Page 403 - Anatomy and Physiology of Farm Animals, 8th Edition

388 / Anatomy and Physiology of Farm Animals

the descending colon to the rectum. As in colon. The right dorsal colon becomes the
transverse colon which crosses the midline
other animals, the rectum terminates at
VetBooks.ir the anus. cranial to the root of the great mesentery.
The descending colon in the horse (also
called the small colon to distinguish it from
Horse the great colon) is the direct continuation
of the transverse colon. The descending
The horse has the largest and most complex colon is arranged in undulations within the
large intestine of any of the domestic ani­ mesocolon, much like the small intestine in
mals (Fig. 20‐15). The equine diet of grasses the mesentery. The small colon, however, is
necessitates the assistance of microbes for somewhat larger in diameter than the small
digestion of celluloses, but unlike that of intestine, and, unlike the smooth wall of
ruminants, the horse’s digestive system small intestine, its wall is prominently sac­
defers this fermentation until ingested food culated. The small colon is usually located
reaches the cecum. For this reason, horses near the middle of the caudal part of the
are often called postgastric fermentators. abdominal cavity. It terminates within the
The cecum in the horse is a large, pelvic cavity as the rectum.
comma‐shaped structure extending from The equine great (ascending) colon is
its base in the right side of the pelvic inlet to attached to the body wall only dorsally
the floor of the abdominal cavity, where the on the right side. Most of its considera-
apex lies just caudal to the diaphragm near ble length is mobile within the abdomen.
the xiphoid cartilage of the sternum. The It is therefore susceptible to abnormal
ileum enters the cecum near its base at the displacements and/or torsions, which
ileal orifice. The cecum is the primary site can cause obstruction, gas accumula-
of fermentation in the horse, and its average tion, and strangulation. These cause
capacity is about 33 L (about 9 gallons). extreme abdominal pain – called colic –
The ascending colon of the horse is and are often life threatening unless cor-
highly modified and extremely capacious, rected surgically.
for which reason it is commonly referred
to as the great colon. The initial part
leaves the cecum and passes craniad along Peritoneal Features
the right ventral abdominal wall toward
the sternal part of the diaphragm, where it The parietal and visceral peritonea are
turns sharply to the left and proceeds cau­ continuous with one another at double
dad along the left ventral abdominal wall folds of serosa called mesenteries (see
toward the pelvic inlet. These first parts above). In some locations, visceral perito­
of the large colon are known sequentially neum reflects off one region of the gut,
as the right ventral colon, the sternal spans a short distance, then merges onto
flexure, and the left ventral colon. They the surface of nearby structures. Although
are arranged like a horseshoe, with the toe these double folds of peritoneum are typi­
forward and the branches directed caudad cally very thin, in this configuration they
on either side of the apex of the cecum. are sometimes referred to as ligaments.
At the pelvic inlet, the left ventral colon Some examples include the falciform liga-
turns sharply dorsad to form the pelvic ment, which tethers the liver to the ventral
flexure. The colon then continues craniad midline; the renosplenic (nephrosplenic)
as the left dorsal colon, located just dorsal ligament, spanning between the left kid­
to the left ventral colon. As it approaches ney and spleen; and the hepatoduodenal
the diaphragm (just dorsal to the sternal ligament, connecting the liver and proxi­
flexure), it bends to the left as the dia- mal duodenum.
phragmatic flexure and then continues a Omentum refers to those parts of the
short distance caudad as the right dorsal peritoneum connecting the stomach with