496 THE MALE GENITAL ORGANS OF THE HORSE 



In structure the corpus spongiosum has a general resemblance to the corpus 

 cavernosum, but the trabeculse are finer and more elastic. In the glans the trabe- 

 culse are highly elastic, and the spaces are large and very extensible; the latter are 

 continuous at the extremity of the processus dorsalis 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 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 lymphatics run with the veins and go to the inguinal and luml)ar glands. 

 The nerves are derived chiefly from the pudic nerves and the pelvic plexus of the 

 sympathetic. 



MUSCLES OF THE PENIS (Figs. 183, 45 J, 452, 456) 



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 some- 

 what 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 l^lood-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 sur- 

 face of the first two or three coccygeal vertebrae. 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 form- 

 ing 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 tvmica 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 antl support it during defecation. 



THE PREPUCE 

 The prepuce (Prccputium), popularly called the "sheath," is a double in- 

 vagination of the skin which contains and covers the free or prescrotal portion of 

 the penis when not erect. It consists of two parts, external and internal. The 

 external part or sheath extends from the scrotum forward close to the umbilicus, 

 where the external layer is refiectetl ventrally and laterally, forming the thick 

 margin of the preputial orifice (Ostium prneputiale) ; dorsally it is directly continu- 

 ous with the integument of the abdominal wall. It is marked by a median raphe 

 praeputii, a continuation of the scrotal raphe. At the lower margin of the pre- 

 putial orifice there are often in the stallion two papillae, which are rudimentary 

 teats or nipples. The internal layer passes backward from the preputial orifice 

 a distance of about six to eight inches (ca. 15 to 20 cm.), lining the cavity of the ex- 

 ternal prepuce, and is then reflected forward until it approaches the orifice, where 

 it is again reflected backward. It thus forms within the cavity of the sheath a sec- 

 ondary tubular invagination, the prepuce proper, in which the anterior part of 



