Large Intestine

The large intestine consists of the cecum, a blind sac, and the colon, which consists of ascending, transverse, and descending parts. The descending colon terminates as the rectum and anal canal.

There is considerably more variation in the large intestine from one species to another than in the small intestine (Figs. 20-12 to 20-14). Most of this variation results from modifica­tions of the ascending colon. The transverse colon forms a short connection that runs trans­versely from distal ascending colon to proximal descending colon; it is invariably found running from right to left sides of the abdomen, just cranial to the root of the great mesentery. The descending colon is generally relatively straight, running caudad on the left side of the abdomen to the pelvic cavity, where it terminates as the rectum.

Ruminants

In the ruminant (Fig. 20-13) the cecum is about 12 cm in diameter, and when full, its blind end projects as far caudad as the pelvic inlet. Crani- ally, the cecum is continuous with the colon.

The proximal part of the colon is the ascend­ing colon. it is modified into a series of three loops in the ruminant. The proximal loop (ansa proximalis) forms an s shape that leads to the spiral loop (ansa spiralis). The spiral colon forms an orderly spiraling mass on the left face of the great mesentery. The first portion of the spiral colon coils toward the center of the mesentery (centripetally), reverses direction at the central flexure, then spirals away from the center (centrifugally). The last part of the ascending colon, the distal loop (ansa distalis) connects the spiral colon with the transverse colon. The transverse colon crosses from right to left, cranial to the cranial mesenteric artery, which supplies the small intestine, the cecum, and the ascending colon, and continues caudad as the descending colon to the rectum.

The arrangement of the intestinal tract of the sheep and goat is similar to that of the ox. in small ruminants, however, the last centrifugal loop of the ansa spiralis lies much closer to the jejunum than in the ox.

Pig

The porcine cecum (Fig. 20-14) is a moderately large (1.5-2.2 L) blind sac that projects cranio- ventrad near the midline. The dorsal end of the cecum is continuous with the colon at the ileo- cecocolic junction, where the entrance of the ileum marks the division between the cecum and colon. Unlike those of most domestic species, the bulk of the porcine cecum lies to the left of midline, with its junction with the colon ventral to the left kidney.

The ascending colon of the pig, like that of the ruminant, presents a spiral arrangement of coils, although in the case of the pig the spiral loop is arranged in a cone shape rather than in a flat plane. The transverse colon continues from its junction with the distal end of the spiral loop, passes forward, then crosses to the left side of the abdomen. The bowel continues caudad as the descending colon to the rectum. As in other animals, the rectum terminates at the anus.

Horse

The horse has the largest and most complex large intestine of any of the domestic animals (Fig. 20-12). The equine diet of grasses neces­sitates the assistance of microbes for digestion of celluloses, but unlike that of ruminants, the horse’s digestive system defers this fermenta­tion until ingested food reaches the cecum. For this reason, horses are often called postgastric fermentators.

The cecum in the horse is a comma-shaped structure extending from its base in the right side of the pelvic inlet to the floor of the abdominal cavity, where the apex lies just caudal to the diaphragm near the xiphoid car­tilage of the sternum.

The ascending colon of the horse is highly modified and extremely capacious, for which reason it is commonly referred to as the great colon. The proximal part leaves the cecum and passes craniad along the right ventral abdomi­nal wall toward the sternal part of the dia­phragm, where it turns sharply to the left and proceeds caudad along the left ventral abdomi­nal wall toward the pelvic inlet. These first parts of the large colon are known respectively as the right ventral colon, the sternal fl exure, and the left ventral colon. They are arranged like a horseshoe, with the toe forward and the branches directed caudad on either side of the apex of the cecum.

At the pelvic inlet, the left ventral colon turns sharply dorsad to form the pelvic fl exure. The colon then continues craniad as the left dorsal colon, located just dorsal to the left ventral colon. As it approaches the diaphragm (just dorsal to the sternal flexure), it bends to the left as the diaphragmatic fl exure and then contin­ues a short distance caudad as the right dorsal colon. The right dorsal colon turns again to the left and crosses the midline in front of the root of the great mesentery as the transverse colon.

The descending colon in the horse (also called the small colon to distinguish it from the great colon) is the direct continuation of the transverse colon. The descending colon is arranged in undulations within the mesoco­lon, much like the small intestine in the mesentery. The small colon, however, is some­what larger in diameter than the small intestine. The small colon is usually located near the middle of the caudal part of the abdominal cavity. it terminates within the pelvic cavity as the rectum.