The human body carries on several kinds of manufacture, two of which—­the evolution of muscular force or motion, and intellection with all moral activities—­alone concern us here.  We are somewhat apt to antagonize these two sets of functions, and to look upon the latter, or brain-labor, as alone involving the use or abuse of the nervous system.  But every blow on the anvil is as distinctly an act of the nerve centres as are the highest mental processes.  If this be so, how or why is it that excessive muscular exertion—­I mean such as is violent and continued—­does not cause the same appalling effects as may be 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 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.