Those peripheral nerves which provoke sexual excitation are especially the nerves of the glans. This possesses a skin or mucous membrane which is extremely delicate and is protected against external irritation by a fold of skin called the prepuce, or foreskin. The prepuce is often too narrow so that it cannot be withdrawn behind the glans. It then forms a pocket in which sebaceous matter, semen, urine, etc., accumulate and decompose. This anomaly, called phimosis, does not exist among the Jews owing to circumcision, or the removal of the prepuce in the newly born, which forms part of their religious rites. Hygienic considerations sometimes oblige us to perform this operation in others. The bad habit of masturbation, so common in boys, is often provoked by phimosis, and shows that simple mechanical irritation of the glans, due here to secretions contained in the prepuce, may lead to ejaculation of semen as well as to erection.
We have seen above that the male and female germinal glands arise from the same primitive organ in the embryo. If the embryo becomes male, this organ is transformed into the two testicles which descend gradually in the canal of the groin and become placed in the scrotum. If it becomes female, the two sexual glands remain in the abdominal cavity and are transformed into ovaries.
The Genital Organs of Woman.—The organs described in Chapter II (Figs. 18 and 19), constitute the internal and more important part of the female sexual apparatus. In women, the urethra opens externally on its own account. It is much shorter and wider than in men. At its external extremity is a small cavernous body called the clitoris, which corresponds embryologically to the penis in man, and chiefly to the glans. Like the latter it is specialized for sexual irritation and possesses very sensitive nerves. The opening of the female urethra is situated in front of the vulva directly under the pubic bone, at the same place as the root of the male penis. From this point, on each side of the middle line, extend two longitudinal folds, one external covered with skin and called the larger lip of the vulva (Fig. 18, labia majora), the other internal, hidden under the first, called the lesser lip of the vulva (labia minora), and covered with thin mucous membrane. Between the two lesser lips is the sexual aperture, which, with the labia majora and minora is called the vulva. This opening is distinct from that of the urethra, and leads to the internal cavity or vagina (Fig. 18). The vagina is about ten to twelve centimeters long (2 to 2-½ inches) and terminates in a cul-de-sac which surrounds the vaginal portion of the womb, of which we have spoken above.
In virgins the entrance to the vagina is more or less closed by a delicate transverse membrane called the hymen, which is only perforated by a narrow opening. At the first coitus the hymen is torn, causing a certain amount of pain and bleeding. The walls of the vagina are thrown into transverse folds, which render them somewhat rough. The remains of the hymen torn by the first coitus afterward form behind the vulva small excrescences named carunculæ myrtiformes.
In the first chapter we have spoken of the changes undergone by the fecundated ovule till it becomes the embryo and then the infant. It remains to speak of the mechanism of expulsion of the ovule and of its fecundation, as well as the changes in the womb which result from these phenomena.
Menstruation.—About every four weeks, one or two ovules (rarely more) mature and are discharged into the Fallopian tubes, down which they pass by the movement of the vibratile cilia of the mucous membrane, to the uterus, to the walls of which they become attached if they have been fecundated on the way (Fig. 18). Fecundation or conjugation takes place most often in the Fallopian tube, sometimes in the uterus. The maturation and expulsion of the ovule are generally accompanied in women by a nervous phenomenon closely related to erection in man. The mucous membrane of the cavity of the uterus is very rich in blood vessels which become dilated and gorged with blood under the inhibitory influence of certain nerve centers. As the mucous membrane is very thin, the result is otherwise than in man; the blood transudes through the mucous membrane and flows away. This is called menstruation ("courses" or monthly periods). The object of this is, no doubt, to prepare the mucous membrane of the womb for the fixation of the fecundated egg which will become grafted on its surface. The courses in women generally last three or four days, but are often very irregular. It is necessary to point out that they do not depend on ovulation (expulsion of the egg). The two phenomena may take place independently of each other, for menstruation in itself depends only on nervous irritation, which may be provoked or averted by hypnotic suggestion, for example.
Moreover, there are women who never menstruate and who, in spite of this, not only regularly discharge ovules but may be fecundated and become pregnant. Usually, however, the two phenomena are associated by nervous reflexes, so that menstruation takes place first and then the ovule commences its migration.
The Mechanism of Coitus.—Copulation, or coitus, takes place as follows: After a certain degree of excitation, both mental and sensory, the male introduces the erect and stiffened penis into the vagina. In the case of advanced pregnancy he should place himself behind, so as to avoid injuring the unborn child. Rhythmic movements of the two individuals, especially of the man, gradually increase the excitation of the mucous membrane or skin of the genital organs of each party, till voluptuous sensations, arising chiefly in the glans penis and clitoris, spread to the whole nervous system and the entire body, constituting what is called the venereal orgasm, and terminating in the man by the ejaculation of semen.
The localizations of irritability in woman are multiple, and to the clitoris must be added the nipples, the vulva, and even, it is said, the neck of the womb. In man the parts round the anus may also, besides the glans penis, form an excitable region. At the acme of erection the glans is turgid, and is applied directly against the neck of the womb (Fig. 18). In this way the sperm is ejaculated directly against the neck of the womb.
In the woman an analogous phenomenon takes place; the clitoris becomes turgid and the mild and repeated friction of the mucous membranes, together with contact on other sensitive parts, produces a voluptuous sensation as in the man. Through nervous association, the repeated excitation determines secretion from certain glands of the vagina which lubricate the vulva (glands of Bartholin). At the maximum point of voluptuous feeling the woman experiences something analogous to the venereal orgasm of the man. There is thus manifested in the two sexes an intense and reciprocal desire of penetration one by the other, a desire which powerfully favors fecundation. In the woman as in the man the end of the orgasm is followed by an agreeable relaxation which invites sleep.