The sexual function is regulated by two nerve-centres, one of which is cerebral and the other spinal. The cerebral centre is the seat of the feelings and appetite which prompt the individual to seek sexual congress.

The spinal centre regulates the condition of erection in the sexual organs which is necessary for coitus. These two centres may act independently of each other, e.g. when the spinal cord is cut, but in the normal condition they naturally influence each other, excitement of the spinal centre re-acting on the cerebral centre so as to awaken sexual feelings, and excitement of the cerebral centre re-acting on the spinal so as to produce erection of the genital organs.

Erection is due partly to dilatation of the arteries in the erectile tissues of the genital organs, and partly to compression of the efferent veins. The blood being thus allowed to flow freely into the organs, and prevented from flowing out, distends them so as to render them turgid and more or less rigid. During the orgasm the turgidity is increased by partial stoppage of respiration, which, by rendering the blood venous and thus stimulating the vaso-motor centre, tends to raise the blood-pressure in the body generally, and in the erectile tissues particularly.

Dilatation of the arteries in the genital organs and consequent erection occurs on stimulation, either of the genital centre in the lumbar spinal cord or of the vaso-dilating nerves (nervi erigentes) which pass from it to the genital organs and end in a ganglionic plexus surrounding the arteries.

The lumbar genital centre may be excited either reflexly by stimulation of the sensory nerves of the genital organs and adjoining parts, or by psychical stimuli transmitted to it from the brain.

The exact seat of the cerebral genital centre has not been determined, but Eckhard has found that irritation of the crura cerebri can produce similar effects to stimulation of the nervi erigentes.

The cerebral genital centre may be stimulated and sexual feelings aroused by impressions made on the nerves of special or general sense, e.g. on the eye, ear, nose, on the mammae, and general surface of the body, the genital organs and parts adjoining, as the bladder, prostate, and nates. Thus, sexual excitement may occur in consequence of the sight of persons or pictures, the reading or hearing of licentious stories, or of irritation of the surface of the body either by gentle friction or by pruriginous irritation due to irritating articles of clothing, parasites, or skin diseases. Distension of the bladder has a somewhat similar effect, and the irritation consequent on an enlarged prostate is probably, in part at least, the cause of the great sexual excitement which sometimes occurs in elderly men. A very acid condition of the urine, such as is found in some gouty patients, may possibly have a similar action. Chlorate and nitrate of potassium administered internally are said by Jacobil to render the urine so irritating and to produce such sexual excitement as to lead to onanism. Ascarides in the rectum may cause excitement of the cerebral genital centre and give rise to nocturnal emissions as well as possibly to diurnal excitement, and in females they may cause even greater irritation by passing into the vagina. Irritation of the rectum from the presence of piles or fissure may also give rise to such great sexual excitement as to induce onanism or nymphomania. Faeces in the rectum, and perhaps in the colon, may also cause sexual excitement in some persons or increase it when present.

Fig. 154.   Diagram to illustrate the action of aphrodisiacs and anaphrodisiacs. The darkly shaded spot indicates the genital centre in the brain, and the lighter spot the spinal centre in the lumbar portion of the cord.

Fig. 154. - Diagram to illustrate the action of aphrodisiacs and anaphrodisiacs. The darkly-shaded spot indicates the genital centre in the brain, and the lighter spot the spinal centre in the lumbar portion of the cord. The direction in which impulses are conveyed along the nerves are indicated by the arrows. The nerves from the general surface have been represented as going to the cerebral centre, and acting through it on the spinal centre. It is probable, however, that several of them pass directly to the spinal centre, as represented in the case of the nerves of the nates.

1 Medical Times and Gazette, 1876, vol. i. p. 177.

Such sources of local irritation may sometimes be insufficient to affect the cerebral centre during waking hours, when the attention is otherwise engaged, but may do so powerfully during sleep, or when the cerebral functions are disturbed by cannabis indica, and they may then produce erotic dreams or seminal emissions.

The lumbar centre is most readily excited by mechanical stimulation of the genital organs, but it may be also powerfully stimulated from the mucous membranes of the urinary passages, as is seen in the painful priapism which occurs in poisoning by cantharides.

Fig. 155.   Diagram to illustrate the effects on the genital centres of irritation of the stomach or intestine by flatulence, acrid matters, or faecal accumulations.

Fig. 155. - Diagram to illustrate the effects on the genital centres of irritation of the stomach or intestine by flatulence, acrid matters, or faecal accumulations.

Stimulation of the lumbar centre without stimulation of the cerebral centre may occur from the presence of faeces in the rectum and perhaps in the colon, so as to give rise to seminal emissions during sleep unaccompanied by any dreams of a sexual character. Distension of the stomach or intestines by flatus may have a similar effect (Fig. 155).