From the perspective of histology, the most remarkable feature of the penis is its core of erectile tissue. Erectile tissue contains a specialized arrangement of arteries, shunts, and venous sinusoids within a matrix of connective tissue and smooth muscle, forming "a highly structured criss-crossing of interconnected fibers and spaces that are tensed as the cylinder expands during erection. This creates an internal strength and rigidity that is far greater than that possible in a hollow tube filled to equivalent pressure" (Barreto, Caballer, and Cubilla, in Sternberg's Histology for Pathologists, 2nd. ed., p. 1045).Blood flowing in the deep artery of the penis may flow either into the corpora cavernosa or into an arteriovenous anastomosis, which connects directly with efferent veins and which is usually dilated.
In the flaccid state, almost all the blood from the deep artery passes directly into the dilated arteriovenous shunt. Therefore, minimal amounts of blood pass into the corpora cavernosa. Blood that does enter the corpora divides into two routes -- the helicine arteries that empty directly into the blood spaces of the erectile tissue and the nutritive arteries of the trabeculae, which, after breaking up into a capillary network re-form into small veins that also then empty into the cavernous spaces. Cavernous spaces are drained by veins that pierce the tunica albuginea and constitute the efferent venous return.
During erection, blood flow in the deep artery of the penis increases. The opening of the arteriovenous anastomosis is reduced by active vasoconstriction, resulting in a slightly dilated artery passing through the tunica albuginea into the cavernous body. The helicine arteries then dilate the cavernous spaces and fill them with blood. Blood flow leaving the cavernous body is not reduced.
(Adapted from Weiss's Histology., 5th  edition, p. 1050)
The erectile tissue is organized into paired dorsal corpora cavernosa and one ventral corpus spongiosum. The corpora cavernosa are surrounded by tough fibrous connective tissue, the tunica albuginea. Between this sheath and the overlying skin is a layer of very loose elastic connective tissue (Buck's fascia) that permits the skin of the penis to move freely along the shaft. The skin includes a smooth muscle layer (the dartos). The penile urethra passes through the corpus spongiosum, where it is associated with small mucous glands of Littre.
The slide in our reference sets (e.g., the image at right) may not match this description, presumably because it represents a very immature organ (note the size) in which the erectile tissue is not yet fully developed. Nevertheless, you should be able to observe several prominent features including erectile tissue of the corpora cavernosa with numerous interconnected vascular channels, the surrounding dense connective connective tissue of the tunical albuginea, conspicuous nerves and blood vessels, the urethra lined by transitional epithelium (urothelium), and keratinized epithelium of the skin.
For additional images of erectile tissue and other specialized penile tissues, consult Sternberg's Histology for Pathologists or check out the male reproductive section of UC Davis Histo Quiz-O-Rama.
SIUC / School
of Medicine / Anatomy / David
Last updated: 4 February 2005 / dgk