Basic Human Anatomy
Lesson 4: Skeletal System
Welcome to Lesson 4 of the Basic Human Anatomy Course. Today, we’ll be studying the Human Skeletal System.
I have 13 goals for you in this lesson:
1. Define skeleton.
2. Name four functions of the human skeleton.
3. Name the layers and describe the basic structure of an individual bone, name and describe the parts of an individual long bone, and describe the periosteum and the blood supply of an individual bone.
4. Describe the development of an individual bone.
5. Name four types of bones by shape.
6. Describe major categories used in classification of joints.
7. Name the major parts of a “typical” synovial joint.
8. Name and describe classifications of synovial joints according to the kind of motion and number of axes.
9. Name and define the two major subdivisions of the skeleton.
10. Describe a typical vertebra. Name the regions of the vertebral column and give the number of vertebrae in each region. Describe the intervertebral discs and ligaments that hold vertebrae together.
11. Describe the thoracic cage.
12. Describe the skull.
13. Describe the general pattern of the bones of the upper and lower members.
INTRODUCTION
The skeleton serves as a support or framework of the human body. It is a combination of bones joined together.
FUNCTIONS OF THE HUMAN SKELETON
The human skeleton serves the following functions:
a. Bodily Support. The skeletal system provides a framework for the human body.
b. Protection. The skeleton protects certain soft structures within the human body. An example is the skull, which surrounds the brain.
c. Motion. Muscles are attached to and move the bones. Bones provide leverage for motion.
d. Formation of Blood Cells (Hematopoiesis). Blood cells are manufactured in the red bone marrow, mainly found in flat bones.
PRIMARY STUDY AREAS
In this text, we study the skeletal system from four different viewpoints:
a. Bone As Tissues. This aspect of the human skeletal system was discussed in Lesson 2 and will not be further discussed here.
b. Bone As An Individual Organ. This lesson discusses bone as an individual organ.
c. Articulations (Joints)–Arthrology. This lesson introduces the study of joints, or arthrology.
d. The Human Skeleton. This lesson discusses the human skeleton as a whole in terms of its major subdivisions.
BONE AS AN INDIVIDUAL ORGAN
BASIC STRUCTURE OF AN INDIVIDUAL BONE
See figure 4-1 for the basic structure of an individual bone.
Figure 4-1. A mature long bone (femur).
a. Use of Bony Tissues to Form an Individual Bone.
(1) Cortex. The cortex is the outer layer of the individual bone. It is made up of compact (dense) bony tissue.
(2) Medulla. The medulla is the central portion of the individual bone. It generally consists of cancellous (spongy) bone tissue. In some bones, particularly long bones, the medulla may include a space without any bony tissue. This space is called the medullary or marrow cavity.
b. Marrow. Marrow serves as a filler of the inside of bones. There are two types of bone marrow–yellow bone marrow and red bone marrow. Yellow bone marrow is mostly yellow fat tissue. Red bone marrow is the only site in adults for the formation of red blood cells (hematopoiesis).
c. Named Parts of an Individual Long Bone.
(1) Shaft (diaphysis). The shaft is the central portion of a long bone. Here, the cortex is thickened as required by applied physical stresses.
(2) Ends (epiphyses). The ends of long bones are made up mainly of cancellous (spongy) bone tissue. An articular cartilage covers each area where a bone contacts another bone(s). This articular cartilage is made up of hyaline cartilage tissue and provides a smooth surface for motions.
d. Periosteum. The periosteum is a covering of the bone surface area not covered by articular cartilage. It has two layers–the innermost layer and the fibrous layer.
(1) The innermost layer, which lies against the outer surface of the bone, consists of bone-forming cells (osteoblasts). It is the osteogenic (bone-forming) layer. Basic Human Anatomy Lesson 4: Skeletal System Page 5
(2) The outermost layer is a FCT (fibrous connective tissue) layer.
(3) The periosteum is well supplied with blood vessels and sensory-type nervous tissue.
e. Blood Supply of an Individual Bone. A system of blood vessels enters and spreads out through the periosteum. Additional blood vessels, called “nutrient vessels,” penetrate the cortex of the bone and spread out through the marrow. The passageways for penetration of these vessels are called the nutrient canals.
DEVELOPMENT OF AN INDIVIDUAL BONE
a. General. The human skeleton is “preformed” in the early fetus, but the early form is not of bony material. There are two types of bones according to their preformed basis: membranous bones and cartilage bones. These are in the location and have the general shape of the adult bones they will later become.
(1) Membranous bones. The outer skull bones are an example of membranous bones. Osteoblasts invade a membrane to form a center of ossification (formation of bone). Bone-forming activity spreads out from this center until a full bone plate is formed.
(2) Cartilage bones. In the fetus, many bones, for example, long bones, exist first as models formed of cartilage.
b. Sesamoid Bones. Sesamoid bones are small masses of bone that develop in tendons at points where great forces are applied to the tendons. The most obvious and largest sesamoid bone is the patella, or kneecap.
c. Ossification Centers. An ossification center is a growing mass of actual bone within the preformed material, as noted above. Basic Human Anatomy Lesson 4: Skeletal System Page 6
(1) Initial bone formation involves destruction of the preforming material and replacement with bony tissue.
(2) In the development of long bones, there are two types of ossification centers:
(a) Diaphyseal–in the shaft region.
(b) Epiphyseal–in the end(s).
(3) As a long bone grows in length, the preforming material grows faster than the ossification center can tear it down. Ultimately, with time, the preforming material is overcome and growth ceases.
d. Growth in Bone Width. A bone grows wider through the activity of the osteogenic layer of the periosteum. Remember, the periosteum covers most of the outer surface of the bone.
TYPES OF BONES
Bones of the skeleton can be grouped into the following major types: long, short, flat, and irregular. Each type has a somewhat different construction pattern.
a. Long Bones. The basic structure of a long bone is illustrated in figure 4-1. Example: femur.
b. Short Bones. The short bones, such as those of the wrist and feet, have a thin layer of compact bone surrounding an inner mass of spongy bone. Example: carpal bones.
c. Flat Bones. The flat bones are constructed with two plates of compact bone, which enclose between them a layer of spongy bone. The spongy bone is richly supplied with blood vessels and red marrow. Example: the cranial frontal bone.
d. Irregular Bones. The irregular bones are those that do not fit into the three categories above. Example: a vertebra.
ARTHROLOGY–THE STUDY OF JOINTS (ARTICULATIONS)
DEFINITION
A joint, or articulation, is the location where two or more bones meet.
TYPES OF JOINTS
Joints are classified according to the kind of material holding the bones together and the relative freedom and kind of motion at the particular joint.
a. Fibrous Joints. Varying degrees of motion, from none to some, are possible in fibrous joints.
(1) Syndesmosis. When the bones are held together by FCT (fibrous connective tissue), the joint is referred to as a syndesmosis.
SYN = together
DESMOS = fiber (a tying material)
Example: The inferior tibio-fibular joint. Basic Human Anatomy Lesson 4: Skeletal System Page 8
(2) Suture. When the bones are quite close together with a minimum of FCT, the joint is known as a suture. Example: the joints between the cranial bones.
b. Bony Joints. Should the bones be united by bony material, the joint is referred to as a synosteosis.
SYN = together
OSTEO = bone
Example: The frontal bone. (The frontal bone of the skull is actually a bony fusion of two bones. Approximately 10 percent of the time, this fusion fails to take place; the original suture between the bones remains and is called a metopic suture.)
c. Cartilagenous Joints. These are also nonmovable joints.
(1) Synchondrosis. A cartilagenous joint in which the bones are held together by hyaline cartilage.
SYN = together
CHONDRO = cartilage
Example: Epiphyseal plate.
(2) Symphysis. A cartilagenous joint in which the bones are held together by a disc of fibrocartilage.
Example: Pubic symphysis.
d. Synovial Joints. In the synovial type of joints, the bones move on one another so as to allow various motions of the body parts. The “ovial” part of the name refers to the fact that the fluid substance seen in this type of joint appeared to the old anatomists to be like raw egg white (ovum = egg).
A “TYPICAL” SYNOVIAL JOINT
A “typical” synovial joint is one which has parts common to all of the synovial joints. In a sense, it is imaginary. It is not actually a specific synovial joint. It is a composite. It is illustrated in figure 4-2. The “typical” synovial joint has the following parts:
a. Bones. Bones are the levers of motion. They are the site of attachment for skeletal muscles.
b. Articular Cartilages. The “contact” points of the bones are usually covered with a layer of lubricated cartilage. Where these cartilages end, the synovial membranes begin. Cartilages provide a smooth surface to reduce friction.
Figure 4-2. A “typical synovial joint:–diagrammatic.
c. Synovial Membrane, Space, and Fluid.
(1) Synovial membrane. The synovial membrane lines the inner surface of the capsule. It secretes synovial fluid into the synovial space.
(2) Synovial space. Figure 4-2 exaggerates the amount of space between the bones. The space within the capsule allows movement.
(3) Synovial fluid. Synovial fluid is a colorless, viscous fluid similar in consistency to raw egg white. It lubricates the articulation.
d. Capsule. The “typical” synovial articulation is surrounded by a sleeve of dense FCT known as the capsule. The capsule encloses the articulation.
e. Ligaments. Primarily, ligaments hold bones together. Ligaments also may help restrain motion in certain directions and stabilize the articulation.
f. Muscles. Skeletal muscles apply the forces to produce a given motion.
NOTE: See table 4-1 for a summary of the structures in a “typical” synovial articulation, the tissues composing each structure, and the actions attributed to each structure.
4-10. CLASSIFICATION OF SYNOVIAL JOINTS
Synovial joints are further classified according to the kind of motion and the number of axes of motions used.
a. Uni-Axial Synovial Joints.
(1) In uni-axial synovial joints, motion occurs in only one plane. The joints of the fingers (interphalangeal) flex and extend in the sagittal plane. These are commonly referred to as hinge joints.
(2) If a single rotatory (rotational) motion occurs around a post-like structure, the joint is a pivot joint. The atlas vertebra rotating around the dens (tooth like projection) of the axis vertebra at the top of the neck (base of the skull) is a pivot joint.
b. Bi-Axial Synovial Joints. In bi-axial synovial joints, motion between the bones occurs in two planes. Here the surface in contact is curved or rounded in two directions.
(1) The proximal phalanx of a finger can flex and extend and move from side to side on the rounded head of the metacarpal bone. This is the MP or metacarpophalangeal joint.
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(2) When the two surfaces are curved in directions at right angles to each other, a shape similar to that of a cowboy’s saddle is formed. This type of synovial joint is called a saddle joint. In the human body, the saddle joint is located at the base of the thumb. STRUCTURE |
TISSUE(S) |
FUNCTION(S) |
|
BONE |
BONY |
(a) Serves as site of attachment for the skeletal muscles. (b) Serves as lever of motion. |
|
ARTICULAR CARTILAGE |
HYALINE CARTILAGE |
Serves as smooth surface, over which motion takes place. |
|
FIBROUS CAPSULE |
DENSE FCT |
Encloses articulation |
|
SYNOVIAL MEMBRANE |
SIMPLE SQUAMOUS EPITHELIUM |
(a) Lines capsule. (b) Secretes synovial fluid into synovial space. |
|
SYNOVIAL SPACE |
– |
-Frees articulation for motion. |
|
SYNOVIAL FLUID |
SEROUS FLUID |
Lubricates articulation |
|
LIGAMENT |
(VERY) DENSE FCT |
Holds the bones together |
|
SKELETAL MUSCLE |
STRIATED MUSCLE FIBERS |
Applies force to produce motion |