84 PHYSIOLOGY OF MUSCLES A^D NERVES. 



and from this the amount of carbon which must be con- 

 sumed may be calculated. Whatever does not escape 

 in the form of gas during combustion remains behind 

 as ash. The ash of the fire of the steam-engine is 

 represented by the urea and other matter which passes 

 from the muscles into the urine. The whole amount of 

 both must correspond exactly with the whole amount 

 of the products resulting from combustion within the 

 muscle. 



AlthouGfh the small amount of the non-nitros^enous 

 substances present in the muscle does not, therefore, 

 prevent us from regarding them as the main source of 

 muscular labour, yet in one point the machine called 

 muscle differs from the steam-engine, which it other- 

 wise so strikingly resembles. We found that the ex- 

 cretion of urea undergoes an increase, though this may 

 not be very great, when the muscular labour is in- 

 creased. It is, therefore, evident that there must be a 

 greater destruction of the chief constituents of muscle- 

 substance, of the tissue of which muscle is mainly 

 formed, and which may be compared to the metallic 

 parts of the steam-engine. Even in the latter a waste 

 of the metallic parts occurs ; but this is comparatively 

 very small in degree. The muscular machine is not 

 constructed of such durable material ; during its ac- 

 tivity-it, therefore, continually wastes a comparatively 

 considerable amount of its ow^n substance. As the 

 matter leaves the body in a more highly oxidised form 

 than it had when it was present in the muscle, warmth 

 and work must also be freed during this partial com- 

 bustion of the material of the machine. The muscle- 

 machine works, therefore, partly at the expense of its 

 own form-element; and, if it is to work continuously, not 



