The mouth, or oral cavity, is the beginning of the digestive tract.

Here food taken into the body is broken into small particles and mixed with saliva so that it can be swallowed.

a. Teeth. 

(1) A person develops two sets of teeth during his life, a deciduous (or temporary) set and a permanent set. There are 20 deciduous teeth and these erupt during the first 3 years of life. They are replaced during the period between the 6th and 14th years by permanent teeth. There are 32 permanent teeth in the normal mouth: 4 incisors, 2 cuspids, 4 bicuspids, and 6 molars in each jaw.

Each tooth is divided into two main parts: the crown, that part which is visible above the gums; and the root, that part which is not visible and which is embedded in the bony structure of the jaw. The crown of the tooth is protected by enamel. Tooth decay is from the outside in; once the protective enamel is broken, microorganisms attack the less resistant parts of the tooth.

(2) The primary function of the teeth is to chew or masticate food. The teeth also help modify sound produced by the larynx to form words.

b. Salivary Glands. These glands are the first accessory organs of digestion. There are three pairs of salivary glands. They secrete saliva into the mouth through small ducts. One pair, the parotid glands, is located at the side of the face below and in front of the ears.

The second pair, the submandibular glands, lies on either side of the mandible.

The third pair, the sublingual glands, lies just below the mucous membrane in the floor of the mouth. The flow of saliva is begun in several ways. Placing food in the mouth affects the nerve endings there. These nerve endings stimulate cells of the glands to excrete a small amount of thick fluid. The sight, thought, or smell of food also activates the brain and induces a large flow of saliva. About 1,500 ml. of saliva are secreted daily.

The saliva moistens the food, which makes chewing easier. It lubricates the food mass to aid in the act of swallowing. Saliva contains two enzymes, chemical ferments, which change foods into simpler elements. The enzymes act upon starches and break them down into sugars.

c. Tongue. The tongue is a muscular organ attached at the back of the mouth and projecting upward into the oral cavity. It is utilized for taste, speech, mastication, salivation, and swallowing.

d. Taste Buds. Located on the tongue and at the back of the mouth are special clumps of cells known as taste buds. Taste buds are sensitive to substances that are sweet, sour, bitter, and salty.