hydrogen of the proteins and fats. No rules as to the amount can be given, since it varies so much with temperature and the amount of muscular activity; but the habit of drinking no water between meals and but little at the table, in spite of popular opinion on the subject, is to be deprecated. . . .

"Undue emphasis has been laid upon the danger of drinking water with meals. The reasons given - that such water unduly dilutes the gastric juice or takes the place of a normal secretion of saliva - are questionable. As a matter of fact, the water thus taken is soon discharged into the intestine and absorbed. It is true, however, that the use of too much fluid with the meals is apt to lead to insufficient mastication because it makes it easier to swallow the food; and from this point of view caution is advisable. It is probably also true that much drinking with meals tends to overeating, by facilitating rapid eating."-HOUGH and SEDGWICK'S "Human Mechanism."

147. Effects of alcoholic drinks on the organs of digestion. Alcohol, unlike most of the substances taken into the alimentary canal, requires no digestion. It can, therefore, be absorbed very rapidly by the blood, and hence alcohol is possibly sometimes of great value when administered by physicians, in cases when ordinary food cannot be digested. In health, however, alcoholic drinks must be regarded as an expensive and extremely dangerous source of

energy.

According to the best authorities, small quantities of alcohol (when sufficiently diluted) seem for an adult to stimulate an increased flow of saliva and gastric juice, but even this is doubtful. The time required for the digestion of food, when alcohol is present in these small quantities, does not seem to be increased. Entirely different effects follow, however, when strong distilled liquors are taken,

and alcohol in any large quantity often produces serious disturbances of the organs of digestion. This is especially true when liquors are taken without food; that is, between meals. The constant danger that the moderate use of beer and the light wines will lead to an uncontrollable thirst for alcohol cannot be emphasized too strongly. All authorities agree, too, that the growing youth should let alcohol entirely alone.

CHAPTER VI

CIRCULATION OF THE NUTRIENTS

I. COMPOSITION OF THE BLOOD

149. Food and blood. Thus far in our laboratory studies we have tested various foods, and have found that they all consist of one or more of the nutrients; namely, proteins, fats, carbohydrates (i.e. starch and sugar), fats, mineral matters, and water. We have discussed the way in which each of these nutrients is digested, and thus made ready for absorption into the blood for until the nutrients actually become a part of blood, they cannot be of use to the body. In 7 we described the red and white corpuscles of the blood (Fig. 5) and there stated that the liquid part of blood is known as blood plasma.

150. Composition of blood plasma. Blood plasma contains a large amount of water in which are dissolved the various nutrients obtained by absorption from the alimentary canal. The presence of each of these nutrients has been demonstrated by applying the various food tests given in 23-28, "Plant Biology." Following is the percentage of each nutrient found in the human body:

1 For a laboratory study of blood, see Peabody's "Laboratory Exercises," pp. 50-53.

151. Hygiene of the plasma. All the nutrition of the tissues is derived from the blood, and all the nutrients of the blood come from the foods we eat. If these foods are insufficient or of an improper kind, the blood will, of course, be deprived of necessary ingredients, and the cells must inevitably suffer in consequence. Hunger and thirst are the sensations that tell us that the blood is in need of new material. That this is true is demonstrated by the fact that these sensations disappear when water and liquid food, instead of being swallowed, are injected directly through the skin into the blood vessels.

152. Blood clotting. When blood escapes from the body, it is a liquid of a bright red color. It soon changes to a dark maroon, however, and later this thickens to the consistency of jelly. This dark red mass is called a blood clot, and the process is known as clotting or coagulation. Coagulation is of great practical importance, since it provides a natural means of closing injured blood vessels, and of preventing loss of blood.

II. CIRCULATION AND ITS ORGANS

153. Necessity for the circulation. From our study thus far, we have found that our bodies are composed of complex chemical compounds that are constantly being consumed in the development of heat and other forms of energy. It is evident, then, that every organ of the body, and indeed every living cell, must be supplied with new material to make good these losses and to provide for growth. The source of all this material is the food we eat.

In the last chapter we considered some of the processes by which foods are converted into liquid form and made ready for use in the cells. We found that after being liquefied these