If Liebig’s theory is correct, that the proteids of the body are decomposed only as the result or the accompaniment of muscle work, and the proteids of the food are used up only as they take the place of the organized proteid so metabolized, it follows that with a like degree of muscular activity a given body will always decompose the same amount of proteid. If excess of proteid food is taken, the surplus will be stored in the tissues, or, in other words, the excretion of nitrogen will not be influenced by the amount of proteid consumed in the food. This was the line of argument made use of by various physiologists[18] who were disposed to criticise Liebig’s view, and quite naturally the question was soon made the subject of many experiments. It will suffice here merely to say that many concordant results were obtained, showing that an abundance of proteid food leads to an increase in the excretion of nitrogen, muscle activity remaining at a constant level. Hence, as Voit states, some other ground than muscle work must be sought as the true cause of proteid katabolism. Consequently, we find this hypothesis of Liebig replaced by the theory of “luxus consumption,” in which it is maintained that while whatever proteid is used up by the work of the muscle must be made good from the proteid of the food, any excess of proteid absorbed from the intestinal canal is to be considered as “luxus,” and like the non-nitrogenous foods may be burned up in the blood, by the oxygen therein, without being previously organized. Hence, we see suggested two causes for the decomposition of proteid in the body, viz., the work of the muscle and the oxygen of the blood. Further, as stated by C. Voit,[19] the nitrogen excretion of the hungry or fasting animal affords, according to these views, a measure of the extent to which tissue proteid must be broken down in the maintenance of life, and of the amount of proteid food necessary to be consumed in order to make good the loss; viz., the minimum proteid requirement. Again, since any excess of proteid food beyond this minimal requirement, according to the theory, is destined to be burned up in the blood, or elsewhere, to furnish heat the same as non-nitrogenous foods, it follows that the excess of proteid food can be replaced by non-nitrogenous aliment. [18] See C. Voit: Hermann’s Handbuch der physiologie des Gesammt-Stoffwechsels. Band 6, Theil 1, p. 269, 1881. [19] Loc. cit., p. 270.