Penises as variable-volume hydrostatic skeletons.

Penises are inflatable intromittent organs that transfer sperm to a female during copulation. Most of the time, males store their penises in a flexible detumesced state, but they can rapidly inflate them with blood when an opportunity for reproductive behavior arises. In mammals, the primary erectile tissue is called the corpus cavernosum; its anatomy is a close match to a model hydroskeleton reinforced by an axial orthogonal fiber array. The wall of the corpus cavernosum contains layers of highly organized collagen fibers arranged at 0 degrees and 90 degrees to the penile long axis. Flaccid wall tissue is folded. Collagen fiber straightening during erection expands the tunica albuginea and increases both its stiffness and its second moment of area. These changes make the entire penis larger and harder to bend. Axial orthogonal fiber reinforcement affects the mechanical behavior of the erect corpus cavernosum, making it resistant to tensile, compressive, and bending forces.