Large Intestine

The large intestine of mammals comprises the colon, cecum, and rectum.[6] Cecum and colon have longitudinal fibers in the muscle layer, gathered from their equal distribution at the circumference into bundles to appear as three bands (tenae coli). In turn, contractions of the circular fibers without overhead stabilization create out-pocketings (haustrae) of circular fibers between bands. The ileocolonic sphincter opens only with transfer of indigesta from the small to the large intestine. A low-profile mucosa well covered with mucus aids in providing anaerobic conditions for an extensive microbial population. Gentle motility concentrates solutes and fine particulates in the haustrae, where microbial action on complex polysaccharides leads to VFA production and absorption. Coarse fiber collects in the lumen core and rapidly moves to the rectum. In simple-stomached mammals and ruminants, the colon forms coils that dominate the large intestine, whereas in nonruminant herbivores indigesta enter into an accentuated cecum and are retained in the haustrae before movement through the colon.

Fowl have a large intestinal system that drastically differs from mammals. No haustrae exist in the muscle layer, and two large ceca are connected to a small colon. Each cecum has a small entrance protected by villi that restrict entry to fluid and fines. These microbiologically labile materials are segregated from coarse fiber and forced into both ceca by reverse peristalsis originating at the cloaca. In mammals, coarse fiber collects in the rectum to a critical mass before evacuation. However, the cloaca in fowl not only has a coprodeum for such storage but a separate urodeum for urine. Reverse peristalsis moves urine through the colon to facilitate indigesta segregation for ceca entry while the mucosa actively resorbs salt and water. Microbial action on ceca contents yields volatile fatty acids similar to those in the mammal's cecum colon. Fecal excreta from mammals are a combination of coarse fiber in the core, with haustrae residue appearing on the surface as nodules. Coprodeum excreta are voided from fowl as a fibrous mass covered with a uric acid white cap that accrues with urine dehydration. Ceca excreta are separately voided as a viscous mass and may be eaten by the fowl to provide considerable nutrition, particularly vitamins.

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