This experiment with dog No. 17 has been dwelt upon at some length, because it illustrates a very important principle in the nutrition of a high proteid and carnivorous animal. As before stated, it is not a question of high or low proteid simply, but involves possibly the more subtle question of the relative value of specific forms of proteid food. It will be noted that this statement is made somewhat guardedly, in harmony with the caution necessarily called for in view of our lack of knowledge regarding the possible need of the animal's body for extraneous principles which only meat, milk, or other animal products can supply. Inorganic salts, nitrogenous extractives, and other substances without any appreciable fuel value, are quite likely to be of primary importance in controlling and regulating the various processes of the body, which combine to maintain the condition of normal nutrition. With a diet restricted to one or two vegetable products, it is quite conceivable that something may be lacking which the system demands, though it cannot be measured in terms of nitrogen or calories. It may be said that man thrives on a purely vegetable diet, but while this is unquestionably true, it must be remembered that man with his free choice of food has recourse, as a rule, to a large variety of vegetable products from many sources, and consequently there is great likelihood of his absorbing from these varied products such supplementary matters as may be needed. On this question, we are in a realm of doubt and uncertainty, but the possibilities suggested must not be ignored, for they may contain a germ of truth of the utmost importance. The fact remains, however, that a dog when restricted to a purely vegetable dietary does not thrive; a little animal food seems necessary to keep up health and strength, and this suffices even though the daily nitrogen intake and fuel value of the food are restricted to a level below that of the vegetable dietary.

Subject.No. 17 January 2. 1906.

Subject No. 17. February 27, 1906.

Subject No. 17 April 24, 1906.

Subject No. 17 June 27, 1906.

With these facts before us, it is difficult to avoid the conclusion that some significance may attach to the specific nature of the proteid. Of course, we must not overlook the radical difference in dietary habits of man and dog. Man as an omnivorous creature has for generations been accustomed to partake largely of vegetable foods, and as a result his digestive tract and his system as a whole has become acclimated, as it were, to the nutritive effects of vegetable matter. Dogs, on the other hand, are typical carnivores, and their habits for generations have led in an opposite direction, so that their gastro-intestinal tracts and their systems have become accus-stomed to the effects of a diet in which animal food largely predominates. Whether these deeply ingrained characteristics are responsible in any large measure for the difference in behavior of man, on a purely vegetable diet, and dogs is open to question. It would certainly not be strange if such were the case, but as we look at the facts collected in our study of this subject, it is somewhat impressive to note how well dogs thrive on a relatively large amount of vegetable food, provided there is a modicum of animal food added thereto. In other words, these high proteid consumers are apparently quite able to utilize the vegetable foods, but there is something lacking in such a dietary which the body has great need of. Is it not quite possible, as already suggested, that the specific nature of the proteid counts for something in nutrition? The question cannot be answered definitely at present, but there are certain facts slowly accumulating which make the question a pertinent one in this connection.

Thus, it is becoming evident, as was pointed out in an earlier chapter, that the many proteid substances occurring in the animal and vegetable kingdoms are more or less unlike each other in their chemical make-up. They yield different decomposition products, or the same products in widely different proportion, when broken down by the action of hydrolyz-ing agents; and when we recall that the digestive enzymes of the body convert the proteids of the food into these same end-products, it is plain that in the assimilation and utilization of the proteid foodstuffs the body has to deal with these various chemical units. Hence, an animal suddenly restricted to a dietary in which all of the proteid is furnished by bread might be seriously incommoded, either by the excess of certain amino-acids resulting therefrom, or by a lack of certain other end-products to which its body is accustomed. As an example, we may take the three typical proteids of the wheat kernel, gliadin, glutenin, and leucosin, and note the very striking difference in the proportion of certain of the decomposition products of each, as reported by Osborne and Clapp.1

1 See Osborne and Clapp: The Chemistry of the Protein Bodies of the Wheat Kernel. American Journal of Physiology, vol. 17, p. 231.

It is obvious from these figures that the three proteids of the wheat kernel are radically different from each other. Contrast, for example, the content of glutaminic acid in gliadin with the amount in leucosin. With such striking differences in chemical make-up, it is reasonable to assume that corresponding differences in physiological action or food values may exist Further, "in respect to the amount of these amino-acids, leucosin more nearly resembles the animal proteins than the seed proteins thus far examined, and in this connection it is interesting to note that leucosin occurs chiefly if not wholly in the embryo of this seed and is probably one of its 'tissue' proteins, in contrast to the 'reserve' proteins of the endosperm of which gliadin and glutenin form the chief part" (Osborne and Clapp). In other words, animal proteids, such as those of meat, are characterized like leucosin by a small content of glutaminic acid and ammonia; while leucin, lysin, aspartic acid, and arginin are relatively more abundant. Until we know more on this subject, however, any broad generalization would be out of place, but certainly there is justification for the supposition that in these differences in chemical constitution are to be found explanation of some of the peculiarities common to certain varieties of proteid food. Wheat flour, aside from its starch, is composed mainly of glutenin and gliadin with their large content of glutaminic acid. Meat proteids, on the other hand, like leucosin, contain only a small fraction of this acid, and, with the other differences indicated, meat proteid and wheat proteid as food for dogs or other high proteid consumers may reasonably be expected to have at the least very unequal values. And if we go a step beyond this and suppose that in the formation of true tissue proteid or the living protoplasm of the cell, certain of these end-products of proteid decomposition are absolutely indispensable, we can easily picture for ourselves a dearth of such building stones in the long-continued use of a diet which lacks that particular proteid from which the necessary building stones can be split off in adequate number.