The studies made by Burge (26) of the catalase content of the tissues of several species of animals (goose, pigeon, sparrow) indicate, however, that this may vary in individuals of the same species and of similar weights and in a normal nutritive condition by as much as 100 per cent or more. It seems questionable, therefore, whether one is justified in attributing much significance to change in the catalase content of the tissues of animals as an index to their capacity for oxidation. The fall in temperature is, perhaps, but one sign of failure of the metabolic rhythm caused by lack of an indispensable substance, just as it may be expected to fail when the living tissues run out of anything essential for their proper functioning. According to Stehle (27) the variation in the catalase content of the blood and tissues is satisfactorily explained as being due to variation in the number of red cells present. He regards this test as of no significance as a measure of the extent of biological oxidations, and expresses the view that there is no relation between the latter and catalase values.

In the light of these facts, it must be admitted that we have no knowledge whatever of the nature of the role the anti-neuritic substance plays in metabolism. We do know that in its absence the metabolic processes fail, and that this failure involves not primarily the nervous system, but all the tissues of the body, the brain and nerves being injured least of all. The suggestion of Dutcher, that under conditions such as result from specific fasting for this substance, the accumulation of incompletely metabolized products, may affect the nervous system and account for the striking loss of function, seems a probable one.

We have already mentioned the fact that the chemical nature of the substance or substances lacking in the diets which induce the syndrome generally designated as polyneuritis, is entirely unknown. Furthermore, we have no test for its presence or absence, other than a properly planned feeding trial in which a basal diet is employed known to be entirely satisfactory except for this one nutritive principle. The test animal is fed exclusively such a diet, to which is added a definite quantity of a natural food, or of an extract to be tested for the anti-neuritic factor. The success or failure of the nutrition of the animal will turn upon the presence or absence in the food or extract of the dietary principle in question.