72 THE DATA OF BIOLOGY. 



this admission, however, it should be added that possibly 

 out of the exclusively nitrogenous food, glycogen or sugar 

 has to be obtained by partial decomposition before muscular 

 action can take place. But when we pass to animals having 

 food consisting mainly of carbo-hydrates, several difficulties 

 stand in the way of the hypothesis that, by further com- 

 pounding, proteids must be formed from the carbo-hydrates 

 before muscular energy can be evolved. In the first place 

 the anabolic change through which, by the addition of nitro- 

 gen, &c., a proteid is formed from a carbo-hydrate, must 

 absorb an energy equal to a moiety, of that which is given 

 out in the subsequent katabolic change. There can be no 

 dynamic profit on such part of the transaction as effects the 

 composition and subsequent decomposition of the proteid, 

 but only on such part of the transaction as effects the decom- 

 position of the carbo-hydrate. In the second place there 

 arises the question — whence comes the nitrogen required for 

 the compounding of the carbo-hydrates into proteids ? There 

 is none save that contained in the serum-albumen or other 

 proteid which the blood brings; and there can be no gain in 

 robbing this proteid of nitrogen for the purpose of forming 

 another proteid. Hence the nitrogenizing of the surplus 

 carbo-hydrates is not accounted for. One more difficulty 

 remains. If the energy given out by a muscle results from 

 the katabolic consumption of its proteids, then the quantity 

 of nitrogenous waste matters formed should be proportionate 

 to the quantity of work done. But experiments have proved 

 that this is not the case. Long ago it was shown that the 

 amount of urea excreted does not increase in anything like 

 proportion to the amount of muscular energy expended; and 

 recently this has been again shown. 



On this statement a criticism has been made to the follow- 

 ing effect : — Considering that muscle will contract when 

 deprived of ox3'gen and blood and must therefore contain 

 matter from which the energy is derived; and considering 

 that since carbonic acid is given out the required carbon and 



