If a person experiences a sudden mental shock, occasioned, it may be, by sad intelligence of some kind, or if he passes suddenly through some great personal peril, or if he has made unusual mental effort in some abstruse study, or in the conduct of business, which has been long protracted, he becomes conscious of what is called mental fatigue. The brain refuses to respond to any calls which may be made upon it for further action, or partially refuses; it becomes confused and bewildered, and unsteady in its action; it is difficult to force it to further application, or to connected lines of thought, and if, by a strong effort of the will, this is done during a short time, it is inclined soon to wander from the subject, and there exists a more or less distinctly recognized sensation of pain as well as inability to regulate its action. In this case, also, as in the others, there may result any degree of effect, from a sensation of fatigue up to mental spasm, and the amount will depend on the condition of the brain during the time the effect continues, and its intensity. The two conditions of instability and pain may be less marked in the case of the brain than in the nerves of motion, but there can be little doubt there has resulted something of a similar character in both. Precisely in what this change which has taken place consists, may not be easily determined. There may have occurred actual lesion of nerve element, or only impairment or exhaustion of functional power without lesion; probably the latter condition only, in the primary stages of the operation. Certain portions of the brain have been over-stimulated in action, and consequently their power of normal activity and stability is impaired, as was seen in the cases of nerves distributed to the muscles of the arm and other portions of the body. II. Again, if the arm or hand of a vigorous person, which has long been trained to make either large efforts, or the more delicate movements, or again, to make but little effort in any direction, be placed in splints, or be suspended in a sling so that there is little or no activity for a few weeks, more or less, there results a failure of nerve-function; and there may be any degree of impairment, from that of slight degree, such as may be evinced by spasm, to that of paralysis. The same is true in reference to any portion of the nervous system which can be placed in a state of inactivity. A few sentences of recapitulation in our process of reasoning will now be in order.