456 COMPARATIVE PHYSIOLOGY. 



vagi leads to degeneration of the cardiac structure. We now 

 know that this nerve contains fibers which have a diverse 

 action on the metabolism of the heart, and that, according 

 as the one or the other set is stimulated, so does the electri- 

 cal condition vary ; and everywhere, so far as known, a differ- 

 ence in electrical conditions seems to he associated with a 

 difference in metabolism, which may be one of degree only, 

 perhaps, in many instances — still a difference. The facts as 

 brought to light by experimental stimulation harmonize with 

 the facts of degeneration of the cardiac tissue on section of the 

 vagi ; but this is only clear on the view we are now presenting, 

 that the action of the nervous system is not only tiniversal, 

 but that it is constant ; that function is not an isolated and 

 independent condition of an organ or tissue, but a part of a 

 long series of metabolic changes. It is true that one or more 

 of such changes may be arrested, just as all of them may go 

 on at a less rate, if this actual outpouring of pancreatic secre- 

 tion is not constant ; but secretion is not summed up in dis- 

 charge merely ; and, on the other hand, it would seem that in 

 some animals the granules of the digestive glands are being 

 renewed while they are being used up, in secreting cells. The 

 processes may be simultaneous or successive. Nor do we wish 

 to imply that the nervous system merely holds in check or in 

 a very general sense co-ordinates processes that go on unorigi- 

 nated by it. We think the facts warrant the view that they are 

 in the highest mammals either directly (mostly) or iadirectly 

 originated by it, that they would not take place in the absence 

 of this constant nervous influence. The facts of common ob- 

 servation, as well as the facts of disease, point lq the strongest 

 way to such a conclusion. Every one has observed the in- 

 fluence, on not one but many functions of the animal, we might 

 say the entire metabolism, of depressing or exalting emotions. 

 The failure of appetite and loss of flesh under the influence of 

 grief or worry, tell a plain story. Such broad facts are of infi- 

 nitely more value in .settling such a question as that now dis- 

 cussed than any single experiment. The best test of any theory- 

 is the extent to which it will explain the whole round of facts. 

 Take another instance of the influence over metabolism of the 

 nervous system. 



Every trainer of race-horses knows that he may overwork 

 his beast— i. e., he may use his muscles so much as to disturb 

 the balance of his powers somewhere — ^very frequently his di- 



