The skin is the archaeological field of human and prehuman experience, the foundation on which all forms of sensory perception have grown up, and as sexual sensibility is among the most ancient of all forms of sensibility, the sexual instinct is necessarily, in the main, a comparatively slightly modified form of general touch sensibility.  This primitive character of the great region of tactile sensation, its vagueness and diffusion, the comparatively unintellectual as well as unaesthetic nature of the mental conceptions which arise on the tactile basis make it difficult to deal precisely with the psychology of touch.  The very same qualities, however, serve greatly to heighten the emotional intensity of skin sensations.  So that, of all the great sensory fields, the field of touch is at once the least intellectual and the most massively emotional.  These qualities, as well as its intimate and primitive association with the apparatus of tumescence and detumescence, make touch the readiest and most powerful channel by which the sexual sphere may be reached.

In disentangling the phenomena of tactile sensibility ticklishness has been selected for special consideration as a kind of sensation, founded on reflexes developing even before birth, which is very closely related to sexual phenomena.  It is, as it were, a play of tumescence, on which laughter supervenes as a play of detumescence.  It leads on to the more serious phenomena of tumescence, and it tends to die out after adolescence, at the period during which sexual relationships normally begin.  Such a view of ticklishness, as a kind of modesty of the skin, existing merely to be destroyed, need only be regarded as one of its aspects.  Ticklishness certainly arose from a non-sexual starting-point, and may well have protective uses in the young animal.

The readiness with which tactile sensibility takes on a sexual character and forms reflex channels of communication with the sexual sphere proper is illustrated by the existence of certain secondary sexual foci only inferior in sexual excitability to the genital region.  We have seen that the chief of these normal foci are situated in the orificial regions where skin and mucous membrane meet, and that the contact of any two orificial regions between two persons of different sex brought together under favorable conditions is apt, when prolonged, to produce a very intense degree of sexual erethism.  This is a normal phenomenon in so far as it is a part of tumescence, and not a method of obtaining detumescence.  The kiss is a typical example of these contacts, while the nipple is of special interest in this connection, because we are thereby enabled to bring the psychology of lactation into intimate relationship with the psychology of sexual love.