1. The =ischio-cavernosus= (or erector penis) is a short but strong
paired muscle, which arises from the tuber ischii and the adjacent part
of the sacro-sciatic ligament, and is inserted on the crus and adjacent
part of the body of the penis. It is somewhat fusiform, incloses the
crus as in a sheath, and is situated in a deep depression in the
semimembranosus muscle. It pulls the penis against the pelvis, and
assists in producing and maintaining erection by compressing the dorsal
veins of the penis. Its blood-supply is derived from the obturator
artery, and the nerve-supply from the pudic nerve.
2. The =retractor penis= is an unstriped muscle which arises on the
ventral surface of the first two or three coccygeal vertebræ. It divides
into two flat bands, half an inch or more (ca. 1.5 cm.) in width, which
pass downward over the sides of the rectum to meet below the anus. Here
there is a decussation of fibers, thus forming a sort of suspensory
apparatus for the posterior part of the rectum and the anus. From the
decussation the muscle passes for a short distance between superficial
and deep layers of the bulbo-cavernosus, and then along the ventral
surface of the penis, to which it is loosely attached. Near the glans
penis it splits up into bundles which pass through the bulbo-cavernosus
and are attached to the tunica albuginea. Below the anus the muscle is
attached to the sphincter ani externus. On the penis the two muscles are
intimately united to each other. Their action is to withdraw the penis
into the sheath after erection or protrusion. The proximal part would
draw the anus upward and forward and support it during defecation.