structures of the body. It has many variations and is the most widespread tissue of the body. It is highly vascular, surrounds other cells, encases internal organs, sheathes muscles, wraps bones, encloses joints, and provides the supporting framework of the body. Structures of connective tissue differ widely, ranging from delicate tissue-paper membranes to strong cords and rigid bones. Connective tissue is composed of few cells and large amounts of intracellular material; the reverse is true of epithelial tissue. Some of the more predominant types of connective tissues are:

1. Areolar. This tissue connects the various tissues of the organs. It is continuous throughout the body. Nerves, blood, and lymph vessels are found in this tissue (fig. 3-6).

2. Adipose. This tissue is generally called "fatty tissue." It acts as a reservoir for energy-producing foods; helps to reduce body heat loss because of its poor heat conductivity; and serves as support for various organs and fragile structures, such as the kidneys, blood vessels, and nerves.  

3. Osseous. This type is a dense fibrous connective tissue that forms tendons, ligaments, cartilage, and bone (fig. 3-7). These tissues form the supporting framework of the body.

MUSCULAR. Muscular tissue provides for all body movement. There are two types, voluntary and involuntary.

1. Voluntary muscle fibers are striated, or striped, and are under the control of the individual's will (fig. 3-8). Muscles that attach to bone are voluntary muscles.  

2. Involuntary muscle fibers are smooth, or nonstriated, and are not under the control of the individual's will (fig. 3-9). The muscles of the stomach are an example.

   NOTE: Composed of a special branched type of cell, the heart (cardiac) muscle is an involuntary muscles that is striated (fig. 3-10).

LIQUID. Liquid tissues act as a medium for supplying the body with nutrients and as a vehicle for eliminating waste material. They form the blood, lymph, and tissue fluids.

NERVOUS. Nervous tissue is the most complex tissue in the body. It is the substance of the brain, spinal cord, and nerves. Nervous tissue requires more oxygen and nutrients than any other body tissue. The basic cell of the nervous tissue is the neuron (fig. 3-11). This highly specialized cell receives stimuli from, and conducts impulses to, all parts of the body.

As a group of similar cells form tissues, similar tissues form organs such as the heart, liver, and kidneys. These organs are grouped together to form systems, such as the urinary system that is composed of the kidneys, ureters, bladder, and urethra.

The Skeletal System

The skeleton is the bony framework of the body, composed of 206 bones (fig. 3-12). It supports and gives shape to the body; protects vital organs; and provides sites of attachment for tendons, muscles, and ligaments. The skeletal bones are joined members that make muscle movement possible.

Anatomy of Bones

Osteology is the study of the structure of bone. Bone is made up of inorganic mineral salts, calcium and phosphorus being the most prevalent, and an organic substance called ossein. When human bone is soaked in dilute acid until all inorganic mineral salts are washed out, all that remains is a flexible piece of tissue that can easily be bent and twisted. The inorganic mineral salts give bone its strength and hardness.

Bone consists of a hard outer shell, called compact tissue, and an inner spongy, porous portion, called cancellous tissue (fig. 3-13). In the center of the bone is the MEDULLARY CANAL, which contains marrow. There are two types of marrow, red and yellow. Yellow marrow is ordinary bone marrow in which fat cells predominate. It is found in the medullary canals and cancellous tissue of long bones. Red marrow is one of the manufacturing centers of red blood cells and is found in the articular ends of long bones and in cancellous tissue. At the ends of the long bones is a smooth, glossy tissue that forms the joint surfaces. This tissue is called articular cartilage because it articulates (joins) with, fits into, or moves in contact with similar surfaces of other bones. The thin outer membrane surrounding the bone is called the PERIOSTEUM. An important function of the periosteum is to supply nourishment to the bone. Capillaries and blood vessels run through the periosteum and dip into the bone surface, supplying it with blood and nourishment. The periosteum is the pain center of the bone. When there is a fracture, the pain you feel comes from the periosteum, not the bone proper. Periosteum also forms new bone.

Classification Bones are classified according to shape, as follows:

• Long-femur and humerus

• Short-wrist and ankle bones

• Flat-skull, sternum, and scapula

• Irregular-vertebrae, mandible, hyoid, and pelvic bones

Divisions of Skeleton

The human skeleton is divided into two main divisions, the axial skeleton and the appendicular skeleton. The axial skeleton consists of the skull, the vertebral column, and the thorax. The skull bones are further divided into the cranial and facial bones. The appendicular skeleton consists of the bones of the upper and lower extremities.

Axial Skeleton

Skull - Consists of 28 bones (figs. 3-14 and 3-15), 22 of which form the framework of the head and provide protection for the brain, eyes, and ears; six are ear bones. With the exception of the lower jaw bone (mandible) and the ear bones, all skull bones are joined together and fixed in one position. The seams where they join are known as sutures.

Cranial Bones - The cranium is formed by eight cranial bones, six of which are essential to know. The FRONTAL BONE, which forms the forehead, contains the frontal sinuses and helps form the eye socket and nasal cavity. The two PARIETAL BONES form the roof of the skull. The two TEMPORAL BONES, which help form the sides and base of the skull, also house the auditory, or hearing organs. The OCCIPITAL BONE forms part of the base and back of the skull and contains a large hole, called the FORAMEN MAGNUM. This opening permits passage of the spinal cord from the cranium into the spinal column.

Facial Bones - The two MAXILLARY BONES form the upper jaw, nasal walls, and part of the eye socket. These bones contain large cavities called maxillary sinuses. Frequently these sinuses become infected, causing the individual much discomfort. The lower jaw is called the MANDIBLE. Its main function is mastication. Other bones of the face are the LACRIMAL and NASAL BONES.

Vertebral (Spinal) Column - It consists of 24 movable or true vertebrae, the sacrum, and the coccyx, or tail bone (fig. 3-16). The spinal column is divided into five regions in the following order: cervical (neck), thoracic (chest), lumbar (lower back), and sacral and coccygeal (pelvis).

The vertebrae protect the spinal cord and the nerves arising from the spinal cord. Each vertebra has an anterior portion, the body, which is the large solid segment of the bone (fig. 3-17). This body is for support, not only for the spinal cord, but for other structures of the body as well. Many of the main muscles are attached to the vertebrae. The vertebral foramen is a hole directly behind the body of the vertebrae and forms the passage for the spinal cord. The vertebral projections are for the attachments of muscles and ligaments and to facilitate movement of one vertebra over another.

There are seven cervical vertebrae in the neck. The first is called the ATLAS and resembles a bony ring. It supports the head. The second is the highly specialized AXIS. It has a bony prominence that fits into the ring of the atlas, thus permitting the head to rotate from side to side. The atlas and the axis are the only named vertebrae, all others are numbered. Each cervical vertebrae has a transverse foramen to allow passage of nerves, the vertebral artery, and a vein. The seventh cervical vertebra has an especially prominent projection that can easily be felt at the nape of the neck. This makes it possible for physicians to count and identify the vertebrae above and below it.

There are 12 vertebrae in the thoracic region. These articulate with the posterior portion of the 12 ribs to form the posterior wall of the thoracic, or chest, cage.

There are five lumbar vertebrae, which are the largest segments of the vertebral column.

The SACRUM is the triangular bone immediately below the lumbar vertebrae, formed by the fusion of five false vertebrae. It articulates on each side with the hip bone and with the COCCYX to form the posterior wall of the PELVIS.

Thorax - It is a cone-shaped bony cage, about as wide as it is deep (fig. 3-18). It is formed by 12 ribs on each side, which articulate posteriorly with the thoracic vertebrae. The first seven pairs of ribs are attached to the sternum by cartilage and are called true ribs. The eighth, ninth, and tenth ribs are united by their cartilages to the cartilage of the seventh rib and are called false ribs. The STERNUM is an elongated flat bone, forming the middle portion of the upper half of the chest wall in front. The xiphoid process, located at the inferior aspect of the sternum, serves as a landmark in the administration of cardiopulmonary resuscitation.

Appendicular Skeleton

Upper Extremity - The upper extremity consists of the bones of the shoulder, the arm, the forearm, the wrist, and the hand (figs. 3-19 and 3-20). The specific bones that form the framework for the upper extremity are:

The CLAVICLE forms the front part of the shoulder girdle. It lies nearly horizontally just above the first rib and is shaped like a flat letter S. The clavicle is a thin brace bone that fractures easily. Its inner end is round and attached to the sternum; its outer end is flattened and fixed to the scapula.

The SCAPULA is a triangular bone that lies in the upper part of the back on both sides, between the second and seventh ribs, forming the posterior portion of the shoulder girdle. Its lateral corner forms part of the shoulder joint, articulating with the humerus.

The HUMERUS is the longest bone of the upper extremity and is often called the arm bone. It articulates with the shoulder girdle to form the shoulder joint and with the bones of the forearm to form the elbow. Its anatomical portions include a head, a rounded portion that fits into a recess of the scapula called the glenoid fossa; the greater and lesser tuberosities, which are enlargements of the superior end; the surgical neck, a slight narrowing distal to the tuberosities and a frequent site of fractures; the shaft, which is the main part of the humerus; and the distal end, which includes the prominences called epicondyles and the surfaces that articulate with the bones of the forearm.

When the arm is in the anatomical position with the palm turned forward, the RADIUS is on the lateral, or thumb, side and the ULNA is on the medial, or little finger, side of the forearm. When the hand is pronated (palm turned downward), the bones rotate on each other and cross in the middle. This makes it possible to turn the wrist and hand as in opening doors. The ulna and the radius articulate at their proximal ends with the humerus, at their distal ends with some of the carpal bones, and with each other at both ends.

There are eight CARPAL bones, arranged in two rows, forming the wrist.

The METACARPAL bones are numbered one to five corresponding with the five fingers, or digits, with which they articulate. The fingers are named as follows: 1st - thumb; 2nd - index; 3rd - middle; 4th - ring; and 5th - little.

The small bones of the fingers are called PHALANGES, and each one of these bones is called a PHALANX. Each finger has three phalanges, except the thumb which has two. The phalanges are named for their anatomical position proximal phalanx is the bone closest to the hand; the distal phalanx is the bone at the end of the finger; and the middle phalanx, the bone located between the proximal and distal phalanges.

Lower Extremity - The lower extremity includes the bones of the hip, thigh, leg, ankle, and foot (figs. 3-21 and 3-22). The specific bones that form the framework of the lower extremity are:

The hip, or INNOMINATE, is a large irregularly shaped bone composed of three parts: the ilium, ischium, and pubis. In children these three parts are separate bones, but in adults they are firmly united to form a cuplike structure, called the ACETABULUM, into which the head of the femur fits. The ILIUM forms the outer prominence of the hip bone (crest of the ilium), the ISCHIUM forms the hard lower part, and the PUBIS forms the front part of the pelvis.

The area where the two pubic bones meet is called the SYMPHYSIS PUBIS and is often used in anatomical measurements. The largest foramen (opening) is located in the hip bone, between the ischium and the pubis, and is called the OBTURATOR FORAMEN. The crest of the ilium is used in making anatomical and surgical measurements (e.g., location of the appendix, which is approximately halfway between the crest of the ilium and the umbilicus).

The FEMUR, or thigh bone, is the longest bone in the body. The proximal end is rounded and has a head supported by a constricted neck that fits into the acetabulum. Two processes called the GREATER and LESSER TROCHANTERS are at the proximal end for the attachment of muscles. The neck of the femur, located between the head and the trochanters, is the site most frequently fractured. At the distal end are two bony prominences called the LATERAL and MEDIAL CONDYLES, which articulate with the tibia and the patella.

The PATELLA is a small oval-shaped bone overlying the knee joint. It is enclosed within the tendon of the quadriceps muscle of the thigh. Bones like the patella that develop within a tendon are known as SESAMOID bones.

The TIBIA, or shin bone, is the larger of the two leg bones and lies at the medial side. The proximal end articulates with the femur and the fibula. Its distal end articulates with the talus (one of the foot bones) and the fibula. A prominence easily felt on the inner aspect of the ankle is called the MEDIAL MALLEOLUS.

The FIBULA, the smaller of the two leg bones, is located on the lateral side of the leg, parallel to the tibia. The prominence at the distal end forms the outer ankle, known as the LATERAL MALLEOLUS.

The TARSUS, or ankle, is formed by seven tarsal bones. The strongest of these is the heel bone or CALCANEUS.

The sole and instep of the foot is called the METATARSUS and is made up of five METATARSAL bones. They are similar in arrangement to the metacarpals of the hand.

The PHALANGES are the bones of the toes and are similar in number, structure, and arrangement to the bones of the fingers.

  Joints

Whenever two bones are attached to each other, a joint is formed. In a freely movable joint, such as the knee or elbow joint, the ends of the bones are covered with a smooth layer of cartilage. The whole joint is enclosed in a watertight sac or membrane containing a small amount of lubricating fluid. This enables the joint to work with little friction. The ligaments that reach across the joints from one bone to another keep them from getting out of place. When ligaments are accidentally torn, we call the injury a sprain; when bones are out of place, there is a dislocation; and when bones are chipped or broken, the injury is called a fracture.

Joints are classified according to the amount of movement they permit (fig. 3-23). They may be:

1. Immovable - Bones of the skull are rigidly interlocked along immovable joint lines known as sutures.

2. Slightly Movable- In these joints the bones are held together by broad flattened disks of cartilage and ligaments (e.g., vertebrae and symphysis pubis).  

3. Freely Movable - Such joints include the knee, hip, and shoulder. These joints are further subdivided into (figs. 3-23A and 3-23B):

   Figure 3-23A
   Hinge Joints	-	elbow, finger and knee
   Figure 3-23B
   Ball-and-Socket Joint	-	Shoulder and hip

Joint Movements

Joint movements are generally divided into four types:

GLIDING is the simplest type of motion. It is one surface moving over another without any rotary or angular motion. This motion exists between two contiguous or adjacent surfaces.

ANGULAR motion decreases or increases the angle between two adjoining bones. The more common types of angular motion are:

• Flexion-bending the arm or leg.  

• Extension-straightening or unbending, as in straightening the forearm, leg, or fingers.  

• Abduction-moving an extremity away from the body.  

• Adduction-bringing an extremity toward the body.

ROTATION is a movement in which the bone moves around a central point without being displaced, such as turning the head from side to side.

CIRCUMDUCTION is movement of the hips and shoulders.

Other types of movement generally used to indicate specific anatomical positions include the following:

• SUPINATION is turning upward, as in placing the palm of the hand

• PRONATION is turning downward, as in placing the palm of the hand down.

• EVERSION is turning outward, as in turning the sole of the foot to the outside

• INVERSION is turning inward, as in turning the sole of the foot inward.

  Muscles

Muscles make up about one-half of the total body weight. Their main functions are threefold:

• Providing movement, including internal functions such as peristalsis in the intestines.

• Maintaining body posture through muscle tone, as in the muscles of the head, neck and shoulders, which keep the head up.

• Providing heat through chemical changes that take place during muscle activity, such as mild exercise that warms the body on cold days.

In addition, muscles are involved in such essential bodily functions as respiration, blood circulation, digestion, and even such functions as speaking and seeing.

At one end of some muscles are long white TENDONS that attach the muscles to bone. The point of fixed attachment of a muscle to bone is called the ORIGIN. The more flexible attachments, especially to a movable bone, is termed the INSERTION.

Muscle tissue has a highly developed ability to contract. CONTRACTIBILITY enables a muscle to become shorter or thicker, and this ability, along with interaction with other muscles, produces movement in internal and external body parts. Muscle contraction in a tissue or organ produces motion and provides power and speed for body activity. A contracting muscle is referred to as a PRIME MOVER. A muscle that is relaxing while a prime mover is contracting is called the ANTAGONIST. Muscular tone, or TONICITY, is a continual state of partial contraction that gives muscles a certain firmness. ISOMETRIC muscle contraction occurs when the muscle is stimulated and shortens, but no movement occurs, as when a person tenses his or her muscles against an immovable object. Muscles are also capable of stretching when force is applied (EXTENSIBILITY) and regaining their original form when that force is removed (ELASTICITY).

All types of muscles respond to stimulus. This property is called EXCITABILITY or IRRITABILITY. The mechanical muscular action of shortening or thickening is activated by a stimulus sent through a motor nerve. All muscles are linked to nerve fibers that carry messages from the central nervous system.

The chemical action of muscle fibers consists of two stages, CONTRACTION and RECOVERY. In the contraction stage, two protein substances (actin and myosin) react to provide energy through the breakdown of glycogen into lactic acid. In the recovery stage, oxygen reacts with lactic acid to release carbon dioxide and water.

When a muscle contracts, it produces chemical waste products (carbon dioxide, lactic acid, and acid phosphate), which make the muscle more irritable. If contraction is continued, the muscle will finally cramp up and refuse to move. This condition is known as fatigue. If it is carried too far, the muscle cells will not recover and permanent damage will result. Muscles, therefore, need rest to allow the blood to carry away the waste materials and bring in fresh glucose, oxygen, and protein to restore the muscle protoplasm and the energy that was used.

The importance of exercise for normal muscle activity is clear, but excessive muscle strain is damaging. For example, if a gasoline motor stands idle, it eventually becomes rusty and useless. Similarly, a muscle cell that does not work becomes weak and flabby. On the other hand, a motor that is never allowed to stop and is forced to run too fast or to do too much heavy work soon wears out so that it cannot be repaired. In the same way, a muscle cell that is forced to work too hard without proper rest will be damaged beyond repair. Violent exercise is never good. Exercise should be adapted to the individual and should never be carried to the point of extreme fatigue.

During exercise, massage, or ordinary activities, the blood supply of muscles is increased. This brings in fresh nutritional material, carries away waste products more rapidly, and enables the muscles to build up and restore their efficiency and tone.

When a muscle dies, it becomes solid and rigid and no longer reacts. This stiffening, which occurs from 10 minutes to several hours after death, is called RIGOR MORTIS.

Muscles seldom act alone; they usually work in muscle groups held together by sheets of a white fibrous tissue call FASCIA. There are three types of muscle tissue: skeletal, smooth, and cardiac. Each is designed to perform a specific function.

SKELETAL MUSCLES are attached to the bones and give shape to the body. They are responsible for allowing body movement. This type of muscle is sometimes referred to as STRIATED because of the striped appearance of the muscle fibers under a microscope (fig. 3-24). They are also called VOLUNTARY muscles because they are under the control of our conscious will. These muscles can develop great power.

SMOOTH, or NONSTRIATED, muscle tissues are found in the walls of the stomach, intestines, urinary bladder, and blood vessels, as well as in the duct glands and in the skin. Under a microscope, the smooth muscle fiber lacks the striped appearance of other muscle tissue (fig. 3-25). This tissue is also called INVOLUNTARY muscle because it is not under conscious control.

The CARDIAC MUSCLE tissue forms the bulk of the walls and septa (partitions) of the heart, as well as the origins of the great blood vessels. The fibers of the cardiac muscle differ from those of the skeletal and smooth muscles in that they are shorter and branch into a complicated network (fig. 3-26). The cardiac muscle has the most abundant blood supply of any muscle in the body, receiving twice the blood flow of the highly vascular skeletal muscles and far more than the smooth muscles. Cardiac muscles contract to pump blood out of the heart and through the cardiovascular system. Interference with the blood supply to the heart can result in a heart attack.

Important Functional Muscles

These muscles, shown in figures 3-27 and 3-28, are described below.

The MASSETER muscle raises the mandible, or lower jaw, to close the mouth. It is the chewing muscle in the mastication of food. It originates in the zygomatic process and adjacent parts of the maxilla and is inserted in the mandible.

The TEMPORAL muscle assists the masseter and draws the mandible backward. It has its origin in the temporal fossa and is inserted in the coronoid process of the mandible.

The STERNOCLEIDOMASTOID muscles are located on both sides of the neck. Acting individually, these muscles rotate the head left or right. Acting together, they bend the head forward toward the chest. The sternocleidomastoid muscle originates in the sternum and clavicle and is inserted in the mastoid process of the temporal bone. This muscle is commonly affected in cases of stiff neck.

The TRAPEZIUS muscles are a broad, trapezium-shaped pair of muscles on the upper back, which raise or lower the shoulders. They cover approximately one-third of the back. They originate in a large area, which includes the 12 thoracic vertebrae, the seventh cervical vertebra, and the occipital bone. They have their insertion in the clavicle and scapula.

The LATISSIMUS DORSI is a broad flat muscle that covers approximately one-third of the back on each side. It rotates the arm inward and draws the arm down and back. It originates from the upper thoracic vertebrae to the sacrum and the posterior portion of the crest of the ilium. Its fibers converge to form a flat tendon that has its insertion in the humerus.

The PECTORALIS MAJOR is the large triangular muscle that forms the prominent chest muscle. It rotates the arm inward, pulls a raised arm down toward the chest, and draws the arm across the chest. It originates in the clavicle, sternum, and cartilages of the true ribs, and the external oblique muscle. Its insertion is in the greater tubercle of the humerus.

The DIAPHRAGM is an internal muscle that forms the floor of the thoracic cavity and the ceiling of the abdominal cavity. It is the primary muscle of respiration, modifying the size of the thorax and abdomen vertically. It has three openings for the passage of nerves and blood vessels.

The DELTOID muscle raises the arm and has its origin in the clavicle and the spine of the scapula. Its insertion is on the lateral side of the humerus. It fits like a cap over the shoulder and is a frequent site of intramuscular injections.

The BICEPS BRACHII is the prominent muscle on the anterior surface of the upper arm. Its origin is in the outer edge of the glenoid cavity and its insertion in the tuberosity of the radius. This muscle rotates the forearm outward (supination) and, with the aid of the brachial muscle, flexes the forearm at the elbow.

The TRICEPS BRACHII is the primary extensor of the forearm (the antagonist of the biceps brachii). It originates at two points on the humerus and one on the scapula. These three heads join to form the large muscle on the posterior surface of the upper arm. The point of insertion is the olecranon process of the ulna.

The GLUTEALS (MAXIMUS, MINIMUS, and MEDIUS) are the large muscles of the buttocks, which extend and laterally rotate the thigh, as well as abduct and medially rotate it. They arise from the ilium, the posterior surface of the lower sacrum, and the side of the coccyx. Their points of insertion include the greater trochanter and the gluteal tuberosity of the femur. The gluteus maximus is the site of choice for massive intramuscular injections.

The QUADRICEPS is a group of four muscles that make up the anterior portion of the thigh. The rectus femoris originates at the ilium; the vastus femoris, v. lateralis, and v. intermedius originate along the femur. All four are inserted into the tuberosity of the tibia through a tendon passing over the knee joint. The quadriceps serves as a strong extensor of the leg at the knee and flexes the thigh.

The SARTORIUS is the longest muscle in the body. It extends diagonally across the front of the thigh from its origin at the ilium, down to its insertion near the tuberosity of the tibia. Its function is to flex the thigh and rotate it laterally, and to flex the leg and rotate it slightly medially.

The GRACILIS is a long slender muscle located on the inner aspect of the thigh. It adducts the thigh and flexes and medially rotates the leg. Its origin is in the symphysis pubis, and its insertion is in the medial surface of the tibia, below the condyle.

The BICEPS FEMORIS (often called the hamstring muscle) originates at the tuberosity of the ischium and the middle third of the femur. It is inserted on the head of the fibula and the lateral condyle of the tibia. It acts, along with other related muscles, to flex the leg at the knee and to extend the thigh at the hip joint.

The GASTROCNEMIUS and SOLEUS (calf muscles) extend the foot at the ankle. The gastrocnemius originates at two points on the femur; the soleus originates at the head of the fibula and the medial border of the tibia. Both are inserted in a common tendon called the calcaneus, or Achilles tendon.

The TIBIALIS ANTERIOR originates at the upper half of the tibia and inserts at the first metatarsal and cuneiform bones. It flexes the foot.

  The Integumentary System

The skin, or integument, is the outer covering of the body. It consists of two layers, the epidermis and the dermis, and supporting structures and appendages (fig. 3-29).

Skin Function

The skin covers almost every visible part of the human body. Even the hair and nails are outgrowths from it. It protects the underlying structures from injury, drying, and invasion by foreign organisms; it contains the peripheral endings of many sensory nerves; and it has limited excretory and absorbing powers. It also plays an important part in regulating body temperature. In addition, the skin is a waterproof covering that prevents excessive water loss, even in very dry climates.

Skin Structure

The EPIDERMIS is the outer skin layer. It is made up of tough, flat, scalelike epithelial cells. Four different sublayers of epidermal cells have been identified. The uppermost is called the horny layer (stratum corneum). It is composed of scaly dead cells that form a protective surface and are gradually sloughed off naturally or by irritation (e.g., sunburn) or abrasion. This scaly layer, if unbroken, can block the passage of almost every known type of germ; however, its protective powers are reduced if the skin is not cleansed regularly. Two middle layers of cells may be present in a particular area of skin, depending on its thickness (the soles of the feet are the thickest skin, the eyelids the thinnest). In the innermost sublayer, the stratum germinativum, new epidermal cells are constantly being produced to replace the sloughed off cells. These newly formed cells push the older cells outward. As they approach the surface, they become drier or more scalelike. Because of this constant activity of the deeper cells of the epidermis, any injury of the outer layer of the skin is repaired in a few days without leaving a scar. Skin pigment, called melanin, which is responsible for skin color, is found here in this deepest sublayer. The color and quantity of the melanin are the chief factors in determining one's complexion. The pigment can be darkened by exposure to the ultraviolet rays of the sun (tanning). Freckles are patches of melanin.

The DERMIS, or true skin, lies below the epidermis and gradually blends into the deeper tissues. It is a wide area of connective tissue that contains blood vessels, hair follicles, nerve endings, smooth muscles, and sweat and oil glands.

The blood vessels of the dermis can dilate to contain a significant portion of the body's blood supply. This ability, along with the actions of the sweat glands, forms the body's primary temperature regulating mechanism. The constriction or dilation of these blood vessels also affects blood pressure and the volume of blood available to the internal organs.

The skin contains nerve endings that carry impulses to and from the central nervous system. The nerves are distributed to the smooth muscles in the walls of the arteries in the dermis and to the smooth muscles around the sweat glands and hair roots. Through these nerves, messages about the external environment are carried to the brain.

Smooth involuntary muscles are found in the dermis. They are responsible for controlling the skin surface area. When dilated, these muscles allow for maximum skin surface exposure to aid heat loss. When constricted, the skin surface exposure is decreased, thus impeding heat radiation. Repeated muscle contractions (shivering) are also a rapid means of generating body heat.

Skin Appendages

The appendages of the skin are the nails, hairs, sebaceous glands, sweat glands, and ceruminous glands.

The NAILS are composed of horny epidermal scales and are found on the dorsal surfaces of the fingers and toes. They protect the many sensitive nerve endings at the ends of these digits. New formation of nail will occur in the epithelium of the nail bed. As new nail is formed, the whole nail moves forward, becoming longer.

HAIR is an epithelial structure found on almost every part of the surface of the body. Its color depends on the type of melanin present. The hair has two components: the root below the surface and the shaft projecting above the skin. The root is embedded in a pitlike depression called the hair follicle. Hair grows as a result of the division of the cells of the root. A small muscle, the arrector, fastens to the side of the follicle and is responsible for the gooseflesh appearance of the skin as a reaction to cold or fear. Each hair follicle is associated with two or more sebaceous (oil) glands.

SEBACEOUS GLANDS are found in most parts of the skin except in the soles of the feet and the palms of the hand. Their ducts open most frequently into the hair follicles and secrete an oily substance that lubricates the skin and hair, keeping them soft and pliable and preventing bacterial invasion.

SWEAT GLANDS are found in almost every part of the skin. They are control mechanisms to reduce the body's heat by evaporation of water from its surface. The perspiration secreted is a combination of water, salts, fatty acids, and urea. Normally, about one liter of this fluid is excreted daily. However, the amount varies with atmospheric temperature and humidity and the amount of exercise taken. When the outside temperature is high, or upon exercise, the glands secrete excessive amounts to cool the body through evaporation. When evaporation cannot handle all the sweat that has been excreted, the sweat collects in beads on the surface of the skin.

CERUMINOUS GLANDS are modified sweat glands found only in the auditory canal. They secrete a yellow, waxy substance called cerumen that protects the eardrum.

  Circulatory System

The circulatory system, also called the VASCULAR SYSTEM, consists of the heart, blood vessels, and lymphatic system. It is the primary fuel supplier of the body. The transportation media is the blood. This system is a closed circuit. At no place does it have access to other tissues of the body except at the capillaries.

OSMOSIS, the transfer of fluids through the plasma membrane from an area of lower concentration of particles to an area of higher concentration, is the method of feeding body tissues and eliminating waste materials. This occurs in the capillaries, the smallest of the blood vessels.

Blood

Blood is fluid tissue composed of formed elements (cells) suspended in plasma. It is pumped by the heart through miles of arteries, capillaries, and veins to all parts of the body. Total blood volume of the average adult is 5 to 6 liters.

PLASMA is the liquid part of blood; the whole blood minus cells. Plasma constitutes 50 to 60 percent of whole blood. It is a clear, slightly alkaline, straw-colored liquid consisting of about 92 percent water. The remainder is made up mainly of proteins. One of these, fibrinogen, contributes to coagulation.

BLOOD SERUM is a clear, pale yellow liquid. It is the liquid portion of blood after coagulation. Plasma and serum differ in that plasma is whole blood minus the cells, and serum is plasma minus the clotting elements.

Red blood cells (RBCs), or ERYTHROCYTES, are small, biconcave, nonnucleated disks, formed in the red bone marrow. Blood of the average man contains 5 million red cells per cubic millimeter. Women have fewer red cells, 4.5 million per cubic millimeter. Emotional stress, strenuous exercise, high altitudes, and some diseases may cause an increase in the number of RBCs.

During the development of the red blood cell, a substance called hemoglobin is combined with it. HEMOGLOBIN is the key of the red cell's ability to carry oxygen and carbon dioxide. Thus the main function of erythrocytes is the transportation of respiratory gases. The red cells deliver oxygen to the body tissues, holding some oxygen in reserve for an emergency. Carbon dioxide is picked up by the same cells and discharged via the lungs.

The color of the red blood cell is determined by the hemoglobin content. Bright red, or arterial, blood is due to the combination of oxygen and hemoglobin. Dark red, or venous, blood is the result of hemoglobin combining with carbon dioxide.

A red blood cell will live only about 100 to 120 days in the body. There are several reasons for its short life span. This delicate cell has to withstand constant knocking around as it is pumped into the arteries by the heart. It travels through blood vessels at high speed, bumps into other cells, bounces off the walls of arteries and veins, and squeezes through narrow passages. It must adjust to continual pressure changes. Fragments of red blood cells are found in the spleen and other body tissues. The spleen is the "graveyard" where old, worn out cells are removed from the blood stream.

White blood cells (WBCs), or LEUKOCYTES, are almost colorless, nucleated cells originating in the bone marrow and in certain lymphoid tissues of the body. There is only one white cell to every 600 red cells. Normal WBC count is 6,000 to 8,000 per cubic millimeter, although the number of white cells may be 15,000 to 20,000 or higher during infection.

Leukocytes are important for the protection of the body against disease. Their AMEBOID movement permits them to leave the blood stream through the capillary wall and to attack pathogenic bacteria. They can travel anywhere in the body and are often named "the wandering cells." They protect the body tissues by engulfing disease-bearing bacteria and foreign matter, a process called phagocytosis. When white cells are undermanned, more are produced, causing an increase in their number and a condition known as leukocytosis. Another way they protect the body from disease is by the production of bacteriolysins that dissolve the foreign bacteria. The secondary function of WBCs is to aid in blood clotting.

Blood platelets, or THROMBOCYTES, are round bodies in the blood that contain no nucleus, only cytoplasm. They are smaller than red blood cells and average about 250,000 per cubic millimeter of blood. They play an important role in the process of blood coagulation, clumping together in the presence of jagged, torn tissue.

Blood Coagulation

To protect the body from excessive blood loss, blood has its own power to coagulate, or clot. If blood constituents and linings of vessels are normal, circulating blood will not clot. Once blood escapes from its vessels, however, a chemical reaction begins that causes it to become solid. The clot formed is at first fluid but soon becomes thick and then sets into a soft jelly that quickly becomes firm enough to act as a plug. This plug is the result of a swift, sure mechanism that changes one of the soluble blood proteins, fibrinogen, into an insoluble protein, fibrin, whenever injury occurs.

Other necessary elements for blood clotting are calcium salts, a substance called prothrombin, which is formed in the liver, blood platelets, and various factors necessary for the completion of the successive steps in the coagulation process. Once the fibrin plug is formed, it quickly enmeshes red and white blood cells and draws them tightly together. Blood serum, a yellowish clear liquid, is squeezed out of the clot as the mass shrinks. Formation of the clot closes the wound, preventing blood loss. A clot also serves as a network for the growth of new tissues in the process of healing. Normal clotting time is 3 to 5 minutes, but if any of the substances necessary for clotting are absent, severe bleeding will occur.

HEMOPHILIA is an inherited disease characterized by delayed clotting of the blood and consequent difficulty in controlling hemorrhage. Hemophiliacs may bleed to death as a result of even a trivial wound.

  The Heart

The heart is a hollow, muscular organ, somewhat larger than the closed fist, located anteriorly in the chest and to the left of the midline. It is shaped like a cone, its base directed upward and to the right, the apex down and to the left. Lying obliquely in the chest, much of the base of the heart is immediately posterior to the sternum.

The heart is enclosed in a membranous sac, the PERICARDIUM. The smooth surfaces of the heart and pericardium are lubricated by a serous secretion, the pericardial fluid. The inner surface of the heart is lined with a delicate serous membrane, the ENDOCARDIUM, similar to and continuous with that of the inner lining of blood vessels.

The interior of the heart (fig. 3-30) is divided into two parts by a wall called the INTERVENTRICULAR SEPTUM. In each half is an upper chamber, the ATRIUM, which receives blood from the veins, and a lower chamber, the VENTRICLE, which receives blood from the atrium and pumps it out into the arteries. The openings between the chambers on each side of the heart are separated by flaps of tissue that act as valves to prevent backward flow of the continuously forward moving column of blood. The one on the right has three flaps, or cusps, and is called the TRICUSPID VALVE. The one on the left has two flaps and is called the MITRAL, or BICUSPID, VALVE. The outlets of the ventricles are supplied with similar valves. On the right the pulmonary valve is at the origin of the pulmonary artery, and on the left the aortic valve is at the origin of the aorta.

Physiologically, the heart acts as four inter related pumps. The right atrium receives deoxygenated blood from the body via the superior and inferior vena cavae. It pumps this blood through the tricuspid valve to the right ventricle. The right ventricle pumps the blood past the pulmonary valve through the pulmonary artery to the lungs for oxygenation. The left atrium receives the oxygenated blood from the lungs through four pulmonary veins and pumps it to the left ventricle past the mitral valve. The left ventricle pumps the blood to all areas of the body via the aortic valve and the aorta.

The heart muscle, the MYOCARDIUM, is striated like the skeletal muscles of the body, but involuntary in action, like the smooth muscles. The walls of the atria are thin with relatively little muscle fiber because the blood flows from the atria to the ventricles under low pressure. However, the walls of the ventricles, which comprise the bulk of the heart, are thick and muscular. The wall of the left ventricle is considerably thicker than that of the right, because more force is required to pump the blood into the peripheral systemic circulation than into the lungs located only a short distance from the heart.

The heart acts by contraction and relaxation. It contracts with a wringing motion, forcing blood into the arteries. Each contraction is followed by limited relaxation or dilation. Cardiac muscle never completely relaxes, it always maintains a degree of tone. Contraction of the heart is called SYSTOLE and the period of work. Relaxation of the heart with limited dilation is called DIASTOLE and the period of rest. A complete CARDIAC CYCLE is the time from onset of one contraction, or heart beat, to the onset of the next.

The contractions of the heart are stimulated and maintained by the SINOATRIAL NODE, commonly called the PACEMAKER of the heart, which is a group of hundreds of cells in the upper part of the right atrium that sets off electrical impulses, causing both atria to contract simultaneously. The normal heart rate, or number of contractions, is about 72 beats per minute.

The BLOOD PRESSURE is the pressure the blood exerts on the walls of the arteries. The highest pressure is called SYSTOLIC pressure, because it is caused when the heart is in systole, or contraction. A certain amount of blood pressure is maintained in the arteries even when the heart is relaxed. This is the DIASTOLIC pressure, because it is present during diastole, or relaxation of the heart.

Normal blood pressure can vary considerably with age, weight, and general condition of the individual. For young adults the systolic pressure is between 120 and 150 mm of mercury, and the diastolic pressure is between 70 and 90 mm of mercury. Women have a lower blood pressure than men. The difference between systolic and diastolic pressure is known as PULSE PRESSURE.

  Blood Vessels

The blood vessels of the body fall into three distinct classifications:

1. Distributors-arteries and arterioles

2. Exchangers-capillaries

3. Collectors-veins and venules

The ARTERIES are elastic tubes constructed to withstand high pressure. They carry blood away from the heart to all parts of the body. The smallest branches of the arteries are called arterioles.

The AORTA is the large tubelike structure arising from the left ventricle of the heart. It arches upward over the left lung and then down along the spinal column through the thorax and the abdomen, where it divides to send arteries down both legs (fig. 3-31). The CORONARY ARTERIES are branches of what is generally called the ascending aorta, and they supply the heart with blood.

There are certain branches of the aorta with which you should be familiar, since these often must be compressed to control hemorrhage. You will find a discussion of pressure points in the hemorrhage section of the chapter in this manual entitled "First Aid and Emergency Procedures."

Three large arteries arise from the aorta as it arches over the left lung. First is the innominate artery, which divides into the right subclavian artery to supply the right arm, and the right com- mon carotid to supply the right side of the head. The second branch is the left common carotid, which supplies the left side of the head. The third branch from the arch of the aorta is the left subclavian, which supplies the left arm.

The carotids divide into internal and external branches, the external supplying the muscle and skin of the face and the internal supplying the brain and the eyes.

The subclavian arteries are so named because they run underneath the clavicle. They supply the upper extremity, branching off to the back, chest, neck, and brain through the spinal column (fig. 3-31).

The large artery going to the arm is called the axillary. It divides into the ulnar and radial arteries. The radial artery is the one at the wrist that you feel to take the pulse of your patient. It is located on the thumb side (fig. 3-32).

In the abdomen the aorta gives off branches to the abdominal viscera, including the stomach, liver, spleen, kidneys, and intestines. It finally divides into the left and right common iliacs, which supply the lower extremities. On entering the thigh, this artery is called the femoral artery. At the knee it becomes the popliteal artery (fig. 3-33).

At the end of the arterioles is a system of minute vessels that vary in structure, but which are spoken of collectively as CAPILLARIES. It is from these capillaries that the tissues of the body are fed. There are approximately 60,000 miles of capillaries in the body. As the blood passes through the capillaries, it releases oxygen and nutritive substances to the tissues and takes up various waste products to be carried away by the veins.

VEINS comprise a system of vessels that collect blood from the capillaries and carry it back to the heart. Veins begin as tiny venules formed from the capillaries. Joining together as tiny rivulets, they connect and form a small stream. The force of muscles contracting adjacent to veins aids in the forward propulsion of blood on its return to the heart. Valves, spaced frequently along the larger veins, prevent the backflow of blood.

  Blood Collection System (Venous Circulation)

Since arterial blood arises at the heart, we trace arteries from the heart. To return blood to the heart, we trace veins from the small venules back through larger veins. There are three principal venous systems in the body: the pulmonary, portal, and systemic.

The PULMONARY SYSTEM comprises four vessels, two from each lung, which empty into the left atrium. These are the only veins in the body that carry freshly oxygenated blood.

The PORTAL SYSTEM consists of the veins that drain venous blood from the abdominal part of the digestive tract (except the lower rectum), spleen, pancreas, and gallbladder and deliver it to the liver. There it is distributed by a set of venous capillaries. The blood in the portal system conveys absorbed substances from the intestinal tract to the liver for storage, alteration, or detoxification. From the liver the blood flows through the hepatic vein to the inferior vena cava.

The SYSTEMIC SYSTEM is divided into the deep and superficial veins. The superficial veins lie immediately under the skin, draining the skin and superficial structures. The deep veins, usually located in the muscle or deeper layers, drain the large muscle masses and various other organs. They usually lie close to the large arteries that supply the various organs of the body (fig. 3-31) and usually have the same name as the artery they accompany.

The superficial veins of the head unite to form the external jugular veins. They drain blood from the scalp, face, and neck, and finally empty into the subclavian veins.

The veins draining the brain and internal facial structures are the internal jugular veins. These combine with the subclavian veins to form the innominate veins, which empty into the superior vena cava (fig. 3-31).

The veins of the upper extremity begin at the hand and extend upward. An extremely valuable vein, the median cubital, crosses the anterior surface of the elbow. It is the vein most commonly used for intravenous injections and infusions.

The deep veins of the upper arm unite to form the axillary vein, which unites with the superficial veins to form the subclavian vein. This later unites with other veins to form the innominate and eventually, after union with still more veins, the superior vena cava.

In the lower extremity (fig. 3-32), a similar system drains the superficial areas. The great saphenous vein originates on the inner aspect of the foot and extends up the inside of the leg and thigh to join the femoral vein in the upper thigh. This vein is sometimes used for intravenous injections at the ankle. The superficial venous system of the leg often becomes varicose, or excessively dilated, particularly in persons whose occupations require long periods of standing. When this develops, the venous valves become incompetent, allowing stagnation of blood in the dependent extremity. Under these circumstances varicose ulcers frequently develop. Ligation at several points along the system will force the venous return into the deep venous system and restore normal venous circulation.

The veins from the lower extremities unite to form the femoral vein in the thigh, which becomes the external iliac vein in the groin. Higher in this region, it unites with the hypogastric vein from the lower pelvic region to form the common iliac vein. The two common iliac veins unite to form the inferior vena cava.

The veins from the abdominal organs, with the exception of those of the portal system, empty directly or indirectly into the inferior vena cava, while those of the thoracic region eventually empty into the superior vena cava.

  Lymphatic System

All tissue cells of the body are continuously bathed in interstitial fluid. This fluid is formed by leakage of blood plasma through minute pores of the capillaries. There is a continual interchange of fluids of the blood and tissue spaces with a free interchange of nutrients and other dissolved substances. Most of the tissue fluid returns to the circulation by means of venous capillaries, which feed into larger veins. Large protein molecules that have escaped from the arterial capillaries cannot reenter the circulation through the small pores of the venous capillaries. However, these large molecules, as well as white blood cells, dead cells, bacterial debris, infected substances, and larger particulate matter, can pass through the larger pores of the lymphatic capillaries and thus enter the lymphatic circulation with the remainder of the tissue fluid.

Lymph

Lymph usually is clear, but following ingestion of a fatty meal the lymph contained in the lymphatics that drain the small intestine appears milky because of the fat globules that have been absorbed. This milky lymph is called CHYLE.

Lymph Vessels

Lymph vessels and lymph nodes form a network throughout the body. Capillaries, like veins, collect lymph from the tissue spaces and, by means of a system in which small vessels unite to form larger ones, carry it toward the heart. As the lymph vessels increase in size, the walls become stronger until they are composed of three layers, like blood vessels. Along the path of the larger lymphatics are valves that prevent backflow of lymph.

Lymphatic channels from the upper half of the right side of the body converge to form the right lymphatic duct, which empties into the right subclavian vein. Drainage from the remainder of the body is by way of the thoracic duct, which empties into the left subclavian vein.

Lymph Nodes

Lymph nodes, which are frequently called glands but are not true glands, are small beanshaped bodies of lymphatic tissue found in groups of two to fifteen along the course of the lymph vessels. They usually occur singly just beneath the skin. Nodes vary in size and act as filters to remove bacteria and particles from the lymph stream. Lymph nodes also participate in the manufacture of white blood cells and thus in the immunity functions of the body.

  The Respiratory System

Respiration (breathing) is the exchange of oxygen and carbon dioxide between the atmosphere nd the cells of the body. There are two phases of respiration:

• Physical, or mechanical, respiration involves the motion of the diaphragm and rib cage. The musculoskeletal action, which resembles that of a bellows, causes air to be inhaled or exhaled.  

• Physiological respiration involves an exchange of gases, oxygen and carbon dioxide, at two points in the body. The first is the transfer that occurs in the lungs between the incoming oxygen and the carbon dioxide present in the capillaries of the lungs (external respiration). The second transfer occurs when the oxygen brought into the body replaces the carbon dioxide build up in the cellular tissue (internal respiration).

Normally, oxygen and carbon dioxide exchange in equal volumes; however, certain physiological conditions may throw this balance off. For example, heavy smokers will find that the ability of their lungs to exchange gases is impaired, leading to shortness of breath and fatigue during even slight physical exertion. This is the direct result of their inability to draw a sufficient amount of oxygen into the body to replace the carbon dioxide build-up and sustain further muscular exertion. On the other side, hyperventilation brings too much oxygen into the body, overloading the system with oxygen and depleting the carbon dioxide needed for balance.

Anatomy of the Respiratory System

Air enters the nasal chambers and the mouth, then passes through the pharynx, larynx, and bronchi into the bronchioles, which form a network around the alveolar air sacs in the lungs (figs. 3-34 and 3-35).

Air enters the NASAL CAVITY through the nostrils (NARES). Lining the nasal passages are hairs, which together with the mucous membrane, entrap and filter out dust and other minute particles that could irritate the lungs. Incoming air is warmed and moistened in the chambers of the nasal cavity to prevent damage to the lungs.

The mouth and nose serve as auxiliary respiratory structures.

The PHARYNX, or throat, serves both the respiratory and digestive systems and aids in speech. It has a mucous membrane lining that traps microscopic particles in the air and aids in adjusting temperature and humidifying inspired air. The pharynx connects with the mouth and nasal chambers posteriorly. According to its location, it is referred to as:

• NASOPHARYNX-posterior to the nasal chambers

• OROPHARYNX-posterior to the mouth

• LARYNGOPHARYNX-posterior to the pharynx

The EPIGLOTTIS is a lidlike, cartilaginous structure that covers the entrance to the larynx and separates it from the pharynx. It acts as a trap door to deflect food particles and liquids from the entrance to the larynx and trachea.

The LARYNX, or voice box, is a triangular cartilaginous structure located between the tongue and the trachea. It is protected anteriorly by the thyroid cartilage (Adam's apple), which is usually larger and more prominent in men than in women. During the act of swallowing, it is pulled upward and forward toward the base of the tongue. The larynx is responsible for the production of voice. This is accomplished by the passing of air over the vocal cords. The ensuing vibrations can be controlled to produce the sounds of speech or singing. The nose, mouth, throat, bone sinuses, and chest serve as resonating chambers to further refine and individualize the voice.

The TRACHEA, or windpipe, begins at the lower end of the larynx and terminates by dividing into the right and left bronchi. It is a long tube composed of 16 to 20 C-shaped cartilaginous rings, embedded in a fibrous membrane, that support its walls, preventing their collapse (fig. 3-35).

The trachea has a ciliated mucous membrane lining that entraps dust and foreign material. It also propels secretions and exudates from the lungs to the pharynx, where they can be expectorated.

The BRONCHI are the terminal branches of the trachea, which carry air to each lung and further divide into the bronchioles (fig. 3-35).

The BRONCHIOLES are much smaller than the bronchi and lack supporting rings of cartilage. They terminate at the alveoli (fig. 3-36).

The ALVEOLI are thin, microscopic air sacs within the lungs. They are in direct contact with the pulmonary capillaries. It is here that fresh oxygen exchanges with carbon dioxide by means of a diffusion process through the alveolar and capillary cell walls (fig. 3-37). The LUNGS are cone-shaped organs that lie in the thoracic cavity. Each lung contains thousands of alveoli with their capillaries. The right lung is larger that the left and is divided into superior, middle, and inferior lobes. The left lung has two lobes, the superior and the inferior.

The PLEURAE are airtight membranes that cover the outer surface of the lungs and line the chest wall. They secrete a serous fluid that prevents friction during movements of respiration. Pleurisy is a painful inflammation of the pleural lining.

The MEDIASTINUM is the interpleural space between the two lungs. It extends from the sternum to the thoracic vertebrae and from the fascia of the neck to the diaphragm. It contains the heart, the great blood vessels, the esophagus, a portion of the trachea, and the primary bronchi.

The DIAPHRAGM is the primary muscle of respiration. It is dome-shaped and separates the thoracic and abdominal cavities. Contraction of the muscle flattens the dome and expands the vertical diameter of the chest cavity.

The INTERCOSTAL MUSCLES are situated between the ribs. Their contraction pulls the ribs upward and outward, resulting in an increase in the transverse diameter of the chest (chest expansion).

INHALATION is the direct result of the expansion caused by the action of the diaphragm and intercostal muscles. The increase in chest volume creates a negative (below atmospheric) pressure in the pleural cavity and lungs. Air rushes into the lungs through the mouth and nose to equalize the pressure. EXHALATION results when the muscles of respiration relax. Pressure is exerted inwardly as muscles and bones return to their normal position, forcing air from the lungs.

The Process of Respiration

The rhythmical movements of breathing are controlled by the respiratory center in the brain. Nerves from the brain pass down through the neck to the chest wall and diaphragm. The nerve to the diaphragm is called the phrenic nerve; the nerve to the larynx is the vagus nerve; and those to the muscles between the ribs are the intercostal nerves.

The respiratory center is stimulated by chemical changes in the blood, especially if it becomes acidic. When too much carbon dioxide accumulates in the blood stream, the respiratory center signals the lungs to breathe faster to get rid of the carbon dioxide.

The respiratory center can also be stimulated or depressed by a signal from the brain. For example, changes in one's emotional state can alter respiration through laughter, crying, emotional shock, or panic.

The muscles of respiration normally act automatically, with normal respiration being 14 to 18 cycles per minute. The lungs, when filled to capacity, hold about 6,500 ml of air, but only 500 ml of air is exchanged with each normal respiration. This exchanged air is called TIDAL AIR. The amount of air left in the lungs after forceful exhalation is about 1,200 ml and is known as RESIDUAL AIR. The existence of this reserve is the basis for administering the abdominal thrust maneuver, described in the chapter entitled "First Aid and Emergency Procedures." In this lifesaving procedure, the residual air is used to force a foreign object out of the trachea.

Abnormalities of Breathing

The following terms are used to describe breathing and significant variations in exchanges of respiratory gases:

• EUPNEA is ordinary quiet respiration.

• BRADYPNEA is abnormal slowness of breathing.

• HYPOPNEA is abnormal decrease in the depth and rate of the respiratory movements.

• DYSPNEA is labored or difficult breathing.

• HYPERPNEA is abnormal increase in the depth and rate of the respiratory movements.

• APNEA is cessation of breathing.

• CHEYNE-STOKES RESPIRATION are respirations that increase with force and frequency up to a certain point, then decrease until they cease altogether. After a short period of apnea, the respirations begin again, and the cycle is repeated.

• STERTOROUS RESPIRATION is breathing with abnormal snoring sounds.

• RALES are abnormal respiratory sounds, either moist or dry depending upon the fluid in the air passages, which are classified according to their location as bronchial or laryngeal rales.

• RHONCHUS is a rattling sound in the throat due to partial obstruction; it is also a dry, coarse rale in the bronchial tubes.

  The Nervous System

To effectively support human life, the activities of all the widely diverse cells, tissues, and organs of the body must be monitored, regulated, and coordinated. The interaction of the nervous and endocrine systems provides the needed control.

The nervous system is specifically adapted to the rapid transmission of impulses from one area of the body to another. On the other hand, the endocrine system, working at a far slower pace, maintains body metabolism at a fairly constant level.

In this section we will study the structure and functions of the nervous system.

The Neuron

The structure and functional unit of the nervous system is the nerve cell, or neuron, which can be classified into three types. The first is the sensory neuron, which conveys sensory impulses inward from the receptors. The second is the motor neuron, which carries command impulses from a central area to the responding muscles or organs. The third type is the interneuron, which links the sensory neurons to the motor neurons.

The neuron is composed of dendrites, a cyton, and an axon (fig. 3-38). The DENDRITES are thin receptive branches, which vary greatly in size, shape, and number with different types of neurons. They serve as receptors, conveying impulses toward the cyton. The CYTON is the cell body containing the nucleus. The single, thin extension of the cell outward from the cyton is called the AXON. It conducts impulses away from the cyton to its terminal filaments, which transmit the impulses to the dendrites of the next neuron.

Impulse Transmission

When dendrites receive a sufficiently strong stimulus, a short and rapid depolarization of the neuron is triggered. Sodium ions rush through the plasma membrane into the cell, potassium ions leave, and an electrical impulse is formed, which is conducted toward the cyton. The cyton receives the impulse and transmits it to the terminal filaments of the axon. At this point a chemical transmitter such as acetylcholine is released into the SYNAPSE, a space between the axon of the activated nerve and the dendrite receptors of another neuron. This transmitter activates the next nerve. In this manner the impulse is passed from neuron to neuron down the nerve line to a central area at approximately the speed of a bullet.

Almost immediately after being activated, the transmitter chemical in the synapse is neutralized by the enzyme acetylcholinesterase, and the first neuron returns to its normal state by pumping out the sodium ions and drawing potassium ions back in through the plasma membrane. When these actions are completed, the nerve is ready to be triggered again. A particularly strong stimulus will cause the nerve to fire in rapid succession, or will trigger many other neurons, thus giving a feeling of intensity to the perceived sensation.

Central Nervous System

The central nervous system consists of the brain and spinal cord (fig. 3-39). The brain is almost entirely enclosed in the skull, but it is connected with the spinal cord, which lies in the canal formed by the vertebral column.

Brain

The brain has two main divisions, the cerebrum and the cerebellum. The cerebrum is the largest and most superiorly situated portion of the brain. It occupies most of the cranial cavity. The outer surface is called the cortex. This portion of the brain is also called gray matter because the nerve fibers are unmyelinated (not covered by a myelin sheath), causing them to appear gray. Beneath this layer is the medulla. This is often called the white matter of the brain, because the nerves are myelinated (covered with a myelin sheath and an outer covering called the neurilemma), which gives them their white appearance (fig. 3-40).

The cortex of the cerebrum is irregular. It bends on itself in folds called convolutions, which are separated from each other by grooves and fissures. The deep sagittal cleft, a longitudinal fissure, divides the cerebrum into two hemispheres. Other fissures further subdivide the cerebrum into lobes, each of which serves a localized, specific brain function (fig. 3-41). For example, the frontal lobe is associated with the higher mental processes such as memory, the parietal lobe is concerned primarily with general sensations, the occipital lobe is related to the sense of sight, and the temporal lobe is concerned with hearing.

The cerebellum is situated posteriorly to the brain stem, which is made up of the pons, midbrain, and medulla oblongata, and inferior to the occipital lobe. It is concerned chiefly with bringing balance, harmony, and coordination to the motions initiated by the cerebrum.

Two smaller divisions of the brain, vital to life, are the pons and the medulla oblongata. The pons consists chiefly of a mass of white fibers connecting the other three parts of the brain-the cerebrum, cerebellum, and medulla oblongata.

The medulla oblongata is the inferior portion of the brain, the last division before the beginning of the spinal cord. It connects to the spinal cord at the upper level of the first cervical vertebra (C-1). In it are the centers for the control of heart action, breathing, circulation, and other vital processes such as blood pressure.

The outer surface of the brain and spinal cord is covered with three layers of membranes called the meninges. The dura mater is the strong outer layer; the arachnoid membrane is the delicate middle layer; and the pia mater is the vascular innermost layer that adheres to the surface of the brain and spinal cord. Inflammation of the meninges is called meningitis. The type depends upon whether the brain, spinal cord, or both are affected.

Cerebrospinal fluid is formed by a plexus (network) of blood vessels in the central ventricles of the brain. It is a clear, watery solution similar to blood plasma. The total quantity bathing the spinal cord is about 75 ml. It is constantly being produced and reabsorbed. It circulates over the surface of the brain and spinal cord and serves as a protective cushion as well as a means of exchange for nutrients and waste materials.

Spinal Cord

The spinal cord is continuous with the medulla oblongata and extends from the foramen magnum, down inside the atlas, to the lower border of the first lumbar vertebra, where it tapers to a point. The cord is surrounded by the bony walls of the vertebral canal (fig. 3-42). It is ensheathed in the three protective meninges and surrounded by adipose tissue and blood vessels. The cord does not completely fill the vertebral canal, nor does it extend the full length of it. The nerve roots serving the lumbar and sacral regions must pass some distance down the canal before making their exit.

A cross section of the spinal cord shows white and gray matter (fig. 3-43). The outer white matter is composed of bundles of myelinated nerve fibers arranged in functionally specialized tracts. It establishes motor communication between the brain and the body parts. The inner gray unmyelinated matter is shaped roughly like the letter H. It establishes sensory communication between the brain and the spinal nerves, conducting sensory impulses from the body parts. It also plays an integral role in the autonomic nervous system and in the reflex arc, both of which will be discussed later.

The spinal cord may be thought of as an electric cable containing many wires (nerves) that connect parts of the body with each other and with the brain. Sensations received by a sensory nerve are brought to the spinal cord, and the impulse is transferred either to the brain or to a motor nerve. The majority of impulses go to the brain for action. However, a system exists for quickly handling emergency situation. It is called the reflex arc.

If you touch a hot stove, you must remove the hand from the heat source immediately or the skin will burn very quickly. But the passage of a sense impulse to the brain and back again to a motor nerve takes time. The reflex arc is set up to respond instantaneously to emergency situations like the one just described. The sensation of hot travels to the spinal cord on a sensory nerve, where it is picked up by an interneuron in the gray matter, which triggers the appropriate nerve to stimulate a muscle reflex drawing the hand away. Another example of the reflex arc is shown in fig. 3-43. The reflex arc works well in simple situations r

equiring no action of the brain. Consider, however, what action is involved if the individual touching the stove pulls back and, in so doing, loses his or her balance and has to grab a chair to regain stability. Then the entire spinal cord is involved. Additional impulses must travel to the brain, then down to the muscles of the legs and arms to enable him or her to maintain balance and to hold on to a steadying object. While all this is going on, the stimulus is relayed through the sympathetic autonomic nerve fibers to the adrenal glands, causing adrenalin to flow, which stimulates heart action, and to the brain, making the individual conscious of pain. In this example the spinal cord has functioned not only as a center for spinal relaxes, but also as a conduction pathway for other areas of the spinal cord to the autonomic nervous system and to the brain.

Peripheral Nervous System

The peripheral nervous system is made up of 12 pairs of cranial nerves and 31 pairs of spinal nerves arising from the brain and spinal cord, respectively. These nerves carry both voluntary and involuntary impulses (fig. 3-44).

Cranial Nerves

The 12 pairs of cranial nerves are sensory, motor, or mixed (sensory and motor).

• The OLFACTORY nerve (sensory) conveys the sense of smell from the mucous membrane in the upper nose to the olfactory center of the brain.

• The OPTIC nerve (sensory) conveys the sensation of sight from the retinal cells of the eye to the visual area of the brain.

• The OCULOMOTOR nerve (motor) controls most muscles that move the eyeball and some of those in the iris of the eye.

• The TROCHLEAR nerve (motor) controls the muscles that turn the eyeball down and to the side.

• The TRIGEMINAL nerve (sensory and some motor) is divided into three branches: ophthalmic, maxillary, and mandibular. It sometimes is called the great sensory nerve of the head because it supplies the sense of touch, pain, heat, and cold to the skin of the face, eyelids, cornea, conjunctiva, tongue, teeth, and mucous membranes of the head. A branch of the mandibular division supplies motor fibers to the muscles of mastication.

• The ABDUCENS nerve (motor) controls the muscles that turn the eye outward.

• The FACIAL nerve (motor and sensory) control the muscles of the face, scalp, and ears. It contains autonomic motor fibers, which cause the salivary glands to secrete, and sensory fibers, which carry taste sensations from the anterior two-thirds of the tongue to the brain.

• The ACOUSTIC (vestibulocochlear) nerve (sensory) is the nerve of hearing and equilibrium.

• The GLOSSOPHARYNGEAL nerve (motor and sensory) carries sensation from the pharynx and posterior one-third of the tongue and transmits motor impulses to the parotid gland and to one of the small muscles of swallowing.

• The VAGUS nerve (motor and some sensory) is composed of motor fibers (some of which are parasympathetic) and sensory fibers. It extends down through the neck to the pharynx, larynx, trachea, esophagus, and thoracic and abdominal viscera.

• The ACCESSORY nerve (motor) supplies nerves to muscles of the neck (sternocleidomastoid, trapezius, pharyngeal, and laryngeal).

• The HYPOGLOSSAL nerve (motor) controls the muscles of the tongue.

Spinal Nerves

Spinal nerves arise from the spinal cord and leave the vertebral canal in the spaces between the vertebrae. These nerves send fibers to sensory surfaces and all muscles of the trunk and extremities. Also, involuntary fibers go to the smooth muscles and glands of the gastrointestinal tract, urogenital system, and cardiovascular system. There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and I coccygeal. The lower spinal nerves going to the legs and feet extend below the level of the spinal cord. The nerve roots arising from the lumbar and sacral regions pass some distance down the canal before making their exit. This bundle of nerve roots is called the cauda equina because it resembles a horse's tail. The various roots emerge through openings in the sacrum and extend to the areas they supply. Spinal nerves contain all types of sensory and motor fibers of both the voluntary and autonomic nervous systems. In some regions of the body they interlace in a thick network called a plexus. The cervical plexus is located in the neck, and the brachial plexus is in the shoulder. In the pelvic region are the lumbar, sacral, and pudendal plexuses.

  Autonomic Nervous System

The autonomic nervous system, as its name implies, functions automatically. It helps to regulate the smooth muscles, cardiac muscle, digestive tube, blood vessels, sweat and digestive glands, and certain endocrine glands. It is not directly under the control of the brain but usually works in harmony with the nerves that are under the brain's control. The autonomic nervous system is divided into the sympathetic and parasympathetic systems (see table 3-1).

Sympathetic Nervous System

Numerous ganglia (nerve centers) located just outside the spinal cord, beside the vertebrae, are the basis of the sympathetic (thoracolumbar) system. These nerve centers connect with the thoracic and lumbar regions of the spinal cord and, through the spinal nerves, with the muscles, organs, and glands they affect.

Because one function of the sympathetic system is to increase the activity of the body to enable it to meet danger or undergo strenuous physical activity, it has been called the "fight or flight" nervous system. The sympathetic nerves, when stimulated, usually discharge as a unit, and the effects can be noticed especially under circumstances of fright or rage; for example, the heart beats faster, blood pressure rises, the spleen discharges red blood cells into the blood, the blood sugar level rises, the pupils dilate, and the peripheral blood vessels constrict. These changes prepare the body for a stressful situation.

Parasympathetic System

The ganglia of the parasympathetic system are located in the midportion of the brain, the medulla oblongata, and the sacral regions. For this reason the parasympathetic system is sometimes called the craniosacral system. The ganglia in the midbrain and medulla oblongata send impulses out along cranial nerves (oculomotor, facial, glossopharyngeal, and vagus). The sacral ganglia stem from the second, third, and fourth sacral nerves. The parasympathetic nerves do not all discharge at once. They aim more toward conserving and restoring energy. Their actions slow the heart beat, lower the blood pressure, stimulate gastrointestinal movements and secretion, aid absorption, constrict the pupils, dilate peripheral blood vessels, and empty the bladder and rectum. Overall they promote the autonomic restoration of body systems to normal functioning after sympathetic stimulation. The sympathetic and parasympathetic systems counterbalance each other to preserve a harmonious balance of body functions and activities.

  The Sensory System

The sensory system functions to inform areas of the cerebral cortex of changes that are taking place within the body or in the external environment. The special sensory receptors are designed to respond only to a special individual stimulus such as sound waves, light, taste, smell, pressure, heat, cold, pain, or touch. Positional changes, balance, hunger, and thirst sensations are also detected and passed on to the brain.

Smell

Odor is perceived upon stimulation of the receptor cells in the olfactory membrane of the nose. The olfactory receptors are very sensitive, but they are also easily fatigued. This explains why odors that are initially very noticeable are not sensed after a short time. Smell is not as well developed in man as in other mammals.

Taste

The taste buds are located in the tongue. The sensation of taste is limited to sour, sweet, bitter, and salty. Many foods and drinks tasted are actually smelled, and their taste depends upon their odor. This can be demonstrated by pinching the nose shut when eating onions. Sight can also affect taste. Several drops of green food coloring in a glass of milk will make it all but unpalatable, even though the true taste has not been affected.

Sight

The eye, the organ of sight, is a specialized structure for the reception of light. It is assisted in its function by accessory structures, such as the ocular muscles, eyelids, conjunctiva, and lacrimal apparatus.

Structure of The Eye

The eye is a hollow ball, or globe, which consists of various tissues that perform specific functions. The globe, or eyeball, is composed of three layers (fig. 3-45).

Outer Layer - The outer layer of the eye is called the sclera. It is the tough, fibrous, protective portion of the globe, commonly called the white of the eye. Anteriorly, the outer layer is transparent and is called the cornea, or the window of the eye. It permits light to enter the globe. The exposed sclera is covered with a mucous membrane, the conjunctiva, which is a continuation of the inner lining of the eyelids. The lacrimal gland produces tears that constantly wash the front part of the eye and the conjunctiva. The tear gland secretions that do not evaporate flow toward the inner angle of the eye where they drain down ducts into the nose.

Middle Layer - The middle layer of the eye is called the choroid. It is a highly vascular, pigmented tissue that provides nourishment to the inner structures. Continuous with the choroid is the ciliary body, whose muscular structure attaches to the lens by means of suspensory ligaments and produces changes in the thickness of the lens. This permits the eye to focus to longrange or close-up vision.

The iris is continuous with the ciliary body. It is a circular, pigmented muscular structure that gives color to the eye. The opening in the iris is called the pupil (fig 3-46). The amount of light entering the pupil is regulated through the constriction of radial/circular muscles in the iris. When strong light is flashed into the eye, the circular muscle fibers of the iris contract, reducing the size of the pupil. If the light is dim, the pupil dilates to allow as much of the light in as possible. The size and reaction of the pupils of the eyes are an important diagnostic tool.

The lens is a transparent, biconvex structure suspended directly behind the iris. It separates the interior eye into anterior and posterior cavities. The anterior cavity contains a watery solution alled aqueous humor, which helps to give the cornea its curved shape. The optic globe posterior to the lens is filled with a jellylike substance called vitreous humor, which helps to maintain the shape of the eyeball and prevents misshaping by maintaining intraocular pressure.

Inner Layer - The inner layer of the eye is called the retina (fig. 3-47). It contains different layers of nerve cells, rods, and cones that are the receptors of the sense of vision. The retina is continuous with the optic nerve, which enters the back of the globe and carries visual impulses received by the rods and cones to the brain. The area where the optic nerve enters the eyeball contains no rods and cones and is called the blind spot.

The rods respond to low intensities of light and are responsible for night vision. They are located in all areas of the retina, except in the small depression called the fovea centralis, where light entering the eye is focused, and which has the clearest vision.

The cones require higher light intensities for stimulation and are most densely concentrated in the fovea centralis. The cones are responsible for daytime vision.

Vision Process

Deflection or bending of light rays results when light passes through substances of varying densities in the eye (cornea, aqueous humor, crystalline lens, and vitreous humor) (fig. 3-48). The deflection is referred to as refraction. Accommodation is the process performed by the lens that increases or decreases its curvature to refract light rays into focus on the fovea.

The constriction of the pupil by the iris regulates the amount of light entering the eye. This process protects the retina from excessive stimulation and prevents a scattering of light rays that would produce blurred vision.

A movement of the globes toward the midline, which causes a viewed object to come into focus on corresponding points of the two retinas, is called convergence. This gives clear, three dimensional vision.

The end receptors or nerve endings in the rods and cones that have been stimulated by light conduct impulses to the occipital lobes of the cerebrum, where they are interpreted into vision (fig. 3-48).

Hearing

The ear is the primary organ of hearing. It is divided into three parts: the external, middle, and inner ear (fig. 3-49 and fig. 3-50).

External Ear

The external, or outer, ear is composed of two parts, the auricle and the external auditory canal. The auricle, or pinna, is a cartilaginous structure located on each side of the head. The auricle collects sound waves from the environment, which are then conducted by the external auditory canal to the eardrum. The lining of the auditory canal contains glands that secrete a waxy substance called cerumen. The cerumen aids in protecting the eardrum against foreign bodies and microorganisms.

The eardrum, or tympanic membrane, is an oval sheet of fibrous epithelial tissue, 10 mm by 9 mm in size, which stretches across the inner end of the external auditory canal and separates the outer and middle ear. The sound waves cause the eardrum to vibrate. This vibration transfers the sounds from the external environment to the auditory ossicles.

Middle Ear

The middle ear is a cavity in the temporal bone, lined with epithelium. It contains three auditory ossicles-the malleus (hammer), the incus (anvil), and the stapes (stirrup)-which transmit vibrations from the tympanic membrane to the fluid in the inner ear. The malleus is attached to the inner surface of the eardrum and connects with the incus, which in turn connects with the stapes. The base of the stapes is attached to the oval window (fenestra ovalis), the membrane-covered opening of the inner ear. These tiny bones link together to span the middle ear. They are suspended from its bony wall by ligaments and provide the mechanical means for transmission of sound vibrations to the inner ear.

The eustachian tube connects the middle ear with the pharynx. It is lined with a mucous membrane and is about 36 mm long. Its function is to equalize internal and external air pressure. For example, while riding an elevator in a tall building, you may experience a feeling of pressure in the ear. This is usually relieved by swallowing, which opens the eustachian tube and allows the pressurized air to escape and equalize with the area of lower pressure. Divers who ascend too fast to allow pressure to adjust may experience rupture of their eardrums. The eustachian tube can also be a pathway for infection of the middle ear.

Inner Ear

The inner ear is filled with a fluid called endolymph. Sound vibrations that cause the stapes to move against the oval window create internal ripples that run through the endolymph. These pressurized ripples move to the cochlea, a small snail-shaped structure housing the organ of Corti, the hearing organ. The cells protruding from the organ of Corti are stimulated by the ripples to convert these mechanical vibrations into nerve impulses, which are relayed through the cochlear (8th cranial) nerve to the auditory area of the cortex in the temporal lobe of the brain. Here they are interpreted as the sounds we hear.

Other structures of the inner ear are the three semicircular canals, situated perpendicular to each other. Movement of the endolymph within the canals, caused by general body movements, stimulates nerve endings, which report these changes in body position to the brain, which in turn uses the information to maintain equilibrium.

The round window (fenestra rotunda) is another membranecovered opening of the inner ear. It contracts the middle ear and flexes to accommodate the inner ear ripples caused by the stapes.

Touch

Until the beginning of the last century, touch (feeling) was treated as a single sense. Thus warmth or coldness, pressure, and pain, were thought to be part of a single sense of touch or feeling. It was then discovered that different types of nerve ending receptors are widely, but unevenly, distributed in the skin and mucous membranes. For example, the skin of the back possesses relatively few touch and pressure receptors while the fingertips have a great many. The skin of the face has relatively few cold receptors, and the mucous membranes have few heat receptors. The cornea of the eye is sensitive to pain, and when pain sensation is abolished by a local anesthetic, a sensation of touch can be experienced.

There are five kinds of receptors. The most important, those for the sense of touch, are bare nerve endings next to hairs and specialized encapsulated nerve endings called Meissner's corpuscles. Cold receptors also have encapsulated nerve endings.

The receptors for pain are naked nerve filaments and are the most numerous; they are also the only kind present in the deeper tissues, although stimulation of these usually causes the pain to be referred to a skin area. Three kinds of pain may be experienced: superficial or cutaneous pain; deep pain from muscles, tendons, joints, and fascia; and visceral pain.

Other Senses

Certain nerve receptors, located in muscles and tendons, are stimulated by changes in tension and pressure and continually inform the brain regarding the position of parts of the body (body sense).

Hunger results from rhythmic contractions of the stomach when it has emptied its contents. Blood sugar levels also influence the feeling of hunger. Habit is another factor; for example, persons who habitually snack in midmorning will feel hunger contractions at normal snacktime. If the snack is not eaten for several days, and adequate food intake continues at mealtime, the hunger contractions at snacktime will diminish. The nervous system also plays a part in controlling hunger, either by depressing the desire for food or by stimulating appetite.

The drying of the membranes in the oral cavity influences the sensation of thirst. Although thirst may be due to a lack of water in body tissues, a reduced salivary flow can produce a sensation of thirst.

Special Functions

SPEECH is controlled by the coordinated action of several nerve functions. The speech center is located deep in the brain, and from it nerve impulses pass out to the larynx, which contains folds of mucous membranes called vocal cords. When air is forced from the lungs past these folds, certain sounds are produced, and, in conjunction with the movements of the throat, lips, tongue, and teeth, articulate speech results.

SLEEP is a period of unconsciousness when the higher physical powers are quiet, although body activities continue. It is usually considered a period of rest in which constructive processes build up and repair the body. Certain changes take place during sleep: respiration is slowed; less blood is sent to the brain and greater amounts to to the extremities; digestion goes on, but at a slower rate; body temperature may drop somewhat; and heart action is slowed.

  The Endocrine System

Homeostasis depends on the nervous and endocrine systems, since both are lines of communication for body functions. The endocrine system sends messages by chemical hormones that are carried in the blood stream. These messages aid in the control, development, and integration of body functions.

The endocrine system is made up of glands of internal secretion. These are called ductless glands because they have no ducts to carry away their secretions. The secretion of an endocrine gland is called a hormone. It enters directly into the blood or lymph circulation and eventually reaches the gland, tissue, or organ it controls or influences. Very small quantities of hormones are produced, since only a trace amount is needed to produce the desired effect.

Most hormones can be extracted from the glands of animals, and some can be produced synthetically. Medical officers may prescribe these isolated or synthetic hormones for patients who are deficient in them or who might otherwise benefit from their use. The hormone-producing glands include the pituitary, thyroid, parathyroids, adrenals, gonads, and pancreas (fig. 3-51).

Hypothalamus

The hypothalamus, a structure in the brain, synthesizes chemicals that are secreted to the pituitary gland to stimulate the release of its hormones.

Pituitary Gland

The pituitary is a small, pea-sized gland located at the base of the brain in the sella turcica of the sphenoid bone. It is often called the master gland of the body, because it influences most other endocrine glands. It is divided into two lobes, an anterior and a posterior. The anterior lobe plays the more important role in influencing body functions. The hormones it produces have a broad and significant range of effects.

SOMATOTROPIN, the growth hormone, influences body growth and development. During he growth years an overproduction of somatotropin causes giantism while the lack of it causes dwarfism. An overproduction after the growth years causes acromegaly, which is characterized by the development of abnormally large hands, feet, and jaw.

THYROTROPIN, or the thyroid-stimulating hormone (TSH), influences the growth, development, and secreting activities of the thyroid gland.

GONADOTROPIN influences the gonads (ovaries or testes) and is essential for the normal development and functioning of both male and female reproductive systems.

The ADRENOCORTICOTROPIC hormone (ACTH) acts primarily on the adrenal cortex, stimulating its growth and its secretion of corticosteroids. Removal of the pituitary leads to rapid atrophy of the adrenal cortex.

The posterior lobe of the pituitary produces at least two hormones, vasopressin and oxytocin. VASOPRESSIN acts as an antidiuretic hormone (ADH), promoting the conservation of water by the kidney. When ADH is not produced in adequate amounts, the daily urine volume is between 10 and 15 liters instead of the normal 1.5 liters. This condition is known as diabetes insipidus.

OXYTOCIN stimulates contraction of the muscles of the uterus, particularly during pregnancy. It also plays an important role in the production of milk in the mammary glands of nursing mothers.

Thyroid Gland

The thyroid, shaped like a butterfly, lies in the anterior part of the neck, below the larynx. It consists of two lobes, one on each side of the upper trachea, connected by a strip of tissue called the isthmus. The thyroid secretes the iodinecontaining hormone THYROXIN, which controls the rate of cell metabolism. Excessive secretion of thyroxin raises the metabolic rate and causes hyperthyroidism, a condition characterized by a fast pulse rate, dizziness, increased basal metabolism, profuse sweating, tremors, nervousness, and a tremendous appetite yet a loss of weight. The thyroid may become enlarged.

Iodine is essential for the formation of thyroxin. To prevent simple goiter, iodine-containing foods, such as vegetables, iodized salt, and seafood, are eaten.

Hypothyroidism, on the other hand, is caused by an insufficient secretion of thyroxin. The patient exhibits a decrease in basal metabolism, and sweating is almost absent. There may be a weight gain and constant fatigue. The heart rate may be slow, and there may be an enlargement of the gland, called a simple goiter. There may also be personality changes characterized by slow, lethargic mental functioning. Hypothyroidism in an infant may result in cretinism with impaired mental and physical development.

Parathyroid Glands

Parathyroid glands are four small round bodies located just posterior to the thyroid gland. Their hormone, PARATHORMONE, regulates the calcium and phosphorus content of the blood and bones. The amount of calcium is important in certain tissue activities, such as bone formation, coagulation of blood, maintenance of normal muscular excitability, and milk production in the nursing mother. Diminished function or removal of the parathyroid glands results in a low calcium level in the blood, and in extreme cases death may occur, preceded by strong contraction of the muscles (tetany) and convulsions.

Hyperparathyroidism, an excess of parathyroid hormone in the blood, causes calcium levels in the blood to become elevated by the withdrawal of calcium from the bones, leaving the skeleton demineralized and subject to spontaneous fractures. The excess calcium may be deposited as stones in the kidneys.

Adrenal Glands

The adrenal glands are located on the superior surface of each kidney, fitting like a cap. They consist of an outer portion, the cortex, and an inner portion, the medulla.

Adrenal Cortex

Specialized cells in the outer layer of the adrenal cortex produce three types of steroid hormones that are of vital importance.

MINERALOCORTICOIDS are regulators of fluid and electrolyte balance. They are sometimes called salt and water hormone because they regulate the excretion and absorption of sodium, chlorine, potassium, and water.

GLUCOCORTICOIDS are essential to metabolism. They increase certain liver functions and have an anti-inflammatory effect. Clinically they are used to suppress inflammatory reactions, to promote healing, and to treat rheumatoid arthritis.

The adrenal cortex also produces sex hormones, some with male characteristics (ANDROGENS), others with female characteristics (ESTROGENS). These hormones appear in different concentrations in both men and women.

Adrenal Medulla

The adrenal medulla secretes EPINEPHRINE (ADRENALIN) in the presence of emotional crises, hypoglycemia (low blood sugar), or low blood pressure. Epinephrine constricts the peripheral vascular system and dilates the blood vessels to the skeletal muscles. Heart rate, respiration rate and depth, blood pressure, blood sugar levels, and metabolism are all increased by epinephrine. It also stimulates the production of other adrenal cortical hormones.

NOREPINEPHRINE is also produced in the adrenal medulla. It is a chemical precursor to epinephrine. Its effects are similar to those of epinephrine, but its action differs.

Despite these marked influences, the medullary tissue of the adrenal gland is not essential to life, because its various functions can be assumed by other regulatory mechanisms.

Gonads

The male gonads secrete the hormone TESTOSTERONE, which influences the development and maintenance of the accessory organs and the secondary sex characteristics of the male.

The female gonads, the OVARIES, produce ESTROGEN and PROGESTERONE. Estrogen influences the development and maintenance of the female accessory organs and the secondary sex characteristics and promotes changes in the mucous lining of the uterus (endometrium) during the menstrual cycle. Progesterone prepares the uterus for the reception and development of the fertilized ovum and maintains the lining during pregnancy. It is used in many birth control pills.

Pancreas

The islands of Langerhans in the pancreas contain two types of endocrine cells, alpha and beta. The alpha cells secrete glucagon, which causes a temporary rise in blood sugar levels. The beta cells secrete insulin, which is essential for carbohydrate metabolism. Insulin lowers blood sugar levels by increasing tissue utilization of glucose and stimulating the formation and storage of glycogen in the liver. Together, glucagon and insulin act to regulate sugar metabolism in the body.

When the islet cells are destroyed or stop functioning, the sugar absorbed from the intestine remains in the blood and is excreted by the kidneys into the urine. It is not used by the body or stored. This condition is called diabetes mellitus, or sugar diabetes. Insulin is given to patients having this disease as part of their ongoing treatment.

  The Digestive System

The digestive system (fig. 3-52) consists of the alimentary tract-mouth, pharynx, esophagus, stomach, intestines, and certain accessory organs of digestion. As food passes through the 9-meter- long alimentary tract, digestion and absorption occur, and eventually waste material is eliminated. Secretions of the accessory organs assist in preparing food for absorption and use by the tissues of the body (table 3-2).

Table 3-2 - Principal Digestive Juices, Source and Action

Digestion is both mechanical and chemical. Mechanical digestion occurs when food is chewed, swallowed, and churned by peristalsis. Waste is evacuated when the bowels move. Chemical digestion consists of changing the various foods, with the aid of digestive enzymes, into solutions and simple compounds. Carbohydrates (starches and sugars) change into simple sugars (glucose); fats change into fatty acids; and proteins change into amino acids.

Mouth

In the mouth the TEETH mechanically break up food into small particles before it is swallowed. The salivary glands-parotid, submaxillary, and sublingual-secrete saliva, which moistens the food, makes it easier to chew, and lubricates the food mass to aid in swallowing. About 1,500 ml of saliva are secreted daily. Saliva contains one principal enzyme, ptyalin, which initiates chemical digestion of starches, breaking them into the complex sugar maltose.

The TONGUE is a muscular organ attached to the lower jaw at the back of the mouth and is the chief organ of taste. It assists in mastication, swallowing, and speech. PHARYNX

The pharynx (see The Respiratory System) is the passageway between the mouth and the esophagus and is shared with the respiratory tract. The EPIGLOTTIS is a cartilaginous flap that closes the opening to the larynx when food is being swallowed down the pharynx. Food is deflected away from the trachea to prevent particle aspiration.

Esophagus

The esophagus is a muscular tube about 25 cm (10 inches) long. It is the passageway between the pharynx and the stomach. By means of waves of muscular contractions (peristalsis), food is pushed along this tube to the stomach. When peristalsis is reversed, vomiting occurs.

Stomach

The stomach is a saccular enlargement of the gastrointestinal tube, connecting the lower end of the esophagus and the first portion of the small intestine (duodenum). It lies in the left upper quadrant of the abdomen. Muscular rings, or sphincters, at each end of the stomach form valves to close off the stomach and to prevent its contents from escaping in either direction while they are being mixed by peristaltic muscular contractions of the stomach wall. The sphincter at the esophageal end is the cardiac sphincter; at the duodenal end it is the pyloric sphincter.

The stomach acts as an initial storehouse for swallowed material and helps in the chemical breakdown of food substances. Small glands in the wall of the stomach secrete gastric juice, the principal components of which are hydrochloric acid and pepsinogen. Hydrochloric acid activates pepsin from pepsinogen, kills bacteria that enter the stomach, inhibits the digestive action of ptyalin, and helps regulate the opening and closing of the pyloric sphincter. The action of pepsin is confined to protein, which it splits. The stomach is half-empty within 1 hour of a normal meal and completely empty in 6 hours.

Most food absorption takes place in the small intestine. There is little food absorption in the stomach. One exception is alcohol, which is absorbed directly through the stomach wall. For this reason intoxication happens quickly when alcohol is taken on an empty stomach.

Abdominal Cavity

The stomach and intestines are enclosed in the abdominal cavity, the space between the diaphragm and the pelvis. This cavity is lined with serous membrane, the PERITONEUM. The peritoneum covers the intestines and the organs and, by secreting a serous fluid, prevents friction between adjacent organs. The MESENTERY (double folds of peritoneum) extends from the cavity walls to the organs of the abdominal cavity, suspending them in position and carrying blood vessels to the organs.

Small Intestine

The small intestine is a muscular, convoluted (coiled) tube, about 7 meters (23 feet) long and attached to the posterior abdominal wall by its mesentery. The mesentery is gathered together like a folding fan, permitting coiling of the intestine, allowing this long organ to be contained in a relatively small space.

The small intestine is divided into three contin

uous parts: the duodenum, jejunum, and ileum. It receives digestive juices from three accessory organs of digestion: the pancreas, liver, and gallbladder.

The DUODENUM is about 25 cm (10 inches) long and forms a C-shaped curve around the head of the pancreas, posterior to the liver. It is lined with a mucous membrane that contains small glands. These glands secrete intestinal juices containing the enzymes carbohydrase, peptidase, and lipase.

The JEJUNUM is the middle part of the small intestine and is about 2.5 meters (7.5 feet) long. Its enzymes continue the digestive process.

The ILEUM is the last and longest part of the small intestine. Most of the absorption of food occurs in the ileum where fingerlike projections (villi) provide a large absorption surface. After ingestion it takes 20 minutes to 2 hours for the first portion of the food to pass through the small intestine to the beginning of the large intestine.

Large Intestine

The large intestine is so called because it is larger in diameter than the small intestine. It is considerably shorter, however, being about 1.5 meters (5 feet) long. It is divided into three distinct parts: the cecum, colon, and rectum.

The unabsorbed food or waste material passes through the CECUM into the COLON. The cecum is a pouch at the beginning of the large intestine, located in the lower right portion of the abdominal cavity. Twelve hours after the meal, most of the waste material passes through the colon slowly, building in mass and reaching the rectum 24 hours after the food is ingested.

The APPENDIX, a long narrow tube with a blind end, is an outpouching of the cecum located near the junction of the ileum and the cecum. It has no known function but frequently becomes infected and an inflammation known as appendicitis develops.

The RECTUM is 12.5 cm (5 inches) long and follows the contour of the sacrum and coccyx until it curves back into the short (2.5 to 4 cm) anal canal. The ANUS is the external opening at the lower end of the digestive system. Except during bowel movement (defecation), it is kept closed by a strong muscular ring, the ANAL SPHINCTER.

Accessory Organs of Digestion

The PANCREAS is a large, elongated gland lying posteriorly to the stomach. Its digestive juices amylase, proteinase, and lipase are secreted through the pancreatic duct into the duodenum and act on all types of food. The pancreas contains a special group of cells called the islands of Langerhans, which secrete the hormone insulin needed for use of sugar by the body.

The LIVER is the largest gland in the body. It is located in the upper abdomen on the right side, just under the diaphragm and superior to the duodenum and pylorus.

Of its many functions, the following are im- portant to remember:

• Metabolism of carbohydrates, fats, and proteins preparatory to their use or excretion.  

• Formation and excretion of bile salts and pigment from bilirubin, a waste product of red blood cell destruction.  

• Storage of blood and water and the products of carbohydrate, protein, and fat metablolism.  

• Detoxification of end products of protein digestion and drugs.  

• Production of antibodies and essential elements of the blood-clotting mechanism.  

• Production of heat and formation of vitamin A from carotin.

The GALLBLADDER is a pear-shaped sac, usually stained dark green by the bile it contains. It is located in a hollow on the underside of the liver. Its duct, the cystic duct, joins the hepatic duct from the liver to form the common bile duct, which enters the duodenum. The gallbladder receives bile from the liver and then concentrates and stores it. It secretes bile when the small intestine is stimulated by the entrance of fats.

  The Urinary System

The urinary system is the primary filtering system of the body. It consists of two glands, the kidneys, which produce urine; two tubes, the ureters, which drain the urine from the kidneys; a large reservoir, the bladder, where the urine is temporarily stored before it is excreted from the body; and a tube, the urethra, which carries the urine from the bladder to the outside of the body. All these parts, except the length of the urethra, are the same in both sexes.

Kidneys

The importance of the kidney can be realized only when its structure and function are understood. It is the only part of the urinary system in which any changes occur. The bladder, ureters, and urethra store and pass only the products of the kidneys.

The kidneys are two large, bean-shaped organs designed to filter waste materials from the blood (fig. 3-53). They are located in the upper posterior part of the abdominal cavity, outside the peritoneal sac, one on each side of the spinal column. The upper end of each kidney reaches above the level of the 12th rib. The suprarenal (adrenal) gland sits like a cap on top of each kidney. Each kidney weighs about 125 to 170 grams. It is protected by a considerable amount of fat and supported by connective tissue and the peritoneum. Attached to the hollow side of each kidney is the dilated upper end of the ureter, forming the renal pelvis.

Structure The kidney is composed of an external cortical and an internal medullary substance. The cortex or cortical substance is soft and granular and reddish brown. The urine is formed in the cortex. The medulla or medullary substance is a pyramid-shaped mass of tubes or tubules that drain the urine to the pelvis of the kidney. Blood enters the kidney through the renal artery and is distributed to the glomerulus (fig. 3-54).

The GLOMERULUS, lying in the cortex, consists of a tuft of capillaries. This tuft is surrounded by the glomerular capsule, which is a cuplike dilation of the end of the renal tubule. This combination is called a MALPIGHIAN BODY.

The renal tubule begins with the malpighian body, takes multiple turns, forming the proximal convoluted tubule, extends toward the hilum in the medullary portion to form the descending LOOP OF HENLE, doubles back on itself as the ascending loop of Henle, and goes through several more turns as the distal convoluted tubule. The structural and functional unit of the kidney is called a NEPHRON, of which there are about 1 million in each kidney. For this reason a large portion of the kidney may be destroyed without serious body damage. In addition, the loss or medical donation of one kidney does not seriously affect the body's welfare if the remaining kidney is healthy.

Several of the nephrons terminate in one collecting tubule. Several collecting tubules unite to form a renal pyramid, which drains the urine into a branch (calyx) of the renal pelvis.

Function

The kidneys are effective blood purifiers and fluid balance regulators. Besides maintaining a normal pH of the blood (acid-base balance), they keep the blood slightly alkaline by removing excess substances from the blood. For example, if the blood becomes too acid, they will remove acid in the form of salts; if the blood is too alkaline, they will remove alkaline salts.

The main function of the kidneys is to remove the nitrogenous waste products that result when products of proteins are broken up. They also remove excess sugar.

The second important function of the kidney is reabsorption of water, salts, sugar, and protein elements of the blood. This selective reabsorption keeps the blood at an acid-base balance and also at a constant concentration of water, salts, and proteins. This delicate balance is necessary for normal life processes. Controlled reabsorption accounts for the amount of urine that is finally passed from the kidneys. The glomerulus filters gallons of blood each day. It is estimated that 10,000 quarts of blood pass through the kidneys in 24 hours and about 80 gallons of glomerular filtrate are formed. All the water from this filtrate is reabsorbed in the renal tubules except that containing the concentrated waste products. The amount of urine a normal person excretes varies from 1,000 to 1,500 ml per day, but a person can function normally by excreting only 500 ml per day.

The amount of urine excreted varies greatly with temperature, water intake, and state of health. No matter how much water you drink, the blood will always remain at a constant concentration, and the excess water will be excreted by the kidneys. A large water intake does not put a strain on the kidneys as one might think, instead it eases the load of concentration placed on the kidneys.

In blood plasma there is normally 0.03 percent of urea, while in the urine there is normally 67 times as much, or about 2 percent. This great increase is caused by the concentration of urea contained in a large kidney area in a relatively small quantity of urine.

Beside removing waste products normally found in the body, the kidneys also remove toxic substances, such as certain barbituric acid derivatives, mercury, alcohol, and other drugs.

One of the familiar diseases associated with the kidneys is glomerulonephritis, which is caused by protein loss from the body due to damaged glomeruli. Another disease, though not as prevalent, is uremia. This is caused by failure of the kidneys to remove the waste products from the blood, which then accumulate in high concentrations. This condition is serious and sometimes fatal.

Ureters

The ureters are two membranous tubes 1 mm to 1 cm in diameter and about 28 to 34 cm in length. Their only function is to carry urine from each kidney to the urinary bladder.

Bladder

The urinary bladder is a musculomembranous sac located in the pelvic girdle. It functions as a reservoir for urine until it empties through the urethra.

Urethra

The urethra is the tube that carries the urine from the bladder to the exterior. The urinary meatus is the external urethral opening. In the male the urethra is common to the urinary and reproductive systems; in the female it belongs only to the urinary system.

The female urethra is about 4 cm long, extending from the bladder to the external orifice in the vestibule. It is embedded in the anterior wall of the vagina and surrounded by the sphincter urethrae.

The male urethra is about 20 cm long and is divided into three parts: the prostatic, membranous, and penile portions. The prostatic urethra is surrounded by the prostate gland; it contains the orifices of the prostatic and ejaculatory ducts. This portion of the male urethra is about 2.5 cm long. The membranous urethra is about 2 cm in length and is surrounded by the external sphincter. The penile urethra, the longest portion, is about 15 cm long. It lies in the ventral portion of the penis, extending to its external opening.

  Male Reproductive System The male organs of reproduction are the penis and testes (testicles), and associated ducts and glands (fig. 3-55).

Scrotum

The SCROTUM is a cutaneous pouch containing the testes and part of the spermatic cord. Immediately beneath the skin is a thin layer of muscular fibers (cremaster), which is controlled by temperature and contracts or relaxes to lower or raise the testes in relation to the body. This muscular activity of the scrotum is necessary to regulate the temperature of the testes, which is important in the maturation of sperm cells.

Testes

The TESTES are oval glands suspended by the spermatic cord in a pouch. They perform two functions: production of spermatozoa (sperm) and secretion of the male sex hormone testosterone.

Lying close to the superior pole of each testis is the EPIDIDYMIS, a ductal system that collects and transmits sperm from the testes.

Spermatic Cords

The two spermatic cords, each of which suspends and supplies a testis, are formed by the ductus deferens, arteries, veins, lymphatics, and nerves, bound together by connective tissue.

Ductus Deferens (Vas Deferens)

The ductus deferens is a small tube that connects the epididymis and ejaculatory duct. It ascends as part of the spermatic cord through the inguinal canal into the pelvic cavity and transmits the sperm to the ejaculatory duct.

Seminal Vesicles

The seminal vesicles are two pouches that lie between the bladder and the rectum. They secrete and store a fluid to be added to the secretion of the testes at the time of ejaculation.

Ejaculatory Duct

The ductus deferens and the ducts from the seminal vesicles converge to form the short ejaculatory duct that leads into the prostatic urethra. Its function is the transportation of secretions.

Penis

The penis is composed of three cylindrical bodies of spongy cavernous tissue, bound together by connective tissue and loosely covered by a layer of skin. Two of the bodies, the corpora cavernosa, lie superiorly side by side; the third body, the corpus spongiosum, is median, lying in the groove between the other two. The dilated distal end of the corpus spongiosum is known as the glans penis. The cavernous tissue becomes greatly distended with blood during sexual excitement, causing erection of the penis. The loose skin of the penis folds back on itself at the distal end, forming the prepuce, or foreskin, and covers the glans. Frequently, the prepuce is surgically removed (circumcision) to prevent irritation and to facilitate cleanliness.

Prostate Gland

The prostate gland is made of smooth muscle and glandular tissue that surrounds the first part of the urethra. It resembles a chestnut in shape and size. It secretes an alkaline fluid to keep the sperm mobile and protect it from the acid secretions of the female vagina. This substance is discharged into the urethra as part of the ejaculate, or semen, during the sexual act.

Bulbourethral Glands (Cowper's Glands)

Cowper's glands are two pea-sized bodies, one on either side of the membranous portion of the urethra, the excretory ducts of which open into the urethra. They secrete a mucouslike alkaline fluid during the sexual act to provide lubrication.

Semen

Semen is composed of sperm and secretions from the seminal vesicles, prostate, and Cowper's glands. It is discharged as the ejaculate during sexual intercourse. There are millions of sperm cells in the semen of each ejaculation, but only one is needed to fertilize the ovum. It is generally con-sidered that fertilization of the ovum occurs while it is still in the uterine tube. Therefore, it is apparent that sperm cells can move actively in the seminal fluid deposited in the vagina and through the layers of the secretion lining the uterus and the uterine tube.

Although the prostate, seminal vesicles, and bulbourethral glands secrete most actively during sexual intercourse, a certain amount is being formed continuously. During periods of prolonged sexual abstinence, discharge of this accumulation may occur during sleep as a nocturnal emission or "wet dream." This is an entirely normal condition and does not constitute a harmful or disease state; on the other hand, retention of these secretions in no way impairs health or the mental state.

At times, what appears to be semen may drip from the penis on straining to move bowels. This is the secretion of the prostate and seminal vesicles being forced out by the increased pressure within the abdominal cavity and forceful passage of feces through the rectum, which lies close to these structures. This occurrence does not indicate disease or infection if urethral discharge is present only during such acts of straining. Any continuing discharge should be examined for evidence of infection.

  Female Reproductive System

The female reproductive system (fig. 3-56) includes the ovaries, the fallopian (uterine) tubes, the uterus, the vagina, the external genitalia (vulva), and the breasts (mammary glands), which are not illustrated but will be discussed.

External Genitalia

The external genital organs, referred to collectively as the vulva, include the mons pubis, labia majora, labia minora, clitoris, vestibule, Bartholin's glands, and hymen. The mons pubis is the pad of fatty tissue beneath the skin, anterior to the symphysis pubis. The labia majora are two folds of skin extending from the mons pubis anteriorly to the perineum (the region between the vaginal orifice and the anus). Within these two folds of skin are two smaller folds, called the labia minora, extending from the clitoris to either side of the vaginal orifice. The clitoris is a small body richly endowed with nerves, highly sensitive, and of significance in sexual stimulation. The clitoris becomes engorged with blood during sexual excitement, but, unlike its male counterpart, the penis, it does not become erect. It is located at the point where the two labia minora meet. The vestibule is the area between the labia minora into which the urethral and vaginal orifices open. The urinary meatus is the external urethral orifice situated inferior to the clitoris and superior to the vaginal orifice. The vaginal orifice is situated inferior to the urethra. The Bartholin's glands are the female counterparts of the Cowper's glands in the male. They consist of two small roundish bodies on either side of the vaginal opening. Each gland is connected with the vagina by means of long ducts and secretes a viscid, alkaline fluid lubricant between the labia minora and the hymen. Finally, the hymen is a fold of mucous membrane that extends across the lower part of the vagina. It is not a very reliable indicator of virginity.

  Mammary Glands

The mammary glands, or breasts, are accessory organs of the female reproductive system. They develop during puberty under the influence of the hormones estrogen and progesterone. The breasts are responsible for the secretion of milk (lactation) for the nourishment of newborn infants.

Structurally the breasts resemble sweat glands. At the center is a nipple containing 15 to 20 depressions, into which ducts from the lobes of the gland empty. During pregnancy hormones secreted by the ovaries cause the glandular tissue to grow in preparation for lactation. After childbirth hormones secreted by the anterior lobe of the pituitary gland stimulate production for 6 to 9 months.

Ovaries

The ovaries (female gonads) are two almond-shaped glands suspended by ligaments in the upper pelvic cavity, one on either side of the uterus, posterior and inferior to the fallopian tubes. Their prime function is to produce the ova and the female hormone estrogen and progesterone. Although these hormones are manufactured by the ovaries, their production is controlled by the anterior pituitary gland. These hormones play essential roles in the development of secondary sex characteristics, the reproductive cycle, gestation, and lactation. The graafian follicles are microscopic pockets in the ovaries. One a month, under hormonal influence, a follicle matures, ruptures, and expels its ovum into the uterus. Each ovary normally releases an ovum every 56 days, the right and left ovary alternately discharging an ovum every 28 days. The menstrual cycle in most women is therefore 28 days in length.

Fallopian Tubes

The fallopian (uterine) tubes are composed of internal mucous, middle muscular, and outer serous coats that are continuous with the layers of the uterus. They serve as ducts of the ovaries, providing a passageway to the uterus. These tubes are in contact with the ovaries, but are not continuous with them. Their funnel-shaped openings, called free openings, are fringed with fingerlike processes that pick up an ovum and draw it into the fallopian tubes, where it is transported to the uterus by peristalsis and gravity. Fertilization of an ovum normally takes place in the fallopian tubes.

Uterus The uterus (womb) is a hollow, pear-shaped organ with thick, muscular walls. It is lined with a specialized epithelium, called endometrium, which undergoes partial destruction about every 28 days in the nonpregnant woman.

The uterus averages 7 cm in length and 5 cm in width. It has three openings: the openings of the fallopian tubes laterally and the opening into the vagina. The parts of the uterus are the body, which is the large upper portion, and the cervix, which is the smaller portion that projects into the upper part of the vagina. The cervical opening into the vagina is called the external os. The walls of the uterus are highly flexible and are composed of three layers that are continuous with the respective layers of the fallopian tubes.

In addition to being the focal point of the endometrial (menstrual) cycle, the uterus is the site of implantation, growth, and development of the fertilized ovum. The muscular walls of the uterus produce powerful rhythmic contractions that are important in the expulsion of the fetus at birth.

Vagina

The vagina is a musculomembranous, collapsible tube capable of great distention. It is lined with mucous membrane that extends from the cervix to the vulva. The canal is about 7.5 cm long, and its lining membrane, which is greatly folded, is continuous with the inner lining of the uterus. The vagina is the organ that receives the male sperm during intercourse. It also forms the lower portion of the birth canal, stretching widely during delivery. In addition, it serves as an excretory duct for uterine secretions and menstrual flow.

Recurring Cycles

When females reach puberty, they begin to experience the two recurring female cycles, the ovarian and endometrial.

As previously mentioned, each ovary produces a mature ovum every 56 days. They expel their ova on an alternating basis, approximately one every 28 days. The length of this cycle may vary markedly from individual to individual and between cycles of the same individual. On the first days of menstruation, several ova within the graafian follicles begin to mature, and normally one will be expelled 14 days before the next menstrual flow. This is the ovarian cycle.

The endometrial cycle centers around the periodic development and breakdown of the endometrial lining of the uterus. The first phase of the cycle is the menses, or menstruation. It begins when the endometrial lining starts to slough off from the walls of the uterus, and it is characterized by bleeding from the vagina. This is day 1 of the cycle, and this phase usually lasts through day 5. The time between the last day of the menses and ovulation is known as the postmenstrual phase. It lasts from day 6 through day 13 or 14 and is characterized by proliferation of endometrial cells in the uterus, which develop under the influence of the hormone estrogen. Ovulation is the rupture of a graafian follicle with the release of a mature ovum into the fallopian tubes. It usually occurs on day 14 or 15 of the cycle. The postovulatory (premenstrual) phase is the time between ovulation and the onset of the menses and normally lasts 14 days. During this phase the ovum travels through the fallopian tubes to the uterus. If the ovum becomes fertilized during this passage, it will become implanted in and nurtured by the newly developed endometrial lining. However, if fertilization does not take place, the lining deteriorates and eventually sloughs off, marking day 1 of the next cycle.

References

1. Anthony and Thibodeau: Textbook of Anatomy & Physiology, ed 11. C.V. Mosby.  

2. American Academy of Orthopaedic Surgeons: Emergency Care and Transportation of the Sick and Injured, ed 3.

Approved for public release; Distribution is unlimited.

The listing of any non-Federal product in this CD is not an endorsement of the product itself, but simply an acknowledgement of the source. 

Operational Medicine 2001

Health Care in Military Settings

This web version is provided by The Brookside Associates Medical Education Division.  It contains original contents from the official US Navy NAVMED P-5139, but has been reformatted for web access and includes advertising and links that were not present in the original version. This web version has not been approved by the Department of the Navy or the Department of Defense. The presence of any advertising on these pages does not constitute an endorsement of that product or service by either the US Department of Defense or the Brookside Associates. The Brookside Associates is a private organization, not affiliated with the United States Department of Defense.

Contact Us  ·  ·  Other Brookside Products