In studying digestion it is well to keep in mind the nitrogenous and carbonaceous classification of food, because the process of digestion differs with the different foods.

The use of digestion is to get the food into a liquid form; for food in the stomach is not really in the tissues of the body, and cannot enter the body through the stomach or intestinal surfaces, except in a fluid form.

There are several steps in the process, - mastication, swallowing, and stomach and intestinal digestion. Each portion of the alimentary canal has its own specific work to do, and is furnished with its own distinctive fluid to help it do that work.

All food should be first divided or crushed, if necessary, by the teeth, then mixed with the saliva and thus softened, and above all moistened thoroughly. The saliva is poured into the mouth in large quantities when the presence of food in the mouth excites the salivary glands to secrete it; and sometimes even the sight or thought of food makes the mouth water. The saliva is alkaline, and helps to digest in part the starchy foods by rapidly changing them into sugar, - provided they are kept in the mouth long enough for a thorough mingling with the saliva,- but it does not cause any important change in the nitrogenous foods. Bread, potatoes, rice, and all other starchy foods should therefore be thoroughly masticated and mingled with the saliva. Some substances that are very soft, like thin, starchy gruels, or that become soft and pasty when moistened, like hot fresh bread, are swallowed quickly and almost involuntarily; and although the starch is mostly unchanged, and they may be difficult of digestion by some, no permanent harm comes to healthy people from the absence of salivary digestion, as it is supplemented by the pancreatic.

The stomach carries on the second part of digestion. It pours from its walls an acid fluid, and is furnished with muscles which, by alternate wave-like contraction and relaxation, produce a sort of churning, which helps materially to bring all parts of the food under the action of the gastric juice. This juice dissolves the albumen and fibrin of food, forming peptones, which are very soluble. The starch, sugar, and fat are not changed, except mechanically, the fat being melted, and thus set free.

Such of the albuminoids as are dissolved, and large portions of water, may at once be absorbed into the circulation by the veins of the stomach. The remaining food, in the form of chyme, passes from the stomach into the intestines. Here it meets the bile, originally made in the liver, but stored ready for meal-times in the gall bladder, and also the pancreatic juice, derived directly from the pancreas. These fluids are feebly alkaline, and readily neutralize the weak acid of the gastric juice. They convert the starches into sugar, the nitrogenous foods left over by the stomach (if any) into soluble bodies, and the fats into a finely divided state called an " emulsion," in which the large granules of fat and oil are broken up into minute particles and held in this fluid, very much as cream is held in fresh milk.

The intestinal juice secreted in the mucous membrane the entire length of the intestines has also feeble digestive powers.

The contents of the intestines are now called chyle. The combined amount of the salivary, gastric, pancreatic, biliary, and intestinal fluids secreted daily is said to be about twenty-one pints, of which the gastric juice forms more than one half.

There are mechanical aids to intestinal as well as to stomach digestion. The writhing, worm-like, or "peristaltic" movement of the muscular coats of the intestines, forces the food downward and tends to bring all portions of it in contact with the digestive fluids.

Some of the nutritive and perfectly digested parts of the chyle are next absorbed into the lacteals, which are closely connected with the lining or mucous membrane of the intestines. From these they are emptied into the thoracic duct, and finally into the great veins above the heart. Other portions are carried by the finer branches of the portal vein into the liver, and thence pass into the great veins below the heart.

Thus the venous blood, bringing raw materials from the portal veins and the lacteals, and from the lymphatic vessels waste material, enters the heart through the right auricle, passes through the valves down into the right ventricle, out through the pulmonary artery into the lungs, where as purple venous blood it is driven to the most remote capillaries of the lungs. If the lungs be full of fresh air, the oxygen of the air passes in and changes the purple blood into red oxygenated blood.

This oxygenated blood returns from tlie lungs and enters the heart through the left auricle, then, through the valves, passes into the left ventricle, then out through the aorta, - the great artery, or " main," - from which smaller arteries carry it to the capillaries all over the body. There this new material in the blood is given up to the cells and changed by them as described in Lesson II.