582 GENITAL ORGANS OP THE STALLION 



like that of the corpus cavernosum penis, but the trabeculse 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 trabeculse 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 arterj' 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, AA'hich is drained by the external pudic and obturator veins; 

 from the root the blood is carried by the internal pudic veins. "^ The l3Tnph vessels 

 run with the veins and go to the superficial inguinal glands. The nerves are derived 

 chiefijr 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 

 unstriped muscle of the vessels and the erectile tissue. 



MUSCLES OF THE PENIS (Figs. 272, 576, 577, 58 J) 



1. The ischio-cavemosus- is a short but strong, paired muscle, which arises 

 from the tuber ischii antl the adjacent part of the sacro-sciatic ligament, and is 

 inserted on the crus and adjacent part of the bodj' 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 puchc 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 coccygeal vertebrae and pass doTwaward 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 (Prjeputium), popularly called the "sheath," is a double in- 

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



' It 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. 



^ Also termed the erector penis. 



