The muscular layer which surrounds the membranous portion of the urethra is a veritable sphincter, the contractions or dilatations of which will propel or retain the semen. The semen pouring from the ejaculatory ducts and the secretion of the prostate are steadily forced forward. But, retained by the contracted sphincter externus, the semen is forced into the bulb of the urethra, as the place of least resistance. In this way the bulbous part of the urethra becomes distended and serves as a temporary reservoir for the semen before ejaculation. By reflex excitation, this distention causes spasmodic contractions of the musculus bulbo-cavernosus and clonic contractions of the accelerator urinae. The contractions of these two muscles overcome the contractions of the sphincter, i. e., the muscular layer of the membranous portion of the urethra, and the semen is driven forward and gushes out of the meatus, in several jets, through the open and well-lubricated canal of the urethra. _Nervous control of erection._—The mechanism of erection and ejaculation is controlled by six centres, the three cerebral, one in the medulla oblongata, and the two spinal. Erection may be caused by the stimulation of the cerebral centre of voluptas. Impressions originating in the brain, as sexual thoughts, may evoke vigorous erections. There is no function of the human economy over which the mind exerts a more powerful influence than over that of sex. The imagination, says Hammond, is always a more potent stimulant of sexual desires than the physiological incentive supplied by nature. Besides by imagination, the cerebral centre may be excited by impressions conveyed through the senses. Sight, smell, hearing, and in some pathological states even taste are well known to convey sexual stimulation to the brain. The sense of touch has such an influence over the generative function that it is considered the inseparable companion of sexual activity. [Illustration: CUT XXXIII. _Diagram of nerves and centres._[D] 1, brain; 2, centre of inhibition; 3, centre of libido; 11, centre of voluptas; 4, vasodilatory centre in the medulla oblongata; 16, centre of erection; 7, centre of ejaculation; 10, genitals at the periphery. The broken lines designate centripetal nerves, the solid lines denote centrifugal nerves.] Erection may further be stimulated by excitation of the centre in the spinal cord. The direct electrical excitation of the spinal cord in dogs evokes vigorous erections. The irritation of the spinal cord in some spinal diseases often produces erections. Sleep, while lying on the back, is known to produce vigorous erections, by increasing the flow of blood to the spinal cord and by allowing the blood to settle in the medulla oblongata and in the cord, so as to produce a passive congestion.