568 MUSCLES. 



urine, and found that the work really performed was not by any moans 

 compensated by the consumption of protein. It was therefore proven 

 by this that proteins alone cannot be the source of muscular activity, 

 and that this depends in great measure on the metabolism of non-nitro- 

 genous substances. Many other observations have led to the same result, 

 especially the experiments of VOIT, of PETTENKOFER and VOIT, and of 

 other investigators, whose observations show that while the elimination 

 of nitrogen remains unchanged, the elimination of carbon dioxide during 

 work is very considerably increased. It is also generally considered as 

 positively proven that muscular work is produced, at least in greatest 

 part, by the catabolism of non-nitrogenous substances. Nevertheless 

 there is no warrant for the statement that muscular activity is produced 

 entirely at the cost of the non-nitrogenous substances, and that the 

 protein bodies are without importance as a source of energy. 



The investigations of PFLUGER 1 are of great interest in this connec- 

 tion. He fed a bulldog for more than seven months with meat which 

 alone did not contain sufficient fat and carbohydrates even for the pro- 

 duction of heart activity, and then let him work very hard for periods 

 of 14, 35, and 41 days. The positive result obtained by these series 

 of experiments was that " complete muscular activity may be effected 

 to the greatest extent in the absence of fat and carbohydrates," and the 

 ability of proteins to serve as a source of muscular energy cannot be 

 denied. 



The nitrogenous as well as the non-nitrogenous nutriments may serve 

 as a source of energy; but the views are divided in regard to the relative 

 value of these. PFLUGER claims that no muscular work takes place 

 without a decomposition of protein, and the living cell-substance prefers 

 always the protein and rejects the fat and sugar, contenting itself with 

 these only when proteins are absent. Other investigators, on the con- 

 trary, believe that the muscles first draw on the supply of non-nitrogenous- 

 nutriments, and according to SEEGEN, CHAUVEAU, and LAULANiii 2 the 

 sugar is indeed the only direct source of muscular force. The last- 

 mentioned investigator holds that the fat is not directly utilized for work, 

 but only after a previous conversion into sugar. ZUNTZ and his collabora- 

 tors have made strong objections to the correctness of such a view. If, 

 according to ZUNTZ, the fat must be first transformed into sugar before 

 it can serve as the source of muscular work, a definite expenditure of 

 force must require about 30 per cent more energy with fatty food than 

 it does with carbohydrates ; but this is not the case. The investigations 



1 Pfliiger's Arch., 50. 



2 See Seegen, footnote 4, page 563. The works of Chauveau and his collaborators 

 are found in Compt. rend., 121, 122, and 123; Laulanie, Arch, de Physiol. (5), 8. 



