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 vesicular 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, and impulses, and is the place of origin of the psycho-somatic processes which we designate as sexual life, sexual instinct, and sexual desire. This centre is excitable to both central and peripheral stimuli. Central stimuli, in the form of organic excitation, may be due to diseases of the cerebral cortex. Physiologically they consist of psychical stimuli (memory and sensory perceptions). Under physiological conditions these stimuli are essentially visual perceptions and memory-pictures (_i.e._, lascivious stories) and also tactile impressions (touch, pressure of the hand, kiss, etc.). Within physiological limits auditory and olfactory perceptions certainly play but a very subordinate _rôle_. Under pathological conditions (_v. infra_) the latter have a very decided influence in inducing sexual excitement.