The importance of the lips as a normal erogenous zone is shown by the experiments of Gualino.  He applied a thread, folded on itself several times, to the lips, thus stimulating them in a simple mechanical manner.  Of 20 women, between the ages of 18 and 35, only 8 felt this as a merely mechanical operation, 4 felt a vaguely erotic element in the proceeding, 3 experienced a desire for coitus and in 5 there was actual sexual excitement with emission of mucus.  Of 25 men, between the ages of 20 and 30, in 15 all sexual feeling was absent, in 7 erotic ideas were suggested with congestion of the sexual organs without erection, and in 3 there was the beginning of erection.  It should be added that both the women and the men in whom this sexual reflex was more especially marked were of somewhat nervous temperament; in such persons erotic reactions of all kinds generally occur most easily. (Gualino, “Il Rifflesso Sessuale nell’ eccitamento alle labbre,” Archivio di Psichiatria, 1904, p. 341.)

As tumescence, under the influence of sensory stimulation, proceeds toward the climax when it gives place to detumescence, the physical phenomena become more and more acutely localized in the sexual organs.  The process which was at first predominantly nervous and psychic now becomes more prominently vascular.  The ancient sexual relationship of the skin asserts itself; there is marked surface congestion showing itself in various ways.  The face tends to become red, and exactly the same phenomenon is taking place in the genital organs; “an erection,” it has been said, “is a blushing of the penis.”  The difference is that in the genital organs this heightened vascularity has a definite and specific function to accomplish—­the erection of the male organ which fits it to enter the female parts—­and that consequently there has been developed in the penis that special kind of vascular mechanism, consisting of veins in connective tissue with unstriped muscular fibers, termed erectile tissue.[100]

It is not only the man who is supplied with erectile tissue which in the process of tumescence becomes congested and swollen.  The woman also, in the corresponding external genital region, is likewise supplied with erectile tissue now also charged with blood, and exhibits the same changes as have taken place in her partner, though less conspicuously visible.  In the anthropoid apes, as the gorilla, the large clitoris and the nymphae become prominent in sexual excitement, but the less development of the clitoris in women, together with the specifically human evolution of the mons veneris and larger lips, renders this sexual turgescence practically invisible, though it is perceptible to touch in an increased degree of spongy and elastic tension.  The whole feminine genital canal, including the uterus, indeed, is richly supplied with blood-vessels, and is capable during sexual excitement of a very high degree of turgescence, a kind of erection.