=Influence of Repeated Sexual Excitations.=--The artificial and varied repetition of sexual excitation, by means of objects which provoke it, increases the sexual appetite. This cannot be doubted, for the law of exercise is a general truth in the physiology of the nervous system. This law, which is also called the law of training, shows that every kind of nervous activity is increased by exercise. A man becomes a glutton by accustoming himself to eat too much, a good walker by exercising his legs. The habit of wearing fine clothes or of washing in cold water causes these things to become a necessity. By continually occupying ourselves with a certain thing, we take a liking for it and often become virtuosos. By always thinking of a disease we are led to imagine that we suffer from it. A melody too often repeated often becomes automatic and we whistle or hum it unconsciously. Inversely, inactivity weakens the effect of irritations which correspond to it. By neglecting certain activities or the provocation of certain sensations, these diminish in intensity, and we cease more and more to be affected by them. We become idle when we are inactive, for the cerebral resistance accumulates, and idleness renders the renewal of the corresponding activity more difficult. It is not surprising, therefore, to find this law in the phenomena of the sexual appetite, which diminishes with abstinence and increases with repeated excitation and satisfaction. However, another force, that of the accumulation of semen in the seminal vesicles, associated with an old natural inherited instinct, often counteracts the law of exercise of the nervous system, as the empty stomach excites the instinct of nutrition. But, however imperious the hunger, and however indispensable its satisfaction for the maintenance of life, this does not impair the truth of the old saying, "Appetite comes by eating." The exaggerated desire for sleep experienced by idle people is an analogous phenomenon. Although sufficient sleep is a necessity for healthy and productive cerebral activity, an exaggerated desire for sleep may be artificially developed.