356 THE ANATOMY OF THE HORSE. 



Action. — Acting with the opposite muscle, it forcibly depresses the 

 tail, compressing it over the perinaeum. Acting singly, it inclines the 

 tail to that side. 



Directions. — The preceding two muscles should be entirely removed. 

 Above the rectum there will be found the terminal portion of the pos- 

 terior mesenteric artery, and on its side the pelvic plexus of nerves. 



The Posterior Mesenteric Artery (Plate 46) is a branch of the 

 abdominal aorta. Its terminal portion enters the pelvic cavity between 

 the layers of the meso-rectum ; and, passing backwards above the bowel, 

 it terminates above the anus. In its backward course it detaches 

 numerous branches to the wall of the rectum. 



The Posterior Mesenteric Vein runs in company with the artery. 

 Its initial portion is formed at the posterior part of the rectum, by the 

 union of haemorrhoidal veins, which communicate with like branches of 

 the internal pudic vein. In the abdominal cavity it concurs in the 

 formation of the portal vein. 



The Pelvic Plexus of the sympathetic nerve. This is an intricate 

 network of nerves, placed on the side of the rectum, and distributing 

 branches to the pelvic viscera. It receives in front the offsets from the 

 posterior mesenteric plexus, and above it is joined by branches from 

 the inferior sacral nerves. In both sexes it distributes branches to 

 the rectum and bladder; and, besides, it supplies branches to the 

 prostate, vesicula seminalis, and vas deferens in the male, and to the 

 vagina and uterus in the female. 



The Rectum (Plate 46) is the terminal segment of the large in- 

 testines. At the entrance to the pelvis it is directly continuous with 

 the small colon, and it terminates at the anus. Its initial portion 

 resembles the small colon in being puckered and of comparatively 

 small calibre. Its terminal portion, on the other hand, is dilated 

 and sac-like, forming a large pouch in which the faeces collect. 



In the male it is related inferiorly to the bladder, vesiculae seminales, 

 vasa deferentia, prostate gland, and pelvic part of the urethra. In the 

 female it is related on the same aspect to the vulva, vagina, and uterus. 



Structure. — The wall of the rectum resembles that of the large intes- 

 tine in general (page 319), possessing serous, muscular, submucous, and 

 mucous layers. As already seen, its peritoneal investment is incom- 

 plete, its terminal portion being destitute of peritoneum, and connected 

 by loose areolar tissue to contiguous organs. In front of the anus the 

 longitudinal muscular fibres of the bowel form on each side a band that 

 passes upwards to be inserted into the coccygeal vertebrae. This, which 

 is termed the suspensory ligament of the rectum, forms a prominence at 

 the root of the tail. At the anus the last of the circular muscular fibres 

 form what is termed the internal sphincter. Developed in connection 

 with the termination of the rectum are two striped muscles — the 



