ANUS. 311 



structure the floating colon, but its walls are thicker, and more 

 dilatable. It is related superiorly with the sacrum ; inferiorly 

 with the bladder, vasa deferentia, vesiculae seminales, Cowper's 

 glands, and the prostate gland in the horse ; in the mare, with 

 the vagina and uterus ; laterally it is related with the walls 

 of the pelvic cavity. It is attached to a continuation of the colic 

 mesentery, called the meso-redum ; to the pelvis 'by a reflection 

 of peritoneum ; and to the sacrum by the suspensory ligaments 

 of the penis, which, uniting below, at the posterior extremity, 

 surround it; finally, by two triangular fasciculi of pale muscular 

 fibres, which pass up above the anus, and are inserted to the 

 inferior surface of the coccyx, between its depressor muscles. 

 The serous coat is a reflection of the peritoneum ; the muscular 

 is very strong, consisting of longitudinal bands, with circular 

 fibres beneath them. The mucous coat is similar to that of the 

 colon; its surface presents a number of transverse ridges, similar 

 to thoso of the caecum and large colon. In the spaces between 

 these ridges, the faeces assume their characteristic shape. 



ANUS. 



The anus is the posterior opening of the alimentary ca,na\. 

 Lying below the root of the tail, it forms a round projection, 

 which becomes less prominent with age. It consists of thin, 

 hairless skin externall}', and of mucous membrane internally, the 

 two being continuous. In the fold between them are the muscles, 

 the terminal muscular fibres of the rectum, and a certain quantity 

 of fat. 



The muscles connected with the anus are — 



Sphincter Ani. | Levator -Ani. 



Retractor Ani. 



SPHINCTER ANI. 



(Fig. 100./.) 



Situated round the termination of the rectum, this muscle is 

 •circular, with a central opening, some of the fibres blending with 

 the muscles of the perineal region. It is attached to the fascia 

 covering the perineum, and to tke coccygeal vertebrae, and con- 

 sists of two layers. The inner layer is pale, being part of tho 

 muscular coat of the rectum. 



Action. — To close the anus. 



