Organs and Processes of Digestion : Part 2

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 11 of 31)

In front, above and below, are four chisel-shaped teeth, known as the incisors. Next to these on either side is a tooth longer and thicker than the incisors, called the canine. Back of these are two short, rounded and double pointed teeth, the bicuspids, and back of the bicuspids are three heavy teeth with irregular grinding surfaces, called the molars. Since the molar farthest back in each jaw is usually not cut until maturity, it is called a wisdom tooth. The molars are known as the superadded permanent teeth because they do not take the place of milk teeth, but form farther back as the jaw grows in length.

2. The Tongue. - The tongue is a muscular organ whose fibers extend through it in several directions. Its structure adapts it to a variety of movements. During mastication the tongue transfers the food from one part of the mouth to another, and, with the aid of the cheeks, holds the food between the rows of teeth. (By an outward pressure from the tongue and an inward pressure from the cheek the food is kept between the grinding surfaces.) The tongue has functions in addition to these and is a most useful organ.

3. The Muscles of Mastication. - These are attached to the lower jaw and bring about its different movements. The facial muscles, which are the heavy muscles in the cheeks, and the temporal muscles, located in the region of the temples, raise the lower jaw and supply the force for grinding the food. Small muscles situated below the chin depress the jaw and open the mouth.

4. The Salivary Glands. - These glands are situated in the tissues surrounding the mouth, and communicate with it by means of ducts. They secrete the saliva. The salivary glands are six in number and are arranged in three pairs. The largest, called the parotid glands, lie, one on either side, in front of and below the ears. A duct from each gland passes forward along the cheek until it opens in the interior of the mouth, opposite the second molar tooth in the upper jaw. Next in size to the parotids are the submaxillary glands. These are located, one on either side, just below and in front of the triangular bend in the lower jaw. The smallest of the salivary glands are the sublingual. They are situated in the floor of the mouth, on either side, at the front and base of the tongue. Ducts from the submaxillary and sublingual glands open into the mouth below the tip of the tongue.

The Saliva and its Uses. - The saliva is a transparent and somewhat slimy liquid which is slightly alkaline. It consists chiefly of water (about 99 percent), but in this are dissolved certain salts and an active chemical agent, or enzyme, called ptyalin, which acts on the starch. The ptyalin changes starch into a form of sugar (maltose), while the water in the saliva dissolves the soluble portions of the food. In addition to this the saliva moistens and lubricates the food which it does not dissolve, and prepares it in this way for its passage to the stomach. The last is considered the most important use of the saliva, and dry substances, such as crackers, which require a considerable amount of this liquid, cannot be eaten rapidly without choking. Slow mastication favors the secretion and action of the saliva.

Deglutition. - Deglutition, or swallowing, is the process by which food is transferred from the mouth to the stomach. Though this is not, strictly speaking, a digestive process, it is, nevertheless, necessary for the further digestion of the food. Mastication and insalivation, which are largely mechanical, prepare the food for certain chemical processes by which it is dissolved. The first of these occurs in the stomach and to this organ the food is transferred from the mouth. The chief organs concerned in deglutition are the tongue, the pharynx, and the esophagus.

The Pharynx is a round and somewhat cone-shaped cavity, about four and one half inches in length, which lies just back of the nostrils, mouth, and larynx. It is remarkable for its openings, seven in number, by means of which it communicates with other cavities and tubes of the body. One of these openings is into the mouth, one into the esophagus, one into the larynx, and one into each of the nostrils, while two small tubes (the eustachian) pass from the upper part of the pharynx to the middle ears.

The pharynx is the part of the food canal that is crossed by the passageway for the air. To keep the food from passing out of its natural channel, the openings into the air passages have to be carefully guarded. This is accomplished through the soft palate and epiglottis, which are operated somewhat as valves. The muscular coat of the pharynx is made up of a series of overlapping muscles which, by their contractions, draw the sides together and diminish the cavity. The mucous membrane lining the pharynx is smooth, like that of the mouth, being covered with a layer of flat epithelial cells.

The Esophagus, or gullet, is a tube eight or nine inches long, connecting the pharynx with the stomach. It lies for the most part in the thoracic cavity and consists chiefly of a thick mucous lining surrounded by a heavy coat of muscle. The muscular coat is composed of two layers - an inner layer whose fibers encircle the tube and an outer layer whose fibers run lengthwise.

Steps in Deglutition. - The process of deglutition varies with the kind of food. With bulky food it consists of three steps, or stages, as follows: 1. By the contraction of the muscles of the cheeks, the food ball, or bolus, is pressed into the center of the mouth and upon the upper surface of the tongue. Then the tongue, by an upward and backward movement, pushes the food under the soft palate and into the pharynx.

2. As the food passes from the mouth, the pharynx is drawn up to receive it. At the same time the soft palate is pushed upward and backward, closing the opening into the upper pharynx, while the epiglottis is made to close the opening into the larynx. By this means all communication between the food canal and the air passages is temporarily closed. The upper muscles of the pharynx now contract upon the food, forcing it downward and into the esophagus.

3. In the esophagus the food is forced along by the successive contractions of muscles, starting at the upper end of the tube, until the stomach is reached.

Swallowing is doubtless aided to some extent by the force of gravity. That it is independent of this force, however, is shown by the fact that one may swallow with the esophagus in a horizontal position, as in lying down.

D.C. Heath and Co. - Publishers
Original copyright 1909