inner surface of the tube being thrown into transverse folds which not only retard the rapidity with which food passes down the intestine, but also give more absorbing surface. But far more important for absorption are millions of little projections which cover the inner surface of the small intestine.

The Villi. —So numerous are these projections that the whole surface presents a velvety appearance. Collectively, these structures are called the villi (singular villus). They form the chief organs of absorption in the intestine, several thousand being distributed over every square inch of surface. By means of the folds and villi the small intestine is estimated to have an absorbing surface equal to twice that of the surface of the body. Between the villi are found the openings of many small tubelike glands, the intestinal glands. These glands manufacture a digestive fluid, strongly alkaline, which aids in diges ing fats, and acts somewhat like the pancreatic fluid.

Internal Structure of a Villus.

vet Hto arm

The internal structure of a villus

is best seen in a longitudinal section. We find the outer wall made up of a thin layer of cells, the epithelial layer. It is the duty of these jugular vein cells to absorb the semifluid food from within the intestine. Underneath these cells lies a network of very tiny blood vessels, while inside of these, occupying the core of the villus, are found spaces which, because of their white appearance after absorption of fats, have been called lacteals.

vena cava

heart

sugars

and

proteids

lacteals

fats

thoracic duct

Diagram showing how food reaches the circulation.

Absorption of Foods. Let us now attempt to find out exactly how foods are passed from the intestines into the blood. Food substances in solution may be soaked up as a sponge would take up water, or they may pass by osmosis into the cells lining the villus. These cells are alive, and therefore have the power of selecting certain substances and rejecting others. Once within the villus, the sugars and digested proteids pass through tiny blood vessels into the larger vessels comprising the portal circulation. These pass through the liver, where,

as we have seen, sugar is taken from the blood and stored as glycogen. From the liver, the food within the blood is sent to the heart, from there is pumped to the lungs, from there returns to the heart, and is pumped to the tissues of the body. A large amount of water and some salts are also absorbed through the walls of the stomach and intestine as the food passes on its course. The fats in the form of soaps and fatty acids pass into the space in the center of the villus. Later they are changed into fats again, probably in certain groups of gland cells known as mesenteric glands, and eventually reach the blood by way of the thoracic duct without passing through the liver.

Large Intestine. The large intestine has somewhat the same structure as the small intestine, except that the diameter is much greater. It also contains no villi. Considerable absorption, however, takes place through its walls as the mass of food and refuse material is slowly pushed along by the muscles within its walls.

In this portion of the intestine live millions of bacteria, some of which manufacture poisonous substances from the foods on which they live. These substances are easily absorbed through the walls of the large intestine, and passing into the blood, cause headaches or sometimes serious trouble. Hence it follows that the lower bowel should be emptied of this matter as frequently as possible, at least once a day. Constipation is one of the most serious evils the American people have to deal with, and it is largely brought about by the artificial life which we lead, with its lack of exercise, fresh air, and sleep.

Vermiform Appendix. At the point where the small intestine widens to form the large intestine, a baglike pouch is formed. From one side of this pouch is given off a small tube about four inches long, closed at the lower end. This tube, the function of which in man is unknown, is called the vermiform appendix. It has come to have unpleasant notoriety in late years, as the site of serious inflammation. It often becomes necessary to remove the appendix in order to prevent this inflammation from spreading to the surrounding tissues.

Hygienic Habits of Eating; the Causes and Prevention of Dyspepsia. From the contents of the foregoing chapter it is evident that the object of the process of digestion is to break up solid food so that it may be absorbed to form part of the blood. Any habits we may form of thoroughly chewing our food will evidently aid in this process. Undoubtedly much of the distress known as dyspepsia is due to too hasty meals with consequent lack of proper

mastication of food. The message of Mr. Fletcher in bringing before us the need of proper mastication of food and the attendant evils of overeating is one which we cannot afford to ignore. It is a good rule to go away from the table feeling hungry. Eating too much overtaxes the digestive organs and prevents their working to the best advantage. Still another cause of dyspepsia is eating when in a fatigued condition. It is always a good plan to rest a short time before eating, especially after any hard manual work. Eating between meals is also condemned by physicians because it calls the blood to the digestive organs at a time when it should be in other parts of the body.

Effect of Alcohol on Digestion. It is a well-known fact that alcohol extracts water from tissues with which it is in contact. This fact works much harm to the interior surface of the food tube, especially the walls of the stomach, which in the case of a hard drinker are likely to become irritated and much toughened. In small amounts alcohol stimulates the secretion of the salivary and gastric glands, and thus seems to aid in digestion. It is doubtful, however, whether this aid is real.

The following results of experiments on dogs, published in the American Journal of Physiology, Vol. I, Professor Chittenden of Yale University gives as "strictly comparable," because "they were carried out in succession on the same day." They show that alcohol retards rather than aids in digestion:

fo LB. MEAT WITH DILUTE ALCOHOL

Digested in 3: 15 hours
Digested in 3: 00 hours

XIX 9:00 A.M.

Digested in 2:30 hours

As a result of his experiments, Professor Chittenden remarks: "We believe that the results obtained justify the conclusion that gastric digestion as a whole is not materially modified by the introduction of alcoholic fluids with the food. In other words, the unquestionable acceleration of gastric secretion which follows the ingestion of alcoholic beverages is, as a rule, counterbalanced by the inhibitory effect of the alcoholic fluids upon the chemical process of gastric digestion, with perhaps at times a tendency towards preponderance of inhibitory action." Dr. Kellogg, Sir William Roberts, and others have come to the same or stronger conclusions as to the undesirable action of alcohol on digestion, as a result of their own experiments.

Horsley and Sturge say: "Hundreds of men and women who haunt the out-patient departments of hospitals suffer from chronic atony and slight dilatation of the stomach, which arise in part from the badly cooked food they eat, but chiefly owe their origin to the debilitating effect of alcohol upon the muscular walls of this organ and the fermentation of its retained contents."

XXVI. THE BLOOD AND ITS CIRCULATION

Problem XLVIII. To study the composition of the blood. (Laboratory Manual, Prob. XLVIII.)

Function of the Blood. The chief function of the digestive tract is to change foods to such form that they can be absorbed through the walls of the food tube and become part of the blood.1 By means of a system of closed tubes, this fluid tissue circulates to all parts of the body, equalizing the body temperature by depositing its burden of food in places where it is most needed and where it will be used, either in the repair and building of tissues or for oxidation within the cells of the body to release energy.

If we examine under the microscope a drop of blood taken from the frog or man, we find it made up of a fluid called plasma and two kinds of bodies, the so-called red corpuscles and colorless corpuscles, floating in this plasma.

Composition of Plasma. The plasma of blood (when chemically examined in man) is found to be largely (about 90 per cent) water. It also contains a considerable amount of proteid, some sugar, fat, and mineral material. It is, then, the medium which holds the fluid food (or at least part of it) that has been absorbed from within the intestine. The almost constant temperature of the body is also due, as we shall see, to the blood which brings to the surface of the body much of the heat given off by oxidation of food in the muscles and glands within. When the blood returns from the tissues where the food is oxidized, the plasma brings back with it to the lungs the carbon dioxide liberated from the tissues of the body where oxidation has taken place. Blood returning from the tissues of the body has from 45 to 50 c.c. of carbon dioxide

1 This change is due to the action of certain enzymes upon the nutrients in various foods. But we also find that peptones are changed back again to proteids when once in the blood. This appears to be due to the reversible action of the enzymes (See page 72.)

acting upon them.