THORAX AND ABDOMEN OF THE HORSE 169 



the internal pudendal artery to the sciatic arch, where it becomes the 

 dorsal nerve of the penis. From it the following branches take 

 origin : — 



(1) To the coccygeus and levator ani muscles ; (2) the middle 

 hcemorrhoidal nerve (n. hipmorrhoidalis medius) which furnishes twigs 

 to the levator ani muscle and the rectum ; (3) the perineal nerve to 

 the skin of the perineum and about the anus. The last-named nerve 

 receives fibres from the posterior cutaneous nerve of the thigh. 



The dorsal nerve of the penis (n. dorsalis penis) turns round the 

 sciatic arch, and has already been traced along the dorsum of the penis 

 in company with the dorsal artery and vein. The nerve is the most 

 lateral of the three structures. 



N. H^MORRHOiDALis cAUDALis. — The caudal hemorrhoidal nerve is 

 mainly formed by roots from the third and fourth sacral, but a slender 

 root from the fifth nerve also is not uncommon. Passing is a caudo- 

 ventral direction medial to the sacro-tuberous ligament and the coccygeus 

 muscle, the nerve ends in filaments distributed to the terminal part of 

 the rectum and the skin surrounding the anus. 



Plexus hypogastrici. — Accompanying all the vessels of the pelvis 

 are filaments of sympathetic nerve plexuses. These are continuous with 

 the aortic and caudal mesenteric plexuses, and connected with them are 

 two, or possibly more, small nerves that leave the caudal mesenteric 

 ganglion and enter the pelvis ventral to the large blood vessels. They 

 are also joined by filaments from the third and fourth sacral nerves. 



The hypogastric ^ plexuses are associated with the pelvic organs, as 

 well as with the penis in the male. 



Dissection. — Remove the pelvic organs. Open the rectum longitudin- 

 ally and observe the character of its lining. Open the urinary bladder 

 by making an incision along the mid-ventral line from the vertex to the 

 neck. Continue this incision along the ventral border of the urethra. 



The interior of the urinary bladder is lined by a pale 

 mucous membrane of distinctly rugose character ; but towards the neck 

 the rugse are generally less pronounced than elsewhere. In the dorsal 

 part of the neck the opening of the ureter (orificium ureteris) will be 

 found, and if a probe be passed along the tube, its very oblique course 

 through the wall of the bladder can be determined. An indistinct fold 

 of mucous membrane (plica ureterica) stretches from the ureteral open- 

 ing into the urethra, where the folds of the two sides of the bladder 

 meet and fuse into the urethral crest (crista urethralis). The term 

 trigone of the bladder (trigonum vesicae) is applied to the area bounded 



1 uiro (hypo) [Gr.], under -f- TacTijp (gaster) [Gr.], the stomach. In the erect 

 posture assumed by man, the hypogastrium is below the stomach. 



