It is a well-known physiological fact, that nerves which are inactive lose their excitability and degenerate. This holds good for motor nerves, and we can also note degeneration of muscles and organs which are not used. Thus, the nerves of an extremity, after amputation, undergo a process of degeneration. The lower limbs of people affected with infantile paralysis, or of persons obliged to remain in bed for a long time, show atrophic changes. Hence we can easily see the necessity for exercise, which increases the blood-supply to the muscles and nerves. Work of any kind, even mental work alone, is a means of preventing precocious senility; if manual exercise is combined with it, it is still more efficacious. Plutarch, in his “De educatione puerorum,” mentions that a certain amount of work improves the mind, but excess of work is prejudicial. We see the best illustration of this fact in American business men. There are no men in any country who do such an amount of work, and at the same time take so little recreation or exercise. They sit in their offices till dusk, with a few minutes’ interval for a hasty meal, consisting mainly of meat that has often been kept in cold storage for a long time, after which business goes on again, at high pressure, until the evening. Then, instead of walking home and taking exercise, they take a car or carriage to their house or club, and pass the evening in smoking and drinking, sometimes to excess. Day after day the same killing of body and nerves goes on till these people look old long before fifty, if, indeed, they reach that age. Arteriosclerosis, diabetes, gout, and obesity find many victims among such men. It is sad to think how many thousands of these splendid people, full of genius and talent, could be saved for their native country if only they had been taught in their youth the most elementary rules of hygienics. What joy does money afford without health? Some of the most powerful agencies in producing old age are frequent mental emotions, especially sorrow and grief. It is a common fact that after such emotions people soon look older. To mention an example, there is positive evidence that young persons, after a mental shock, have become gray in a single night, thus developing abruptly one of the most typical symptoms of old age. That mental emotions, especially anger, grief, sorrow, fright, anxiety, etc., are very harmful to glands with an internal secretion, is shown by a series of clinical observations. Sajous has in fact termed _sensorium commune_, i.e., the center which receives all shocks, the governing center of the ductless glands, located in the pituitary body. With mental emotion there is often disturbance of a function, interference with which is very liable to hasten the onset of old age, and this is sleep.