Mention has already been made of the several factors that must be furnished by the diet in order to render it adequate for the nutrition of the rat (14). It has not been found possible to produce experimental scurvy in this species, and it is necessary, therefore, to conclude that the rat either does not require the anti-scorbutic substance in its metabolic processes, or has the capacity to produce the substance synthetically from some other complexes in the food. If the latter assumption be true the nutrition of this animal would be independent of this factor.

In order to determine which of these explanations holds for the immunity of the rat from scurvy several decisive experiments were carried out in my laboratory in the following way (22): A group of young rats were fed a diet of purified food-stuffs supplemented with an alcoholic extract of wheat germ to furnish the anti-beri-beri factor, water-soluble B. They, therefore, did not have access to the anti-scorbutic factor C, since this is absent or nearly so from wheat germ. To make sure of this fact the extracts were heated so as to destroy any traces which may have been present in the beginning. These rats grew to the full adult size, and remained in good health to the age of fifteen months on a diet which would induce within ten to twenty-five days the severest type of scurvy in young guinea pigs. At this point some of the rats were killed and their fresh livers were fed to young guinea pigs which had been caused to develop scurvy as the result of a faulty diet. In this manner several animals were completely cured of scurvy by the livers of rats which lacked the anti-scorbutic substance in their diet. There can be but one conclusion from such a result. The rats were able to synthesize the necessary anti-scorbutic substance they were unable to secure from their food. Otherwise it could not have been so abundant in the liver. Man, guinea pig and monkey, however, are all susceptible to scurvy because they lack the synthetic capacity possessed by the rat. Another example of a species which is able to subsist without the anti-scorbutic substance is the prairie dog of the western plains. In my laboratory a young prairie dog born in captivity was restricted soon after weaning, to a diet which would certainly produce severe scurvy in a guinea pig within thirty days. Yet the animal was able to grow in a nearly normal manner and escaped the development of scurvy during a period of ten months. It has not been actually demonstrated that this species, like the rat, has the power to synthesize the missing water-soluble C, for the possibility remains that it does without it without detriment to its health. The idea that its case is analogous to that of the rat seems most plausible.

It has been pointed out that the application of the biological method for the analysis of a food-stuff to the study of the cereal grains has shown that these are deficient in three important respects and are individually or collectively unsatisfactory for the nutrition of the rat. The quality of their proteins is below the optimum; the mineral content is deficient in calcium, phosphorus, sodium, and chlorin, and with the exception of yellow maize and millet seed, they are decidedly low in fat-soluble A. Even when the latter seeds are abundant in the diet the content of fat-soluble A will fall decidedly below the requirement for the maintenance of nutrition over a prolonged interval, or the amount necessary to meet the needs of a female during the period of gestation or nursing.

It was to be expected, therefore, that investigators should, in the light of these studies, appreciate the shortcomings of experiments of the older type in which it was customary to produce experimentally, scurvy in guinea pigs by restricting the diet to cereals. These are deficient from the dietary standpoint as already discussed and in addition are not of a physical texture suitable to the guinea pig. The digestive tract of this animal is fitted only for a diet of succulent foods such as green grass, roots and leafy vegetables. When these are eaten in liberal amounts, grains may form a valuable addition to their diet, but not otherwise. Any animal which possesses so highly specialized a digestive tract as does the guinea pig, in which the stomach and cecum are very large and delicate, has not been shown to be satisfactorily nourished during long periods on a diet which is thoroughly digested and absorbed, and which leaves little residue, or one which forms hard or pasty feces. The cereal grains leave little residue, and oats and milk form hard and pasty feces.