Digestion and Food : Part 2

Hygienic Physiology: with Special Reference to the Use of Alcoholic Drinks and Narcotics
by Joel Dorman Steele, Ph.D.

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The General Plan of Digestion. - Nature has provided for this purpose an entire laboratory, furnished with a chemist's outfit of knives, mortars, baths, chemicals, filters, etc. The food is (1) chewed, mixed with the saliva in the mouth, and swallowed; (2) it is acted upon by the gastric juice in the stomach; (3) it is passed into the intestines, where it receives the bile, pancreatic juice, and other liquids which completely dissolve it; (4) the nourishing part is absorbed in the stomach and intestines, and thence thrown into the blood vessels, whence it is whirled through the body by the torrent of the circulation. These processes take place within the alimentary canal, a narrow tortuous tube which commences at the mouth, and is about thirty feet long.

I. Mastication and Insalivations.

l. The Saliva. - The food while being cut and ground by the teeth is mixed with the saliva. This is a thin, colorless, frothy, slightly alkaline liquid, secreted by the mucous membrane lining the mouth, and by three pairs of salivary glands (parotid, sub maxillary, and sublingual) opening into the mouth through ducts, or tubes. The amount varies, but on the average is about three pounds per day, and in health is always sufficient to keep the mouth moist. It softens and dissolves the food, an. Therefore enables us to get the flavor or taste of what we eat. It contains a peculiar organic principle called ptyalin, which, acting upon the starch of the food, changes it into glucose or grape sugar.

2. The Process of Swallowing. - The food therefore finely pulverized, softened, and so lubricated by the viscid saliva as to prevent friction as it passes over the delicate membranes, is conveyed by the tongue and cheek to the back of the mouth. The soft palate lifts to close the nasal opening; the epiglottis shuts down, and along this bridge the food is borne, without danger of falling into the windpipe or escaping into the nose. The muscular bands of the throat now seize it and take it beyond our control. The fibers of the esophagus contract above, while they are lax below, and convey the food by a worm-like motion into the stomach.

II. Gastric Digestion.

1. The Stomach is an irregular expansion of the digestive tube. Its shape has been compared to that of a bagpipe. It holds about three pints, though it is susceptible of some distension. It is composed of an inner, mucous membrane, which secretes the digestive fluids; an outer, smooth, well-lubricated serous one, which prevents friction, and between them a stout, muscular coat. The last consists of two principal layers of longitudinal and circular fibers. When these contract, they produce a peculiar churning motion, called the peristaltic movement, which thoroughly mixes the contents of the stomach. At the farther end, the muscular fibers contract and form a gateway, the pylorus (a gate), as it is called, which carefully guards the exit, and allows no food to pass from the stomach until properly prepared.

2. The Gastric Juice. - The lining of the stomach is soft, velvety, and of a pinkish hue; but, as soon as food is admitted, the blood vessels fill, the surface becomes of a bright red, and soon there exudes from the gastric glands a thin, colorless fluid - the gastric juice. This is secreted to the amount of twelve pounds per day. Its acidity is probably due to muriatic or lactic acid - the acid of sour milk. It contains a peculiar organic principle called pepsin which acts as a ferment to produce changes in the food, without being itself modified.

The flow of gastric juice is influenced by various circumstances. Cold water checks it for a time, and ice for a longer period. Anger, fatigue, and anxiety delay and even suspend the secretion. The gastric juice has no effect on the fats or the sugars of the food; its influence being mainly confined to the albuminous bodies, which it so changes that they become soluble in water.

The food, reduced by the action of the gastric juice to a grayish, soupy mass, called chyme, escapes through that jealously guarded door, the pylorus.

III. Intestinal Digestion

The structure of the intestines is like that of the stomach. There is the same outer, smooth, serous membrane (peritoneum) to prevent friction, the lining of mucous membrane to secrete the digestive fluids, and the muscular coating to push the food forward. The intestines are divided into the small and the large. The first part of the former opens out of the stomach, and is called the du-o-de'-num, as its length is equal to the breadth of twelve fingers. Here the chyme is acted upon by the bile, and the pancreatic juice.

1. The Bile is secreted by the liver. This gland weighs about four pounds, and is the largest in the body. It is located on the right side, below the diaphragm. The bile is of a dark, golden color, and bitter taste. About three pounds are secreted per day. When not needed for digestion, it is stored in the gall cyst.

2. The Pancreatic Juice is a secretion of the pancreas, or "sweetbread" - a gland nearly as large as the hand, lying behind the stomach. It is alkaline, and contains a ferment called trypsin. This juice has the power of changing starch to sugar. Its main work, however, is in breaking up the globules of fat into myriads of minute particles, that mix freely with water, and remain suspended in it like butter in new milk. The whole mass now assumes a milky look, whence it is termed chyle and passes on to the small intestine.

3. The Small Intestine is an intricately folded tube, about twenty feet long, and from an inch to an inch and one half in diameter. As the chyle passes through this tortuous channel, it receives along the entire route secretions which seem to combine the action of all the previous ones - starch, fat, and albumen being equally affected.

IV. Absorption is performed in two ways, by the veins, and the lacteals. (1.) The veins in the stomach immediately begin to take up the water, salt, grape sugar, and other substances that need no special preparation. The starch and the albuminous bodies are also absorbed as they are properly digested, and this process continues along the whole length of the alimentary canal. In the small intestine, there is a multitude of tiny projections from the folds of the mucous membrane, more than seven thousand to the square inch, giving it a soft, velvety look. These little rootlets, reaching out into the milky fluid, drink into their minute blood vessels the nutritious part of every sort of food. (2.)The lacteals, a set of vessels starting in the villi side by side with the veins, absorb the principal part of the fat. They convey the chyle through the lymphatics and the thoracic duct to the veins, and so within the sweep of the circulation.