Under pathological conditions the anus may become an “erogenous” area. Thus anal auto-masturbation, which seems to be only too frequent, and passive pederasty would be explained. (Comp. Gamier, “Anomalies sexuelles,” Paris, p. 514; F. Moll, “Conträre Sexualempfindung,” p. 163.)
The psycho-physiological process comprehended in the idea of sexual instinct is composed of (1) concepts awakened centrally or peripherally; (2) the pleasurable feelings associated with them.
The longing for sexual satisfaction (libido sexualis) arises from them. This desire grows stronger constantly, in proportion as the excitation of the cerebral sphere accentuates the feeling of pleasure by appropriate concepts and activity of the imagination; and the pleasurable sensations are increased to lustful feeling by excitation of the erection centre and the consequent hyperæmia of the genitals (entrance of liquor prostaticus into the urethra, etc.).
If circumstances are favorable for the performance of the sexual act satisfactorily, the constantly-increasing desire is complied with; if, however, conditions are unfavorable, inhibitory concepts occur, overcome the sexual longing, and prevent the sexual act.
To civilized man cultivation of a readiness with ideas which inhibit sexual desire is necessary and distinctive. The moral freedom of the individual, and the decision whether, under certain circumstances, excess, and even crime, be committed or not, depend, on the one hand, upon the strength of the instinctive concepts and the accompanying organic sensations; on the other, upon the power of the inhibitory concepts. Constitution and, especially, organic influences have a marked effect upon the instinctive impulses; education and cultivation of self-control have a decisive influence on the opposing concepts.
The exciting and inhibitory powers are variable quantities. Over-indulgence in alcohol in this respect is very fatal, since it awakens and increases libido sexualis, while at the same time it reduces moral resistance.
The Act of Cohabitation.[[27]]
The essential condition for the man is sufficient erection. Anjel (Arch. für Psych., viii, H. 2) calls attention to the fact that in sexual excitement the erection centre is not alone influenced,—the nervous excitement is distributed to the entire vasomotor system of nerves. The proof of this is the turgescence of the organs in the sexual act, injection of the conjunctiva, prominence of the eyes, dilatation of the pupils, and cardiac palpitation (resulting from paralysis of the vasomotor nerves of the heart, which arise from the cervical sympathetic, and the consequent dilatation of the cardiac arteries, and the increased stimulation of the cardiac ganglia induced by the consequent hyperæmia of the cardiac walls). The sexual act is accompanied by a pleasurable feeling, which, in the male, is conditioned by the passage of semen through the ductus ejaculatorii to the urethra, caused by sensory stimulation of the genitals. The pleasurable sensation occurs earlier in the male than in the female, grows rapidly in intensity until the moment of commencement of ejaculation, reaching its height in the instant of free emission, and disappears quickly post ejaculationem.
In the female the pleasurable feeling occurs later and comes on more slowly, and generally outlasts the act of ejaculation.
The distinctive event in coitus is ejaculation. This function is dependent on a centre (genito-spinal), which Budge has shown to be situated at the level of the fourth lumbar vertebra. It is a reflex centre. The stimulus that excites it is the ejection of sperma from the vesiculæ seminales into the pars membranacea urethræ, which follows reflexly from stimulation of the glans penis. As soon as the collection of semen, with ever-increasing pleasurable sensation, has reached a sufficient amount to be effectual as a stimulus of the ejaculation-centre, the centre acts. The reflex motor path lies in the fourth and fifth lumbar nerves. The action consists of a convulsive excitation of the bulbo-cavernosus muscle (innervated by the third and fourth sacral nerves), which forces the semen out.