Copora cavernosa

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 (especially venous) structures, along with fibrous supporting tissue. The are essential in achieving an erection of the male penis. This happens through a complex mechanism involving psychologic and physiologic phenomenea resulting in engorgement of these structures with blood, resulting in an increase in length, grith and rigidity of the penis.

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