_Penis._—The penis is chiefly made up of three erectile bodies, the corpus cavernosum urethrae and the two porpora cavernosa penis. The latter arise each from the ramus descendens ossis pubis, by strong fibrous processes, the crura. The crura converse and coalesce at the inferior margin of the symphysis to which they are fastened by the ligamentum suspensorium penis mediale. The anterior ends of the corpora cavernosa are rounded. They are situated in the furrow of the glans of the penis. After their coalescence the two corpora are divided only by a fibrous septum. The lower surface of the corpora shows a deep furrow which serves as a receptacle for the corpus cavernosum urethrae. The latter thus occupies the same relation to the corpora cavernosa penis as does the ramrod to a double-barreled gun. The corpus cavernosum urethrae forms two expansions. The anterior expansion is represented by the glans penis, the posterior expansion by the bulb, a tuberous enlargement, situated between the diverging crura of the corpora cavernosa penis. The bulb is covered by the musculus bulbo-cavernosus. _Structure of the corpora cavernosa._—The walls of the corpora cavernosa are made up of a dense fibrous elastic membrane, the tunica albuginea. From this tunica arise numerous trabeculae, composed of fibrous tissue and non-striped muscular fibres. In this way a sponge-like tissue is formed in which the spongy superstructure consists of elastic fibres. The mesh-spaces are filled with circulating blood. The spaces of these cavernous bodies are covered with an endothelial lining like blood-vessels. The caverns communicate with each other by the arteriae helicinae, short arterial branches, anastomosing in the cavernous spaces. The arteries, capillaries, and veins pass along the trabeculae of the spongy tissue and open into the caverns. The caverns may thus be considered as enlarged capillaries. The arteria and vena profundae penis run through the middle of the corpus. The arteries within the trabeculae possess not only circular muscular fibres, as all other arteries, but also longitudinal fibres. These muscular fibres are normally contracted and thus prevent the blood from flowing into the cavernous spaces. If by some inhibitory influence the muscles are relaxed, the caverns are immediately filled with blood. This blood has to return by the vena profunda communis which passes through the unstriped muscular fibres of the musculus transversus perinei profundus. Hence by the contraction of this muscle the veins are compressed, and the blood is prevented from flowing off. As a result, the spongy and cavernous bodies become turgid and the corpora cavernosa penis become stone hard. The blood of the corpus cavernosum urethra, on the other hand, returns through the vena dorsalis penis which enters the abdomen through connective tissue parts, beneath the symphysis. For this reason the corpus cavernosum urethrae remains compressible even during erection.