The corpora carvernosa (singular is corpus cavernosum) are two tubular shaped structures located on the dorsal, or back side of the male penis. They lie just underneath of the skin, and consist mostly of vascular (esoecially venous) structures, along with fibrous supporting tissue. The are essential in acheiving an erection of the male penis. This happens through a complex mechanism involving pyschologic and physiologic phenomenea resulting in engorgement of these structures with blood, resulting in an increase in length, grith and rigidity of the penis. The penis (plural penises or penes) or phallus is the external male copulatory organ of some animals, and, in mammals, the external male organ of urination. ... This article needs to be cleaned up to conform to a higher standard of quality. ... The penis (plural penises or penes) or phallus is the external male copulatory organ of some animals, and, in mammals, the external male organ of urination. ...

Source: Moore, Keith L Clinically Oriented Anatomy (2nd ed.). Wiliams & Wilkins ISBN 0-0-683-06132-1