80 PHYSIOLOGY OF MUSCLES AND NERVES. 



within the muscle are expended during its activity. 

 As muscle consists principally of albuminous bodies, it 

 has been assumed that it is to the decomposition of 

 these that the labour accomplished is due. We have, 

 however, seen that non-nitrogenous bodies, such as 

 glycogen and muscle-sugar, are also contained in the 

 muscle, and that lactic acid Avhich must orioinate 

 from the latter, is formed during the active state. 

 Although it is impossible to determine the products of 

 decomposition within a single muscle, yet this may be 

 done in the case of the whole mass of the muscles of 

 the body during an activity of long continuance ; for 

 the products of decomposition finally pass into the ex- 

 cretions, and it is evident that the whole amount of 

 addition to the excretions may be regarded as a 

 measure of the decomposition in the active muscles. 

 The nitrogenous constituents of muscle are almost 

 without exception excreted in the form of urea with 

 the urine. At least the amount of nitrogen contained 

 in the other excretory products is so very small that it 

 may safely be disregarded. Now, the amount of urea 

 contained in the urine may be determined with very 

 great accuracy. Even when the body is in a state of 

 complete rest ^though even then a considerable amount 

 of work is performed in the body, in the action of the 

 heart and of the respiratory muscles — the excretion of 

 urea depends entirely on the amount of nitrogen intro- 

 duced in food. If entirely non-nitrogenous food is 

 taken, then the excretion of urea decreases to a definite 

 point, at which it remains constant for some time. If 

 a larger amount of work is performed, a slight increase 

 in the excretion of urea in fact usually occurs. The 

 amount of albuminous matter which must be modified 



