The functional morphology of the mammalian corpus cavernosum with special reference to erection in the nine-banded armadillo (Dasypus novemcinctus)

This project examines how the morphological design of a hydrostat called the corpus cavernosum within the mammalian penis allows it to inflate to its intromittent shape and increase its flexural stiffness during erection. By examining the gross morphology and histology of flaccid and artificially erected penises from the nine-banded armadillo (Dasypus novemcinctus), I confirmed previous reports that (1) the corpus cavernosum is a biological hydrostat reinforced by an array of collagen fibers arranged at 0;Artificial inflation of armadillo corpora shows that folded tissue and crimped collagen fibers in the wall of the flaccid corpus cavernosum allow it to expand in response to a small (1 kPa) rise in intracavernosal pressure, then increase in stiffness by three to four orders of magnitude at strains larger than 15%. Wall expansion also increases the second moment of area of the corpus cavernosum. Increasing both tissue stiffness and second moment of area contribute to the increase in the flexural stiffness of the corpus cavernosum during erection, although the presence of trabeculae reduces the maximum potential flexural stiffness of the erect corpus cavernosum in favor of protecting the urethra from compression. Once the penis is erect, the axial orthogonal array of collagen fibers gives the corpus cavernosum a fixed shape and size and allows it to resist tensile, compressive, and bending forces.;By examining the gross morphology and histology of corpora from animals in eight additional mammalian orders, I infer that the orthogonal collagen fiber array and the collagenous trabeculae are shared by all therian mammals. Differences in the size of the tunica albuginea's axial orthogonal array and differences in the length and arrangement of the trabeculae may be correlated with length and cross-sectional shape differences in the erect corpora cavernosa among mammalian taxa. The particular shape of the erect corpus cavernosum affects both its relative stiffness to Euler and local buckling and its relative flexural stiffness in different bending planes.