1. We have seen that imperfection and instability of nerve-function may be inherited; that weakness, or impairment, exists with many persons from the time of birth, which affects more or less the functions of the various portions of the nervous system; and that this is seen not only in the mechanical operations of the hand and arm, in the execution of all the more delicate movements of which it may be susceptible, but also in the mental operations. By no possible amount of training can the nerves of the arm in certain persons be educated to do the finer portions of mechanism, any more than can their brains be educated to continuous or concentrated thought on the solution of problems in the higher mathematics; that the broadest diversity as to strength and steadiness of nerve-function exists from the period of childhood to old age: while the nerves and brains of some bear long-continued and large effort without disturbance or failure of function, these portions of the system in others soon manifest the effects of such treatment, and ere long break down; and there may exist any degree of difference, from the manifestations of genius, to those of imbecility. 2. We have also observed that disturbance and imperfection of nerve-function may be produced in all persons by the action of causes operating from without upon the nervous system, and that the results may be similar in character, whether there has been too great or too little exercise of function. It will further be observed, that we have in a definite and somewhat continuous line passed from the production of disturbance and irregularity of the function of simple motion in the hand and arm, up to the more complicated and less understood activities of the hemispheres of the brain. We have seen that there exists at least a similarity in the exhibition of failure in the functions of execution in both cases, whether acquired or inherited. I think there can be little doubt that there exists some such condition of that portion of the brain which is concerned in mental operations, as I have in a somewhat crude manner attempted to illustrate, which is the primary condition in a large number of persons who become insane. Precisely in what it may consist during the earlier stages we may not be able to explain. There doubtless is no change which can be termed _organic_ in either the nerve cell or any other portion of the substance of the brain during the primary stages of this failure in function, but rather an exhaustion or lack in functional power, which after a longer or shorter period _may_ lead to organic change of structure.