Expansion of the tunica albuginea during penile inflation in the nine-banded armadillo (Dasypus novemcinctus).

Artificial inflation of corpora cavernosa from the nine-banded armadillo (Dasypus novemcinctus) showed that the expansion of the tunica albuginea during erection increases both components of flexural stiffness: the second moment of area and Young's modulus of elasticity. Folded tissue and crimped collagen fibers in the tunica albuginea permit its expansion during erection. As the tunica albuginea's radii increase in size, its second moment of area also increases. The crimped collagen fibers permit the flaccid tunica albuginea to expand to strains of 25 % longitudinally and 15 % circumferentially, after which tissue stiffness increases by 3-4 orders of magnitude. Radial expansion of the corpus cavernosum is limited by collagenous trabeculae. The trabeculae maintain the non-circular cross section of the corpus cavernosum during erection. Restricting expansion appears to protect the corpus spongiosum and urethra from occlusion, but has the side effect of reducing the potential flexural stiffness of the corpus cavernosum by reducing the second moment of area of the tunica albuginea.