have seen that the foods we eat are ground up in the mouth cavity by the teeth and thus made ready for the action of the various digestive juices. We have also demonstrated that sugars and soluble salts are dissolved in the mouth; that insoluble mineral matters are made soluble in the stomach; that starch is changed to sugar by the saliva and pancreatic juice; that proteins are converted into peptones by the pancreatic and gastric juices; and that fats are digested in the intestines by the combined action of bile and pancreatic juice. Were the food to remain within the alimentary canal, however, even though it had been thoroughly digested, it would still be, in a certain sense, outside the body, since this canal is a continuous tube opening to the exterior at either end. In order to furnish material for building and repairing the various tissues, the liquid nutrients must be distributed to the tissues wherever needed. This is accomplished through the agency of the blood system. We have now to consider the process of absorption, which includes the final steps whereby foods become a part of blood. By absorption is meant the passage of the digested food through the lining of the alimentary canal, and through the thin walls of the countless blood vessels that lie close at hand.

139. Absorption in the mouth, throat, gullet, and stomach. While the mouth, throat, and gullet all have a moist lining, generously supplied with thin-walled blood vessels, relatively little absorption takes place in these regions; first, because only a small amount of the food has been digested, and secondly, because the food does not remain long enough in these organs for absorption to take place.

The food usually remains in the stomach for several hours, and one would naturally expect that a good deal of absorption would take place during this time. But we must remember that the contraction of the stomach muscles keeps the food in constant motion,

This movement, while favorable to digestion, diminishes absorp tion, because the liquefied food does not remain long enough in one place to be absorbed by the blood.

140. Absorption in the small intestine. We, therefore, find that most of our food passes through the pylorus before it is absorbed. In the structure of the small intestine, however, we seem to find every possible provision for gathering up the nutrients. In the first place, the lining of this tube at intervals is elevated to form ridges that run two thirds of the way around the interior wall, and some of them project about a third of an inch into the cavity of the intestines (Fig. 2). Like little dams, they delay the onward flow of the food, and they also increase considerably the large surface for absorption.

The absorbing surface is multiplied still further by the villi. If one were to examine with a hand lens the mucous

capillaries cells covering villus

lining of the small intestine, one would see that the ridges and the depressions are covered with tiny, hairlike processes that give a velvety appearance to the surface. Each of these minute elevations is called a villus (Latin, villus = a tuft of hair). The villi are exceedingly numerous in the small intestine of man, the total number being FIG. 32. Two villi, highly estimated at four millions. The abmagnified. sorbent action of the villi may be compared with the absorption that takes place through the walls of the root hairs of plants. In structure, however, a villus is much more complicated than is a root hair.

Each villus (Fig. 32) when highly magnified, is found to contain a network of minute blood vessels, and since they are covered only by a thin layer of cells on the outside of the

villus, the liquefied food is readily absorbed by the blood current. Within the villi, too, are other thin-walled tubes, called lacteals, which are of great importance in the absorption of fats. As the souplike mass of food is pushed slowly along through the small intestine, it becomes less and less in bulk, and more and more solid, owing to the fact that the dissolved salts, sugars, peptones, and fats are largely taken up by the blood vessels and lacteals within the villi.

The amount of absorp

141. Absorption in the large intestine. tion in the large intestine is considerably less, of course, for both villi and ridges are wanting. Yet even here considerable absorption takes place. When the mass reaches the lower end of the intestine, it consists of little but the indigestible cellulose of vegetable foods, some undigested connective tissue, waste substances from the bile, the solids in the mucous secretion, and some raw starch and undigested fats if large quantities of these nutrients have been eaten. This refuse of the food is thrown off from the body.

VIII. THE LIVER AND ITS FUNCTIONS

142. Position, form, size. - The human liver (Fig. 26) is the largest gland of the body, weighing three to four pounds. It lies toward the right side of the body, just beneath the diaphragm, and partially covers the pyloric end of the stomach. It consists of several lobes, and on its under surface there is a small, greenish brown sac called the gall bladder. The deep red color of the liver is partly due to the fact that one fourth of all the blood of the body is found within its tissues.

143. Functions of the liver. The liver performs three important functions. In the first place, it secretes a golden brown liquid called the bile, which is either poured at once through the bile duct into the small intestine or is stored in the gall bladder until needed. If the bile duct becomes stopped up, the bile is absorbed into the blood and gives to the tissues the yellow tint that is characteristic

of jaundice. The liver, in the second place, serves as a great store house for the carbohydrates when the blood does not need them for immediate use. When, on the other hand, there is a lack of carbohydrates in the blood, some of the supply in the liver is taken up again by the blood. Finally, the liver helps to destroy some of the worn out cells of the blood (the red corpuscles), and the waste materials thus formed are passed off into the intestine as a part of the bile.

IX. HYGIENE OF DIGESTION

144. Hygienic habits of eating. One should form the habit of eating slowly and of thoroughly masticating each mouthful of food. For by this process the food is thoroughly broken up, and thus is prepared for rapid digestion not only in the stomach but in the intestines as well. The process of chewing likewise stimulates the flow of saliva. Saliva not only helps digest food in the mouth, but this juice also, when swallowed with the food, continues for a time the digestion of starch in the stomach and likewise stimulates to greater activity the glands in the walls of the stomach.

At least a half hour should be devoted to the eating of dinner and twenty minutes to breakfast, lunch, or supper. The proper digestion of food depends in no small degree upon one's mental state; worry and disagreeable topics should, therefore, be forgotten as far as possible while one is eating, and the mealtime should be made a season of enjoyment. Regular hours of eating are of great importance, for nothing more commonly deranges the digestive system than the continual nibbling of food or sweetmeats between meals. One should refrain from vigorous exercise or mental exertion for some time after eating; the reason for this will be clear after a study of the blood system.

145. Prevention of disease.

To insure a state of health

the useless residue of the food should be expelled from the

large intestine regularly each day. If this is not done, serious disturbances of the health are sure to follow. By constipation is meant the abnormal retention of waste matter in the intestine. "The causes of constipation are imperfect digestion (due to deficient secretion in the alimentary canal, inaction of the liver, or insufficient contraction of the muscular fibers of the intestines), insufficient exercise, the use of alcohol or drugs, or improper food." 1

Constipation may usually be counteracted by liberal drinking of water, especially a half hour before breakfast, and by eating food with laxative effect, for example, ripe fruits (especially figs), green vegetables (especially salads with oil), and breads made of the coarser graham and rye flours.

Dyspepsia, also, is far too common, and is one of the most discouraging diseases to treat, because it shows itself in so many different ways. It is far easier to prevent than to cure, for it is usually caused by rapid or irregular eating, by taking indigestible foods, by lack of proper exercise, or by worry; and for all of these conditions the individual is, in the main, responsible.

The regulation of diet in time of sickness is a most important aid to recovery. In certain diseases it is necessary that some kinds of food should be forbidden. Whenever the functions of the body are not carried on with their accustomed vigor, the physician prescribes foods that are easily digested -for example, milk, raw oysters, toasted bread, and softboiled eggs.

146. The use of water as a drink. - "Many people, and especially many women, drink too little water. Water is constantly being lost through the lungs, skin, or kidneys, and this loss is only partially made good by the oxidation of the

1 From New International Encyclopedia.