234 INTESTINAL DIGESTION. 



covering is of common integument, destitute of hairs. Lying 

 between the skin and mucous membrane are two circular 

 muscles, whose office it is to keep the anus closed and prevent 

 constant evacuation of faeces, whilst there are other muscular 

 appendages situated externally to these, destined either to 

 elevate or retract the anus; being evidently antagonistic to the 

 sphincters. 



The internal sphincter ani is in contact with the attached 

 surface of the intestinal mucous membrane, and separated 

 from the integument by the external one. It is constituted 

 of the pale circular fibres of the gut, but towards its free edge 

 certain coloured fibres are apparent on it. 



The external sphincter is situated outside the internal one, 

 and within the anal integument : it is circular, and composed 

 of red fibres, attached superiorly under the first coccygeal 

 bone, and inferiorly its fibres blend in the male subject in the 

 accelerator urinae and triangularis penis, and in the female 

 with the constrictor vaginae. 



The levatores ani are two pale muscles, attached on each 

 side of the first bones of the tail, and spreading downwards 

 and forwards on to the rectum, form an attachment for the 

 internal sphincter, and blending with the longitudinal fibres, 

 so as to increase the thickness of the muscular coat of the 

 rectum. The action of these muscles must be that of elevating 

 the anus and shortening the rectum from before backwards. 



The retractors proper to the anus are one on each side 

 attached to the inner surface of the articular extremity of the 

 ischium. Extending from before backwards, and rather up- 

 wards, they blend with the external sphincter. Their action 

 is obviously that of retracting the anal opening. 



In ruminants the intestine is not so capacious as in the 

 horse, and a singular arrangement is noticed in them, as seen 

 in the subjoined Fig. 93. 



