The core of the penis consists primarily of three large sponge-like regions that become engorged with blood during an erection. The pair that form the sides and dorsal region of the penis shaft are each called the Corpus Cavernosum, or Corpora Cavernosa (plural). The third, similar structure is called the Corpus Spongiosum.

Source: Corpus cavernosum penis. (2009, September 9). In Wikipedia, The Free Encyclopedia