34 THE BEGINNINGS OF LIFE. 



muscular action is derived from no peculiar source. We 

 know that heat is set free by nutritive chemical changes 

 taking place in the blood which circulates through the 

 capillaries of the muscular system, and that the sub- 

 stances which undergo these changes are dissolved non- 

 nitrogenous, as well as nitrogenous products of assimi- 

 lation. We know, in fact, that the muscle acts only as 

 a machine for the purpose of converting a portion of 

 the heat thence derived into mechanical energy \ and 

 that the substance of the muscle itself not yielding the 

 force which is to be transformed undergoes merely a 

 molecular wasting by virtue of its own functional acti- 

 vity as a transformative apparatus, just as the parts of 

 a steam-engine are subject to a gradual wear and tear 

 produced by the friction occasioned during its activity 2 . 



1 The machine being called into action, merely, by the nerve, and the 

 stimulus coming through this being partly, though not wholly, to the 

 contraction of the muscle as the spark is to exploding gunpowder. 

 The experiments of Matteucci ('Letture sul 1' elettro-physiologia,' Milan, 

 1867, p. 35) go to show that the mechanical work effected by a muscle 

 during its contraction may be 30,000 times greater than the work ex- 

 pended in the excitation of the nerve. On the other hand, there is 

 abundant evidence to show that the strength and vigour of the muscular 

 contraction varies with the amount or intensity of the nerve-change 

 which calls it into play; The same muscle which, in certain states of 

 the nervous system, may be almost powerless, may in others be made to 

 contract with far more than ordinary energy. 



8 The molecular restitution of muscle, of brain, and of the nitrogenous 

 tissues generally, which are in continual need of repair, make it essential 

 that nitrogenous substances should to a certain extent be consumed as 

 food. But so far as muscular action is concerned the nitrogenous sub- 

 stances are needed for the repair of the machine, and not, as formerly 

 supposed, as a source of the energy which is to be "transformed through 

 the intervention of the machine. 



