Organs and Processes of Digestion : Part 1

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 10 of 31)

The organs of digestion are adapted to the work of dissolving the foods by both their structure and arrangement. Most of them consist either of tubes or cavities and these are so connected, one with the other, as to form a continuous passageway entirely through the body.

The Alimentary Canal. - The alimentary canal has a length of about thirty feet and, while it begins at the mouth, all but about eighteen inches of it is found in the abdominal cavity. On account of its length it lies for the most part in coils, the two largest ones being known as the small intestine and the large intestine. Connected with the alimentary canal are the glands that supply the liquids for acting on the food.

1. An inner coat, or lining, known as the mucous membrane. This membrane is not confined to the alimentary canal, but lines, as we have seen, the different air passages. It covers, in fact, all those internal surfaces of the body that connect with the external surface. It derives its name from the substance which it secretes, called mucus. In structure it resembles the skin, being continuous with the skin where cavities open to the surface. It is made up of two layers - a thick underlayer which contains blood vessels, nerves, and glands, and a thin surface layer, called the epithelium. The epithelium, like the cuticle, is without blood vessels, nerves, or glands.

2. A middle coat, which is muscular and which forms a continuous layer throughout the canal, except at the mouth. (Here its place is taken by the strong muscles of mastication which are separate and distinct from each other.) As a rule the muscles of this coat are involuntary. They surround the canal as thin sheets and at most places form two distinct layers. In the inner layer the fibers encircle the canal, but in the outer layer they run longitudinally, or lengthwise, along the canal.

3. An outer or serous coat, which is limited to those portions of the canal that occupy the abdominal cavity. This coat is not found above the diaphragm. It is a part of the lining membrane of the cavity of the abdomen, called

The Peritoneum. - The peritoneum is to the abdominal cavity what the pleura is to the thoracic cavity. It forms the outer covering for the alimentary canal and other abdominal organs and supplies the inner lining of the cavity itself. It is also the means of holding these organs in place, some of them being suspended by it from the abdominal walls. By the secretion of a small amount of liquid, it prevents friction of the parts upon one another.

Digestive Glands. - The glands which provide the different fluids for acting on the foods derive their constituents from the blood. They are situated either in the mucous membrane or at convenient places outside of the canal and pass their liquids into it by means of small tubes, called ducts. In the canal the food and the digestive fluids come in direct contact - a condition which the dissolving processes require. Each kind of fluid is secreted by a special kind of gland and is emptied into the canal at the place where it is needed.

The Digestive Processes. - Digestion is accomplished by acting upon the food in different ways, as it is passed along the canal, with the final result of reducing it to the form of a solution. Several distinct processes are necessary and they occur in such an order that those preceding are preparatory to those that follow. These processes are known as mastication, insalivations, deglutition, stomach digestion, and intestinal digestion. As the different materials become liquefied they are transferred to the blood, and substances not reduced to the liquid state are passed on through the canal as waste. The first two of the digestive processes occur in

The Mouth. - This is an oval-shaped cavity situated at the very beginning of the canal. It is surrounded by the lips in front, by the cheeks on the sides, by the hard palate above and the soft palate behind, and by the tissues of the lower jaw below. The mucous membrane lining the mouth is, soft and smooth, being covered with flat epithelial cells. The external opening of the mouth is guarded by the lips, and the soft palate forms a movable partition between the mouth and the pharynx. In a condition of repose the mouth space is practically filled by the teeth and the tongue, but the cavity may be enlarged and room provided for food by depressing the lower jaw.

The mouth by its construction is well adapted to carrying on the processes of mastication and insalivations. By the first process the solid food is reduced, by the cutting and grinding action of the teeth, to a finely divided condition. By the second, the saliva becomes mixed with the food and is made to act upon it.

Accessory Organs of the Mouth. - The work of mastication and insalivations is accomplished through organs situated in and around the mouth cavity. These comprise:

1. The Teeth. - The teeth are set in the upper and lower jaws, one row directly over the other, with their hardened surfaces facing. In reducing the food, the teeth of the lower jaw move against those of the upper, while the food is held by the tongue and cheeks between the grinding surfaces. The front teeth are thin and chisel-shaped. They do not meet so squarely as do the back ones, but their edges glide over each other, like the blades of scissors - a condition that adapts them to cutting off and separating the food. The back teeth are broad and irregular, having surfaces that are adapted to crushing and grinding.

Each tooth is composed mainly of a bone-like substance, called dentine, which surrounds a central space, containing blood vessels and nerves, known as the pulp cavity. It is set in a depression in the jaw where it is held firmly in place by a bony substance, known as cement. The part of the tooth exposed above the gum is the crown, the part surrounded by the gum is the neck, and the part which penetrates into the jaw is the root. A hard, protective material, called enamel, covers the exposed surface of the tooth.

The teeth which first appear are known as the temporary, or milk, teeth and are twenty in number, ten in each jaw. They usually begin to appear about the sixth month, and they disappear from the mouth at intervals from the sixth to the thirteenth year. As they leave, teeth of the second, or permanent, set take their place. This set has thirty-two teeth of four different kinds arranged in the two jaws as follows:

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