THE MECHANISM OF LIFE 557 



that life is associated most closely with the construction and 

 destruction of certain highly complex proteids, which for this very 

 reason we have termed biogens. If this be granted, it would be in 

 the highest degree paradoxical, if an increase of the vital process, 

 and so en<.irmous an increase as is expressed in intense muscular 

 activity, should not be necessarily associated with an augmentation 

 of the biogen-transformation in the body. Hence, to the end of his 

 life Liebig, the old master of physiological chemistry, believed 

 indefatigably that we ought to defend the view that the proteids 

 are the substances the decomposition of which constitutes the 

 source of muscle-energy ; and hence also Pflllger, one of the most 

 far-seeing of the physiologists, combats again to-day the view that 

 muscle-activity is able to take place without the decomposition of 

 proteids. But if, during muscular activity, an increased trans- 

 formation of biogens takes place, and if nevertheless, no 

 more nitrogen is then excreted than during rest, the further 

 conclusion is necessary that the nitrogenous residue is again 

 regenerated into the complete biogen molecule. Without such a 

 possibility the simplest and most general vital phenomena cannot 

 be understood. How, for example, can the fact of growth, the fact 

 that living substance is formed from other living substance only, 

 be otherwise conceived than in accordance with the idea that the 

 biogen molecule is capable of uniting to itself definite atoms and 

 groups of atoms, and thus gradually grows into a polymeric 

 molecule ? Polymerisation depends vipon the successive introduc- 

 tion of groups of atoms. Further, regeneration rests in principle 

 upon the same processes as growth. The fact of complete recovery 

 after total fixtigue, also, and man}- other fundamental phenomena 

 of living substance, })resuppose unconditionally the regenerating 

 power of the biogen molecule. 



It is, however, es]K'cially important that the idea here developed 

 is in accord with the two apparently irreconcilable views upon the 

 source of muscle-energy. According to this idea, both the proteids 

 and the carbohydrate.s of the food may serve as the source of this 

 energy. If nmscular activity is focussed in the decomposition and 

 reconstruction of the biogen molecule, and if in its decomposition 

 only non-nitrogenous groups of atoms leave it, it is evident that 

 only non-nitrogenous groups of atoms also are employed for its 

 reconstruction. The facts prove that the proteids as well as the 

 carbohj'drates of the food can serve for this, although, as Pfltiger 

 has shown, with mixed food and sufficient proteid the proteid is 

 preferred. Thus the incontestable tact that in muscular activity 

 the proteid food can be protected to a certain degree by the carbo- 

 hydrates, is comprehensible ; and the claims that the carbohydrates 

 on the one hand, and the proteids on the other, are the source of 

 musele-energv, are equally justitietl. The two substances can 

 l)lay the same r6k, however, only by placing at the disposal of the 



