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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:

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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.

to every 100 c.c.

(See Chapter XXVII.) Some waste products,

to be spoken of later, are also found in the plasma.

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Clotting of Blood. If fresh beef blood is allowed to stand overnight, it will be found to have separated into two parts, a dark red, almost solid clot and a thin, straw-colored liquid called serum. Serum is found to be made up of about 90 per cent water, 8 to 9 per cent proteid, and from 1 to 2 per cent sugars, fats, and mineral matter. In these respects it very closely resembles the fluid food that is absorbed from the intestines.

If another jar of fresh beef blood is poured into a pan and briskly whipped with a bundle of little rods (or with an egg beater), a stringy substance will be found to stick to the rods. This, if washed carefully, is seen to be almost colorless. Tested with nitric acid and ammonia, it is

found to contain a proteid substance called fibrin.

Blood plasma, then, is made up of serum, a fluid portion, and fibrin, which, although in a fluid state in the blood vessels within the body, coagulates when blood is removed from the blood vessels. It is this coagulation which aids in the formation of a blood clot. A clot is simply a mass of fibrin threads with a large number of corpuscles tangled within. The clotting of blood is of great physiological importance, for otherwise we might bleed to death from the smallest wound.

In blood within the circulatory system of the body, the fibrin is held in a fluid state called fibrinogen. An enzyme, acting upon this fibrinogen, the soluble proteid

in the blood, causes it to change to an insoluble form, the fibrin of the clot.

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The Red Blood Corpuscle; its Structure and Functions.-The red corpuscle in the blood of the frog is a true cell of disklike form. red corpuscle of man, however, lacks a nucleus. Its form is that of a biconcave disk. So small and so numerous are these corpuscles that over five million are found in a drop of normal blood. The color, which is found to be a dirty yellow when separate corpuscles are viewed under the microscope, is due to a proteid

Human blood as seen under the high power of the compound microscope; at the extreme right is a colorless corpuscle.

material called hæmoglobin. Hæmoglobin contains a large amount of iron. It has the power of uniting very readily with oxygen whenever that gas is abundant, and, after having absorbed it, of giving it up to the surrounding media, when oxygen is there present in smaller amounts than in the corpuscle. This function of carrying oxygen is the one most important function of the red corpuscle, although the red corpuscle also removes part of the carbon dioxide from the tissues on their return to the lungs. The taking up of oxygen is accompanied by a change in color of the mass of corpuscles from a dull red to a bright scarlet.

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The Colorless Corpuscle; Structure and Functions. A colorless corpuscle is a cell irregular in outline, the shape of which is constantly changing. These corpuscles are somewhat larger than the red corpuscles, but less numerous, there being about one colorless corpuscle to every three hundred red ones. They have the power of movement,

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for they are found not only inside blood vessels, but outside the blood tubes, showing that they have worked their way between the cells that form the walls of the blood vessels.

A Russian zoölogist, Metschnikoff, after studying a number of simple animals, such as medusæ and sponges, found that in such animals some of the cells lining the inside of the food cavity take up or ingulf minute bits of food. Later, this food is changed into the protoplasm of the cell. Metschnikoff believed that the colorless

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