The penis is the male copulatory and urination organ, located in the perineum. It consists primarily of erectile tissue, a spongy connective tissue that increases significantly in size when engorged with blood. When the penis is engorged in this way, it is said to be erect, and its possessor is said to have a boner.
The structure of the penis can be divided into three parts:
- shaft (body)
Each part contains erectile tissue, which can swell as a result of becoming engorged with blood. The dorsal surface of the flaccid (non-erect) penis is continuous with the anterior abdominal wall, while the ventral surface is closest to the scrotum.
The erectile tissue of the root is in three blocks:
- right crus (attaches to the ischiopubic ramus)
- left crus (attaches to the ischiopubic ramus)
- a central bulb which attaches to the undersurface of the urogenital diaphragm
The crura come together as two cylindrical corpora cavernosa to form the dorsal part of the shaft. The crura are enclosed by the ischio-cavernosus muscles which attach to the ischio-pubic rami and compress the crura of the penis, also compressing veins and perhaps assisting in maintaining an erection.
The central bulb receives the membranous urethra after it passes through the urogenital diaphragm. It narrows (still enclosing the urethra) as the corpus spongiosum, which is located ventrally, between the corpora cavernosa.
The shaft is free and suspended from the anterior abdominal wall and pubic arch by ligaments. Each block of erectile tissue has a coat, or tunica albuginea, which is thickest around the corpora cavernosa. The shaft is surrounded by sheaths of both deep (Buck's) and superficial (Colle's) penile fascia.
The skin of the penis is generally loose. In uncircumcised males, it covers the glans of the flaccid penis as the prepuce (foreskin). When the penis becomes erect, the foreskin retracts, leaving the glans uncovered.
The arterial supply is predominantly from the internal pudendal artery. Bulbar and spongiosal branches from each artery supply the bulb and corpus spongiosum, respectively. The deep (cavernosus) artery of the penis enters its corresponding corpus cavernosum, while the dorsal artery of the penis passes along the dorsal surface of the corpus cavernosum, to supply the glans of the penis.
Blood from the cavernous tissue drains into a plexuses of postcavernous venules, lying immediately deep to the tunica albuginea. The venules connect to a series of emissary veins which penetrate the tunica albuginea and pass to the deep dorsal vein of the penis. The deep dorsal vein passes under the pubic arch to join the prostatic venous plexus. Veins draining the proximal part of the penis, including the crura and bulb, enter the internal pudendal veins.
There is both a somatic and an autonomic supply to the penis. The somatic supply is mainly through the pudendal nerves. The perineal branch of the pudendal nerves sends motor fibres to the bulbospongiosus and ischiocavernosus muscles. The dorsal nerve of the penis is a sensory branch of the pudendal nerve. It passes along the dorsal surface of the penis, supplying much of the penile skin, including the highly sensitive glans.