dictum omne animal post coitum triste may not be exact, but it is certain that the effect of coitus on the organism is far more profound than that produced by the far more extensive evacuation of the bladder or bowels.  Again, this definition leaves unexplained all those elaborate preliminaries which, both in man and the lower animals, precede the sexual act, preliminaries which in civilized human beings sometimes themselves constitute a partial satisfaction to the sexual impulse.  It must also be observed that, unlike the ordinary excretions, this discharge of the sexual glands is not always, or in every person, necessary at all.  Moreover, the theory of evacuation at once becomes hopelessly inadequate when we apply it to women; no one will venture to claim that an adequate psychological explanation of the sexual impulse in a woman is to be found in the desire to expel a little bland mucus from the minute glands of the genital tract.  We must undoubtedly reject this view of the sexual impulse.  It has a certain element of truth and it permits an instructive and helpful analogy; but that is all.  The sexual act presents many characters which are absent in an ordinary act of evacuation, and, on the other hand, it lacks the special characteristic of the evacuation proper, the elimination of waste material; the seminal fluid is not a waste material, and its retention is, to some extent perhaps, rather an advantage than a disadvantage to the organism.

Eduard von Hartmann long since remarked that the satisfaction of what we call the sexual instinct through an act carried out with a person of the opposite sex is a very wonderful phenomenon.  It cannot be said, however, that the conception of the sexual act as a simple process of evacuation does anything to explain the wonder.  We are, at most, in the same position as regards the stilling of normal sexual desire as we should be as regards the emptying of the bladder, supposing it were very difficult for either sex to effect this satisfactorily without the aid of a portion of the body of a person of the other sex acting as a catheter.  In such a case our thoughts and ideals would center around persons of opposite sex, and we should court their attention and help precisely as we do now in the case of our sexual needs.  Some such relationship does actually exist in the case of the suckling mother and her infant.  The mother is indebted to the child for the pleasurable relief of her distended breasts; and, while in civilization more subtle pleasures and intelligent reflection render this massive physical satisfaction comparatively unessential to the act of suckling, in more primitive conditions and among animals the need of this pleasurable physical satisfaction is a real bond between the mother and her offspring.  The analogy is indeed very close:  the erectile nipple corresponds to the erectile penis, the eager watery mouth of the infant to the moist and throbbing vagina, the vitally albuminous milk to the vitally albuminous semen.[17] The complete mutual satisfaction, physical and psychic, of mother and child, in the transfer from one to the other of a precious organized fluid, is the one true physiological analogy to the relationship of a man and a woman at the climax of the sexual act.  Even this close analogy, however, fails to cover all the facts of the sexual life.