COMPOSITION OF THE MUSCLES. 569 



of ZUNTZ, (together with) LOEB, HEINEMANN, FRENTZEL and REACH 

 show that all foodstuffs have nearly the same power of serving as the 

 material for the work of the muscles. The extensive metabolism investi- 

 gations of ATWATER and BENEDICT 1 have also led to similar results as 

 to the fats being a source of muscular energy. The law of the substitu- 

 tion of the foodstuffs, according to their combustion equivalents, is also 

 true for muscular work, and fat correspondingly acts with its full amount 

 of energy without previously being transformed into sugar. The question 

 which of the foodstuffs the muscle prefers is dependent upon the relative 

 quantities of the same at the disposal of the muscle. A direct substitu- 

 tion of the body material by the bodies supplied as food does not take 

 place in the muscular activity in the ordinary nutritive condition. Accord- 

 ing to JOHAXSSOX and KoRAEN 2 the CO 2 excretion produced by cer-. 

 tain work is not influenced by the supply of foodstuffs (protein or sugar) 



SIEGFRIED considers, as above stated, the phosphocarnic acid as a source of 

 energy. According to his and KRUGER'S 3 researches, phosphocarnic acid, which 

 yields on cleavage, among other bodies, carbon dioxide, occurs in part preformed 

 m the muscle, and in part as a hypothetical aldehyde compound of the same 

 a compound which forms phosphocarnic acid on oxidation. SIEGFRIED therefore 

 makes the suggestion that in the resting muscle, which requires more oxygen 

 than exists in the carbon dioxide eliminated, this reducing aldehyde substance is 

 gradually oxidized to phosphocarnic acid, which is used in the activity of the 

 muscle with the splitting off of carbon dioxide. 



Quantitative Composition of the Muscle. A large number of analyses 

 have been made of the flesh of various animals for purely practical pur- 

 poses, in order to determine the nutritive value of different varieties 

 of meat; but there are no exact scientific analyses with sufficient regard 

 to the quantity of different protein bodies and the remaining muscle 

 constituents, that is, these anahses are incomplete or of little value. 



To give the reader some idea of the variable composition of muscle- 

 substance the following summary is presented, chiefly obtained from 

 K. B. HoFMANN's 4 book, although it does not correspond to the present 

 demands. The figures are parts per 1000. 



1 Loeb, Arch. f. (Anat. u.) Physiol., 1894; Heinemann, Pfliiger's Arch., 83; Frentzel 

 and Reach, ibid. ; Atwater and Benedict, U. S. Dept. of Agric., Bull. 136, and Ergeb- 

 nisee der Physiologie, 3. 



2 Skand. Arch. f. Physiol., 13. 



3 Zeitschr. f. physiol. Chem., 22. 

 4 Lehrbuch d. Zoochemie (Wien, 1876), 104. 



