THE RECTUM AND ANUS

The shortness of its mesentery is the only additional point that need be made concerning the rectum. Congenital absence of the anus (atresia ani) once was of frequent occurrence; perhaps surprisingly, it may allow afflicted piglets to survive for 3 or 4 weeks without treatment.

Prolapse of the rectum, encountered in somewhat older pigs, requires more sophisticated surgery, espe­cially if the everted bowel has been mutilated by pen mates, as so often happens. Effective surgery requires some knowledge of the muscles associated with the anus. These are more or less as in other species (see Figure 3-47): bundling together of the longitudinal muscle of the rectum creates the rectococcygeus, and thickening of the circular muscle creates the internal anal sphincter. The striated external sphincter presents no features of note; the levator ani runs between the sacrosciatic liga­ment and the lateral aspect of the anal canal; and the retractor penis (or clitoridis) passes lateral to the rectum and with its fellow forms a sling below the rectum before continuing to the penis (or clitoris).