is an inhibitory one. They inhibit the ganglionic nervous mechanism in the corpora cavernosa upon the action of which the smooth muscle-fibres of the corpora cavernosa are dependent (Kölliker and Kohlrausch). Under the influence of the action of the nervi erigentes these fibres of the corpora cavernosa become relaxed and their spaces fill with blood. Simultaneously, as a result of the dilatation of the capillary net-work of the corpora cavernosa, pressure is exerted upon the veins of the penis and the return of blood is impeded. This effect is aided by contraction of the bulbo cavernosus and ischio cavernosus muscles, which are inserted by means of an aponeurosis on the dorsal surface of the penis.
The erection-centre is under the influence of both exciting and inhibitory innervation arising in the cerebrum. Ideas and sense-perceptions of sexual content have an exciting effect. Also, according to observations made on men that have been hung, it is evident that the erection-centre may be excited by excitation of the tract in the spinal cord. Observations on the insane and those suffering with cerebral disease show that this is also possible as a result of organic irritation in the cerebral cortex (psycho-sexual centre?). Spinal diseases (tabes, especially myelitis) affecting the lumbar portion of the cord, in their earlier stages, may directly excite the erection-centre.
Reflex excitation of the centre is possible and frequent in the following ways: by irritation of the (peripheral) sensory nerves of the genitals and surrounding parts by friction; by irritation of the urethra (gonorrhœa), of the rectum (hæmorrhoids, oxyuris), of the bladder (distension with urine, especially in the morning, irritation of calculi); by distension of the vesiculæ seminales with semen; by hyperæmia of the genitals, occasioned by lying on the back, and thus inducing pressure of the intestines upon the blood-vessels of the pelvis.
The erection-centre may also be excited by irritation of the nervous ganglia which are so abundant in the prostatic tissue (prostatitis, introduction of catheter, etc.).
The experiment of Goltz, according to whom, when (in dogs) the lumbar portion of the cord is severed, erection is more easily induced, shows that the erection-centre is also subject to inhibitory influences from the brain.
In men the fact that the will and emotions (fear of unsuccessful coitus, surprise inter actum sexualem, etc.) may inhibit the occurrence of erection, and cause it, when present, to disappear, also indicates this.
The duration of erection is dependent upon the duration of its exciting causes (sensory stimuli), the absence of inhibitory influences, the nervous energy of the centre, and the early or late occurrence of ejaculation (v. infra).
The central and highest portion of the sexual mechanism is the cerebral cortex. It is justifiable to presume that there is a definite region of the cortex (cerebral centre) which gives rise to sexual feelings, ideas,
