=Masturbation.=--In the waking state the unsatisfied sexual appetite may produce such excitation that the boy applies friction to the glans penis, which cause voluptuous sensations. As soon as he has made this discovery he repeats the act and provokes ejaculation of semen artificially. Thus arises the bad habit of masturbation or onanism, a habit which is both depressing and exhausting, which takes an increasing hold on those who practice it. Although from the purely mechanical point of view masturbation causes a more normal ejaculation than nocturnal emissions, which are often interrupted by awakening and the vanishing of the dream which produced them, it has a much more harmful effect, by its frequency and especially by its depressing action on sentiment and will. We shall return to this subject in Chapter VIII. The accumulation of semen in the seminal vesicles strongly excites the sexual appetite of man, and he is momentarily satisfied by their evacuation. But we shall soon see that this purely organic or mechanical excitation, which seems at first to be only adapted for natural wants, does not in man play the principal role. We can easily understand that it cannot be the principal moving power of the sexual act. In fact, for any of the animals in which copulation occurs, the possibility of accomplishing this is not connected solely with the accumulation of semen, for it depends on obtaining a female. It is necessary, therefore, for the accumulated semen to wait, and for the perception of the female by the aid of the senses to excite the male to coitus. =External Signs of the Sexual Appetite.=--Like every other desire the sexual appetite betrays itself by the physionomy. This consists in the play of cerebral activity, that is the thoughts, sentiments and resolutions, on the muscles by means of motor nerves and nerve centers. It is not limited to the face but extends to the whole body. The abdomen, the hands and even the feet have their physionomy; that of the muscles of the face and eyes is, however, the most active and most expressive. Sexual desire betrays itself in looks, by the expression of the face and by certain movements in the presence of the female sex. Men differ greatly in the way in which they betray or hide their sentiments and thoughts by the play of their muscles, so that the inner self is not always reflected without. Moreover, the expression of sexual desire by the play of the physionomy may be confounded with that of other sentiments, so that one who appears libidinous is not always so in reality, and inversely.