The main muscles of the penis are the erector penis and the bulbo-cavernosus. The latter arises from the central perineal tendon and inserts by embracing the bulb. The erector penis muscle arises at the inner surface of the tuberositas ossis ischii and inserts into the sides of the crura.
The penis is covered with an integument which forms the continuation of the skin of the abdominal wall, but it is somewhat darker than the latter. At the anterior end a fold of skin forms the prepuce which covers the glans penis.
Genital nerves.—The genital nerves are of three different kinds. There are first the efferent nerves, or centripetal nerves. They are the sensitive terminal branches of the nervus pudendus and end, or rather arise, in the genital nerve bulbs. They are richly distributed throughout the prostatic part of the urethra. These centripetal nerves are connected with the Pacinian corpuscles, within the prostate, and with the extensive nerve-plexus, interspersed with ganglia, which are found in the superficial layers of the urethral mucosa as well as in the cortical layers of the prostate. These nerves return to the centres in the spinal cord and in the brain through the rami of the nervus pudendus.
The second kind of nerves are the centrifugal nerves, which arise by two roots at the sacral plexus, from the first to the third pair of sacral nerves. This sacral plexus sends off anastomosing branches to the vesical plexus. The latter is made up of branches from the hypogastric plexus of the sympathetic nerve and of filaments from the sacral ganglia, the pudendal plexus and the sacral nerves. The latter innervates the bladder, seminal vesicles, the urethra, and the prostate. The nervi erigentes are vasodilator-nerves. Their centre, in the lumbar part of the spinal cord, is connected with the centre of the vasodilators of the medulla oblongata by filaments, running within the spinal cord. When these nerves or their centre are irritated, as in diseases of the cord, or excited by electricity, erections will ensue.
The third kind of nerves are those of ejaculation. They are centripetal and centrifugal nerves and run through the nervi perinei, the latter being branches of the nervus pudendus communis. They innervate the ejaculatory ducts, the seminal vesicles, vasa deferentia and testicles. The centre of these nerves has also its seat in the lumbar portion of the spinal cord.
Genital centres.—The centres for the genital nerves are six in number, three cerebral, one in the medulla oblongata and two spinal centres. There is first the cerebral centre of voluptas or cupido. It is the seat of the sexual instinct or of sex desire. The second centre is the centre of libido. It is the centre for experiencing pleasure. The third centre is that of inhibition. It is the centre where, under certain circumstances, sexual activity is prevented. The vasodilator centre in the medulla oblongata and the two spinal centres regulate erection and ejaculation. Connecting fibres are passing between the cerebral centres and the spinal centres. Hence psychic stimulation may cause erection and ejaculation.
[CHAPTER IV]
THE FEMALE ORGANS OF GENERATION
Vulva.—The main female generative organs are entirely situated within the pelvis. The less important external organs are those comprised in the name vulva (derived from the Latin word valva, the folding door). When the woman is in erect position, the vulva runs horizontally from the mons veneris to the frenulum of the fossa navicularis.
CUT XII.