Of the whole of this group of phenomena, the most typical and the most widespread example is certainly the kiss. We have in the lips a highly sensitive frontier region between skin and mucous membrane, in many respects analogous to the vulvo-vaginal orifice, and reinforcible, moreover, by the active movements of the still more highly sensitive tongue. Close and prolonged contact of these regions, therefore, under conditions favorable to tumescence sets up a powerful current of nervous stimulation. After those contacts in which the sexual regions themselves take a direct part, there is certainly no such channel for directing nervous force into the sexual sphere as the kiss. This is nowhere so well recognized as in France, where a young girl's lips are religiously kept for her lover, to such an extent, indeed, that young girls sometimes come to believe that the whole physical side of love is comprehended in a kiss on the mouth; so highly intelligent a woman as Madam Adam has described the agony she felt as a girl when kissed on the lips by a man, owing to the conviction that she had thereby lost her virtue. Although the lips occupy this highly important position as a secondary sexual focus in the sphere of touch, the kiss is--unlike _cunnilingus_ and _fellatio_--confined to man and, indeed, to a large extent, to civilized man. It is the outcome of a compound evolution which had its beginning outside the sphere of touch, and it would therefore be out of place to deal with the interesting question of its development in this place. It will be discussed elsewhere.[18] There is yet another orificial frontier region which is a highly important tactile sexual focus: the nipple. The breasts raise, indeed, several interesting questions in their intimate connection with the sexual sphere and it may be worth while to consider them at this point. The breasts have from the present point of view this special significance among the sexual centres that they primarily exist, not for the contact of the lover, but the contact of the child. This is doubtless, indeed, the fundamental fact on which all the touch contacts we are here concerned with have grown up. The sexual sensitivity of the lover's lips to orificial contacts has been developed from the sensitivity of the infant's lips to contact with his mother's nipple. It is on the ground of that evolution that we are bound to consider here the precise position of the breasts as a sexual centre.