on hereafter, the centres which originate or
evolve muscular power do sometimes suffer from undue taxation; but it is
certainly true that when this happens, the evil result is rarely as
severe or as lasting as when it is the organs of mental power that have
suffered.

In either form of work, physical or mental, the will acts to start the
needed processes, and afterwards is chiefly regulative. In the case of
bodily labor, the spinal nerve-centres are most largely called into
action. Where mental or moral processes are involved, the active organs
lie within the cranium. As I said just now, when we talk of an overtaxed
nervous system it is usually the brain we refer to, and not the spine;
and the question therefore arises, Why is it that an excess of physical
labor is better borne than a like excess of mental labor? The simple
answer is, that mental overwork is harder, because as a rule it is
closet or counting-room or at least in-door work--sedentary, in a word.
The man who is intensely using his brain is not collaterally employing
any other organs, and the more intense his application the less
locomotive does he become. On the other hand, however a man abuses his
powers of motion in the way of work, he is at all events encouraging
that collateral functional activity which mental labor discourages: he
is quickening the heart, driving the blood through unused channels,
hastening the breathing and increasing the secretions of the skin--all
excellent results, and, even if excessive, better than a too incomplete
use of these functions.

But there is more than this in the question. We do not know as yet what
is the cost in expended material of mental acts as compared with motor
manifestations, and here, therefore, are at fault; because, although it
seems so much slighter a thing to think a little than to hit out with
the power of an athlete, it may prove that the expenditure of nerve
material is in the former case greater than in the latter.

When a man uses his muscles, after a time comes the feeling called
fatigue--a sensation always referred to the muscles, and due most
probably to the deposit in the tissues of certain substances formed
during motor activity. Warned by this weariness, the man takes rest--may
indeed be forced to do so; but, unless I am mistaken, he who is
intensely using the brain does not feel in the common use of it any
sensation referable to the organ itself which warns him that he has
taxed it enough.