THE METABOLISM OF THE BODY. “475 
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 indirectly 
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 in the strongest 
way to such aconclusion. Every one has experienced the in- 
fluence, on not one but many functions of the body, we might 
say the entire metabolism, of depressing or exalting emotions, 
The failure of appetite and loss of flesh and mental power under 
the influence of grief or worry, tell a plain story. Such broad 
facts are of infinitely more value in settling such a question as 
that now discussed 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 athlete knows that he may overtrain—i. e., he may 
use his muscles so much as to disturb the balance of his powers 
somewhere—very frequently his digestion; but often there 
seems to be a general break—the whole metabolism of the body 
seems to be out of gear. If we assume a constant nervous influ- 
ence over the metabolic processes, this is comprehensible. The 
centers can produce only so much of what we may call nerv- 
ous force, using the term in the sense of directive power; and 
if this be unduly diverted to the muscles, other parts must 
suffer. The same holds of excessive mental application. 
On this view also the value of rest or change of occupation 
becomes clear. The nervous centers are not without some re- 
semblance to a battery; at most, the latter can generate only a 
definite quantity of electricity, and, if a portion of this be di- 
verted along one conductor, less must remain to pass by any 
other. 
It is of practical importance to recognize that under great 
excitement unusual discharges from a nerve-center may lead 
to unwonted functional activity: thus, under the stimulus of 
