256 ON THE SOURCE OF MUSCULAR POWER, 



We have seen, therefore, in the above four sets of experiments, in-' 

 terpreted by the data afforded by the combustion of muscle and urea 

 in oxygen, that the transformation of tissue alone cannot account for 

 more than a small fraction of the muscular power developed by ani- 

 mals ; in fact, this transformation goes on .at a rate almost entirely 

 independent of the amount of muscular power developed. If the 

 mechanical work of an animal be doubled or trebled there is no cor- 

 responding increase of nitrogen in the secretions; whilst it was proved 

 on the other hand by Lawes and Gilbert, as early as the year 1854, 

 that animals, under the same conditions as regarded exercise, had the 

 amount of nitrogen in their secretions increased twofold by merely 

 doubling the amount of nitrogen in their food. Whence then comes 

 the muscular power of animals ? What are the substances which, by 

 their oxidation in the body, furnish the actual energy, whereof a part 

 is converted into muscular work ? In the light of the experimental 

 results detailed above, can it be doubted that a large proportion of 

 the muscular power developed in the bodies of animals has its origin 

 in the oxidation of non-nitrogenous substances ? For whilst the se- 

 cretion of nitrogen remains nearly stationary under widely different 

 degrees of muscular exertion, the production of carbonic acid increa- 

 ses most markedly with every augmentation of muscular work, as is 

 shown by the following tabulated results of E. Smith's highly impor- 

 tant experiments regarding the amount of carbonic acid evolved from 

 his own lungs under different circumstances.* 



Excretion of carbonic acid during rest and muscular exertion : — 



Carbonic acid 

 per hour. 



During sleep -. •• 19 grams. 



Lying down and sleep approaching , 23'0 " 



In a sitting posture 29'0 



Walking at rate of 2 miles per hour. ... . , 70-5 " 



u " 3 " « 100-6 '« 



On the treadwheel, ascending at the rate of 28"65 feet per minute. . .189-6 " 



It has been already stated as a proposition upon which all are 

 ao-reed, that food, and food alone, is the ultimate source from which 

 muscular power is derived ; but the above determinations and consi- 

 derations, the speaker believed, prove conclusively, firstly, that the 

 non-nitrogenous constituents of the food, such as starch, fat, &c., are 

 the chief sources of the actual energ}-, which becomes partially trans- 



* Phil. Trans, for 185?, p. 109. 



