occasioned by a like abuse of the nerve-organs in mental actions of
various kinds? This is not an invariable rule, for, as I may point out
in the way of illustration 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