SOURCE OF MUSCLE-FORCE. 83 



production of work, as it cannot be digested, and, owing 

 to the low temperature of the body, cannot be oxidised. 

 But combinations abounding in carbon, such as are at 

 hand in the carbon hydrates (starch, sugar, &c.) and in 

 fats, are fitted for the purpose, and a given weight of 

 these affords a considerably greater amount of work 

 than can an equal weight of nitrogenous albumens. 

 If, therefore, the muscle is capable, by the combustion 

 of the non -nitrogenous bodies which it contains, of ac- 

 complishing labour, it is evident that this relation is 

 similar to that in the case of the steam-engine, in which 

 the work is accomplished by the combustion of carbon. 

 It has been objected that the amount of non-nitro- 

 genous substance within the muscle is very small, but 

 the objection is scarcely tenable. If a wdiole steam- 

 engine with its boiler and the coal in the furnace could 

 be subjected to a chemical analysis, the percentage of 

 coaJ in the whole mass would of course be found to be 

 very small. But it is not by the amount of coal present 

 at any given moment that the work is performed, but by 

 the whole amount which in the course of a considerable 

 time is added little by little by the stoker. In the 

 case of muscle the blood acts the part of the stoker. 

 It continually adds matter to the muscle, and the 

 products of combustion resulting from labour escape 

 from the muscle, just as the carbonic acid does from 

 the chimney of the steam-engine. It is evident that 

 the amount of carbon consumed by a steam-engine 

 might be^ accurately determined by collecting and 

 analysing the carbonic acid which escapes from the 

 chimney. We proceed in exactly the same way in the 

 case of the muscle. The lungs represent the chimney; 

 the carbonic acid escaping from these may be collected, 



