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.