582 GENITAL ORGANS OF THE STALLION 
like that of the corpus cavernosum penis, but the trabecule are much finer; they 
consist of fibrous tissue, much of which is elastic, and of bundles of unstriped muscle 
which are chiefly longitudinal in direction. The spaces are numerous and large. 
In the glans penis the trabecul are highly elastic, and the spaces are large and 
very distensible; the latter are specially wide in the posterior part of the processus 
dorsalis, where they communicate with large veins on the dorsum penis. There is 
a partial septum glandis. The skin covering the glans is thin, destitute of glands, 
and richly supplied with nerves and special nerve-endings. 
Vessels and Nerves.—The penis is supplied with blood by three arteries, viz., 
the internal pudic, obturator, and external pudic. The terminal part of the internal 
pudic artery enters the root as the artery of the bulb and breaks up in the bulb into 
numerous branches. The obturator artery gives off the large arteria profunda 
penis, which enters the crus penis and ramifies in the corpus cavernosum. The 
external pudic artery gives off the dorsal arteries of the penis, branches from which 
pass through the tunica albuginea. The veins form a rich plexus on the dorsum 
and sides of the penis, which is drained by the external pudic and obturator veins; 
from the root the blood is carried by the internal pudic veins.!. The lymph vessels 
run with the veins and go to the superficial inguinal glands. The nerves are derived 
chiefly from the pudic nerves and the pelvic plexus of the sympathetic. The former 
supply the dorsal nerves of the penis; special nerve-endings, the end-bulbs (of 
Krause), occur in the skin of the glans penis. The sympathetic fibers supply the 
unsitriped muscle of the vessels and the erectile tissue. 
MUSCLES OF THE PENIS (Figs. 272, 576, 577, 58!) 
1. The ischio-cavernosus’ 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, encloses 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 is a continuation of the sus- 
pensory ligaments of the anus. The latter arise on the ventral surface of the first 
and second coecygeal vertebrie and 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. Be- 
low 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 PREPUCE 
The prepuce (Preputium), popularly called the ‘sheath,’ is a double in- 
vagination of the skin which contains and covers the free or prescrotal portion of 
1It has been shown that the cavernous spaces of the glans penis receive blood exclusively 
from veins which come from the penile layer of the prepuce. This may account for the fact that 
the glans reaches its extreme size during erection later than the corpus cavernosum penis. 
2 Also termed the erector penis. 
