The Secondary Sexual Skin Centres--Orificial Contacts--Cunnilingus and Fellatio--The Kiss--The Nipples--The Sympathy of the Breasts with the Primary Sexual Centres--This Connection Operative both through the Nerves and through the Blood--The Influence of Lactation on the Sexual Centres--Suckling and Sexual Emotion--The Significance of the Association between Suckling and Sexual Emotion--This Association as a Cause of Sexual Perversity. We have seen that the skin generally has a high degree of sensibility, which frequently tends to be in more or less definite association with the sexual centres. We have seen also that the central and specific sexual sensation, the sexual embrace itself, is, in large measure, a specialized kind of skin reflex. Between the generalized skin sensations and the great primary sexual centre of sensation there are certain secondary sexual centres which, on account of their importance, may here be briefly considered. These secondary centres have in common the fact that they always involve the entrances and the exits of the body--the regions, that is, where skin merges into mucous membrane, and where, in the course of evolution, tactile sensibility has become highly refined. It may, indeed, be said generally of these frontier regions of the body that their contact with the same or a similar frontier region in another person of opposite sex, under conditions otherwise favorable to tumescence, will tend to produce a minimum and even sometimes a maximum degree of sexual excitation. Contact of these regions with each other or with the sexual region itself so closely simulates the central sexual reflex that channels are set up for the same nervous energy and secondary sexual centres are constituted. It is important to remember that the phenomena we are here concerned with are essentially normal. Many of them are commonly spoken of as perversions. In so far, however, as they are aids to tumescence they must be regarded as coming within the range of normal variation. They may be considered unæsthetic, but that is another matter. It has, moreover, to be remembered that æsthetic values are changed under the influence of sexual emotion; from the lover's point of view many things are beautiful which are unbeautiful from the point of view of him who is not a lover, and the greater the degree to which the lover is swayed by his passion the greater the extent to which his normal æsthetic standard is liable to be modified. A broad consideration of the phenomena among civilized and uncivilized peoples amply suffices to show the fallacy of the tendency, so common among unscientific writers on these subjects, to introduce normal æsthetic standards into the sexual sphere. From the normal standpoint of ordinary daily life, indeed, the whole process of sex is unæsthetic, except the earlier stages of tumescence.[17]