THORAX AND ABDOMEN OF THE HORSE 167 



The urethral muscle is continued from the bulbo-urethral glands to 

 the glans of the penis as the bulbo-cavernous muscle (m. bulbocaver- 

 nosus). 



M. iscHiouRETHRALis. — The ischio -urethral muscle consists of a 

 pale band of fibres arising from the sciatic arch and one of the crura of 

 the penis, and blending with the ventral stratum of the urethral muscle. 



A. PUDENDA INTERNA. — The internal pudendal is a branch of the 

 hypogastric artery. On entering the pelvis the vessel runs obliquely 

 across the lateral wall of the cavity along the dorsal border of the iliac 

 head of the internal obturator muscle. Part of its course is related to 

 the inner surface of the sacro-spinous ligament close to the sciatic 

 spine. Sometimes the artery is embedded in the ligament ; at other 

 times it may even pierce the ligament and run for a short distance on 

 its outer surface before again perforating it to re-enter the pelvic 

 cavity. Finally, having crossed the lateral face of the levator ani 

 muscle, the artery reaches the sciatic arch, where it ends by dividing 

 into the perineal artery and the artery of the urethral bulb. The 

 following are its principal branches : — 



(1) A. umhilicalis. — This is the shrivelled remains of a vessel that, 

 in the embryo, carries blood from the hypogastric artery through the 

 umbilicus to the placenta. In the adult it occupies the free edge of the 

 lateral umbilical fold of the urinary bladder, and constitutes the 

 so-called round ligament of the bladder. Only the first part of the 

 vessel possesses any lumen, and from this spring small arteries for the 

 bladder and prostate (aa. vesicales craniales) and the ductus deferens 

 (a. deferentialis). 



(2) A. Jicemorrhoidalis media. — The middle haemorrhoidal ^ artery 

 arises medial to the sciatic spine and passes to the rectum. It gives 

 branches to the bladder, urethra, seminal vesicle, prostate and bulbo- 

 urethral gland. 



(3) A. 'perinei. — In the male the perineal artery is small, and 

 supplies the skin of the perineum and the projecting anal part of the 

 rectum. 



(4) A. hidhi urethixe. — As its name indicates, this vessel supplies 

 the bulb of the urethra. A small branch bends round the sciatic arch 

 and anastomoses with a branch of the obturator artery, 



V. PUDENDA INTERNA. — The internal pudendal vein, in the main, 



1 aifj-oppois (haimorhois) [Gr.], a pile, or vascular tumour of the rectal mucous 

 membrane (Hippocrates). The hsemorrhoidal vessels are involved in piles. 



