While experiments of this nature, on man and animals, all tended to show little or no increase in the excretion of nitrogen, as a result of muscle work; and likewise no increase in the output of sulphur and phosphorus, thus strengthening the view that muscular energy is not the result of proteid disintegration, there was observed marked increase in the consumption of oxygen, and in the excretion of carbon dioxide. Non-nitrogenous matter was thus at once suggested as the material with which muscle chiefly does its work. There is to-day no question of the general truth of this statement, yet there are other aspects of the problem to be considered before we can lay it aside. Pflüger, working with dogs, and Argutinsky, experimenting on himself by arduous mountain climbing, reached conclusions seemingly quite opposed to what has just been said. Their results, however, admit of quite a different interpretation from what they were disposed to attach to them. Thus, Pflüger[44] would go back to the old view that all muscle work is at the expense of proteid material, because lean dogs fed mainly, or entirely, on meat and made to do an excessive amount of work were found by him to excrete nitrogen somewhat in proportion to the amount of work done. Argutinsky,[45] likewise, in his mountain climbing carried to the point of fatigue, and with a high proteid intake likewise, saw in the increased output of nitrogen a suggestion of the same idea. In reality, however, their results merely prove that, under some circumstances, proteid may serve as the chief source of muscular energy; as when the body is poor in fat and carbohydrate, or when the intake consists solely of proteid matter. In other words, muscular work may result in an increased excretion of nitrogen when the work is very severe, and there is not a corresponding increase in the fats or carbohydrates (fuel ingredients) of the food. In the words of Bunge,[46] “we might assume _à priori_, on teleological grounds, that in the performance of its most important functions the organism is to a certain extent independent of the quality of its food. As long as non-nitrogenous food is supplied in adequate quantity or is stored up in the tissues, muscular work is chiefly maintained from this store. When it is gone the proteids are attacked.” [44] Pflüger: Die Quelle der Muskelkraft. Pflüger’s Archiv für die gesammte Physiologie, Band 50, p. 98. [45] Argutinsky: Muskelarbeit und Stickstoffumsatz. Ibid., Band 46, p. 552. [46] Bunge: Textbook of Physiological and Pathological Chemistry. Second English Edition, 1902, p. 352.