The crura of the penis are two elongated structures that extend from the base of the penis and anchor it to the pelvic bones. Each crus is made of erectile tissue, which becomes engorged with blood during sexual arousal, contributing to an erection. These structures play a crucial role in the overall function of the penis, providing stability and support. The crura are part of the corpora cavernosa, which are the two main erectile tissues that fill with blood, allowing for increased size and rigidity during sexual activity.