While the clitoris is anatomically analogous to the penis, its actual mechanism under the stress of sexual excitement is somewhat different. As Liétaud long since pointed out, it cannot rise freely in erection as the penis can; it is apparently bound down by its prepuce and its frenulum. Waldeyer, in his book on the pelvis, states more precisely that, unlike the penis, when erect it retains its angle, only this becomes somewhat rounded so that the organ is to some slight extent lifted and protruded. Waldeyer considered that the clitoris was thus perfectly fitted to fulfill its part as the recipient of erotic stimulation from friction by the penis. Adler, however, has pointed out with considerable justice, that this is not altogether the case. The clitoris was developed in mammals who practiced the posterior mode of coitus; in this position the clitoris was beneath the penis, which was thus easily able in coitus to press it against the pubic bone close beneath which it is situated, and thus impart the compression and friction which the feminine organ craves. But in the human anterior mode of coitus it is not necessarily brought into close contact with the penis during the act of coitus, and thus fails to receive powerful stimulation. Its restricted position, which is an advantage in posterior coitus, is a disadvantage in anterior coitus. Adler observes that it thus comes about that the human method of coitus, while by bringing breast to breast and face to face it has added a new dignity and refinement, a fresh source of enjoyment, to the embrace of the sexes, has not been an unmixed advantage to woman, for while man has lost nothing by the change, woman has now to contend with an increased difficulty in attaining an adequate amount of pressure on that "electric button" which normally sets the whole mechanism in operation.[88]