70 LECTURE IV. 



that the source is to be sought entirely in substances free from nitrogen. 

 At all events, however, Liebig's theory is untenable. 



It remained for C. Voit 1 to establish a more exact proof of the theory 

 that muscular work is performed chiefly at the expense of substances free 

 from nitrogen. He caused a dog to run in a tread-mill, and compared the 

 amount of nitrogen in the urine which was passed during the working 

 period with that passed during rest, both before and after working. It 

 was found that the amount of nitrogen eliminated during twenty-four 

 hours of a working period was but slightly, if at all, in excess of that elimi- 

 nated during the resting periods. O. Kellner 2 tried a similar experiment 

 with horses, and obtained the same results when the animals experi- 

 mented upon were fed with an abundance of carbohydrates. If this was 

 not the case, the amount of nitrogen eliminated was considerably more. 

 Finally, Voit 3 performed corresponding experiments with human beings, 

 and determined not only the amount of nitrogen eliminated, but at the 

 same time estimated the carbon dioxide, and indirectly the absorption of 

 oxygen. 4 



The increased absorption of oxygen and elimination of carbon dioxide 

 have also been observed in direct experiments upon the muscles themselves, 

 by comparing the amount of these gases contained in the venous blood of 

 a resting muscle and of one that has been tetanized. 5 The inactive muscle 

 takes up more oxygen from the blood than it gives back to the latter in 

 the form of carbonic acid gas. Obviously the muscular cells retain oxygen 

 in some form or other. We can, in fact, speak of a storing up of oxygen. 

 This stored-up oxygen again appears after violent exercise, for then the 

 muscle gives up to the blood more oxygen as carbon dioxide than it takes up 

 from the blood as free oxygen. Yet it is also true that the muscle takes 

 up more oxygen from the blood during exercise than in periods of rest, for 

 venous blood contains during work less oxygen and more carbonic acid 

 than when the muscle is at rest. That evidently not only an oxidation 

 process but a hydrolytic decomposition as well should be regarded as the 

 source of muscular work will be shown later. 



We must now answer the question as to how the glycogen is decom- 

 posed and in what way it is utilized in muscular work. Furthermore, 

 we are interested to know what connection there is between the principal 



1 Z. Biol. 2, 307 and 339 (1866). 



2 Landw. Jahrb. 8, 701 (1879), and 9, 651 (1880). 



3 Z. Biol. 2, 307, 488 (1866). 



4 This was known to Lavoisier: Seguin and Lavoisier: M6m. acad Sciences, 688 

 and 696 (1789). See also Sonden and Tigerstedt: Skand. Arch. Physiol. 6, 181 (1895). 

 O. Krummacher: Z. Biol. 33, 117 (1896) Zuntz, Frentzel, and Loeb: Arch. (Anat. 

 und) Physiol. 541 (1894). Speck: ibid. 465 (1895). 



8 Ludwig and Sczelkow: Sitzber. Akad. Wiss. Wien. 46, 171 (1862). Max v. Frey : 

 Arch. Anat. Physiol. 533 (1885). 



