Page 425 - Anatomy and Physiology of Farm Animals, 8th Edition

410 / Anatomy and Physiology of Farm Animals

Cecum and Colon of the Horse epithelium of the ileum, and this lowers the
pH of the cecal contents.
VetBooks.ir The extremely large and complex cecum

and ascending colon (also called the “great
colon” or “large colon”) of the horse are the Rectum and Defecation
primary sites of fermentation and micro-
bial digestion of cellulose. Roughage passes Several times daily, strong and extensive
relatively quickly through the stomach and mass movements of the colon move fecal
small intestine of the horse, but fermenta- material into the rectum. Distension of the
tive digestion and passage through the rectum stimulates the need to defecate.
cecum and ascending colon may take days. The act of defecation requires contractions
Complex movements of the cecum and of smooth muscle in the wall of the rectum,
ascending colon mix the contents to pro- and these result from a spinal reflex stimu-
mote fermentative digestion and to expose lated by distension of the rectum. Conscious
the contents to the epithelial surface for control of defecation involves inhibition of
absorption of volatile fatty acids. The pel- the spinal reflex and contraction of the
vic flexure of the ascending colon and the external anal sphincter, which is composed
junction between the great and small of skeletal muscle. Contraction of abdomi-
colons (see Fig. 20‐15) are relatively small nal muscles contributes to an increase in
in diameter, and it appears that the passage intra‐abdominal pressure, which also
of large particles of roughage is restricted. assists with emptying the rectum.
While these sites retain roughage so that it The variability in character and shape
can be subjected to microbial digestion, of feces among species is primarily a func-
the potential for impaction at these sites is tion of the structural and functional
also increased. features of the more distal segments of the
When consumed with roughage, some colon. In horses relatively strong segmen-
starches and sugars escape digestion in the tation contractions form the characteristic
equine stomach and small intestine and fecal balls.
pass into the cecum and colon. Microbial Colic is a general term for any painful
digestion of these and the cellulose in the condition associated with the abdomen
roughage produces volatile fatty acids that of the horse. Conditions associated with
can be absorbed and used for energy. any segment of the gastrointestinal tract
Microbes in the cecum and colon can also may result in colic, and clinical signs
use nonprotein nitrogen sources (urea) for indicate that the degree of pain can vary
the production of microbial proteins. from mild to severe. Causes of colic in
However, these have limited nutritional horses include ulcers, inappropriate diet
value to the horse, because the gastrointes- (such as an overload of grain or concen-
tinal mechanisms necessary to digest pro- trates), infection, intraluminal impac-
teins and absorb the resulting amino acids tion/obstruction (such as parasites,
are not readily available in the cecum or foreign bodies, feed or foreign material,
colon. Some urea is made available to such as sand), twisted segments of intes-
microbes by diffusing into the cecum and tine, and alterations in intestinal motil-
colon from the blood. ity. Management, including parasite
Fermentation and microbial digestion control, proper diet, access to water, and
produce volatile fatty acids, which could attention to environmental hazards, is
lower the pH of the cecal and colonic con- helpful in preventing the incidence of
tents to potentially harmful levels. The colic. Some causes of colic can be readily
colonic epithelium secretes bicarbonate treated medically, but other causes
ions to buffer the pH of the contents. threaten life and require surgical inter-
Additional bicarbonate is secreted by the vention to resolve.