2. All are more or less familiar with that condition which is usually termed the _writer's cramp_. Sometimes persons of a peculiar nervous organization, who have been accustomed to spend many hours a day in the mechanical process of writing, experience such a loss of nervous energy that the arm fails to respond to the will power. There results spasmodic and irregular movements in the arm, and more especially in the fingers, indicating that some morbid condition of the median nerve has been produced. If the case is neglected, and the exciting causes continue in operation, after awhile the nerve will become less and less responsive to the call of the will, and the arm may become seriously affected, after a longer or shorter period of more or less pain. In this case, as in the former one, there may exist all degrees of impairment of nerve function, from that of slight unsteadiness or instability, up to entire failure. 3. The condition of the nervous system in chorea presents another illustration. Between the ages of ten and fifteen years, in some children, there may occur such changes in the condition of the nervous element, from the altered character of the blood, as to render it more or less unstable in some portion. One arm or one leg, or a hand, or some of the muscles of the face or neck, cannot be kept long at rest by any effort which can be exerted by the individual, and this may be so slight as scarcely to attract attention, or it may be so great as to cause extreme suffering for a long time, and ultimately destroy life. Now the primary condition in the above cases is one of _instability of nerve function_. Such a change has occurred in the elemental tissue of the nerve, as to injure its power of activity so far as it is under the control of the will. The nerve has been stimulated to over-activity, or its energy impaired by other causes arising within the system itself. These illustrations have related to those portions of the nervous system which are more immediately connected with motion, and which, consequently, are located in the muscular system. If, now, we pass from these portions of the nervous system up to its grand centre, or that part of it whose function is connected with mental operations, we shall find similar effects resulting from like causes.