832 INFLUENCE OF NERYES ON NUTRITION. [BOOK n. 



elusion that the influence of the nervous system is not limited to 

 such changes of the muscles as belong to the production of con- 

 tractions or the generation of heat, but bears on the whole 

 nutrition of the muscle. Similar considerations lead us also to 

 conclude that the influence of the nervous system bears on the 

 whole nutrition of the glands, of the blood vessels, of the skin 

 and the connective tissues in general, in fact of nearly the whole 

 body. 



Such an influence of the nervous system has often been spoken 

 as 'trophic;' and the term has often been used as if the growth 

 and nourishment of a tissue were the result of nervous action, or 

 at all events could not be complete without the intervention of 

 nervous impulses. Hence, in this view, the consequences following 

 upon section of the fifth nerve as regarded as due to the falling 

 away of ' trophic ' influences. Such a view has however no sound 

 basis. All biological studies teach us that the growth, repair, and 

 reproduction of living substance may go on quite independently of 

 any nervous system. The white blood corpuscles go through 

 their cycles unmoved by nervous impulses, and the nutrition of 

 the nervous system itself cannot be dependent on the action of 

 that system on itself. All that is really needed to explain these 

 phenomena is an acceptance of the view that a nervous impulse 

 may modify the metabolic events of other tissues than muscles and 

 glands, and may modify them in various ways; and further that 

 the nutrition of each tissue is in the complex animal body so 

 arranged to meet the constantly recurring influences brought to 

 bear on it by the nervous system, that, when those influences 

 are permanently withdrawn, it is thrown out of equilibrium ; its 

 molecular processes, so to speak, then run loose, since the bit has 

 been removed from their mouths. And as our knowledge of 

 metabolic processes on the one hand and of the actions of the 

 nervous system on the other hand increases, these suppositions 

 become more and more reasonable. 



