Overview
The large intestine comprises the colon, caecum, rectum and anal canal. Its primary functions are the absorption of water from its contents and the voiding of waste matter via the anus.
However, there is considerable variation amongst mammals in the anatomy of the large intestine in relation to the diet. Ruminants digest a herbivorous diet primarily in the stomach, whereas the horse digests its herbivorous diet in the large intestine. The dog and cat are carnivores and digest their food in the stomach and small intestine. The pig is omnivorous, and its digestive tract anatomy and function are similar to that of the dog.The caecum is a diverticulum of the proximal colon. There is great variation in the anatomy of the caecum of the domestic mammals. As a rule, the caecum of herbivores is large and that of carnivores is small. This difference is related to the storage and breakdown of cellulose by bacteria that occurs in the caecum of herbivores. The appendix is an appendage to the caecum in humans but absent in the domestic animals; it is usually 6-9 cm in length and contains lymphoid tissue.
The colon is divided into ascending, transverse and descending sections in all the domestic mammals. The descending colon is narrower than the other parts and is continuous with the rectum. All carnivores possess two perianal sacs located one either side between the internal and external anal sphincters (see Figure 18.6). The sacs are oval-shaped and 0.5-1.0 cm in diameter and store the sebaceous secretion of glands in the lining of the sacs. The glands are coiled, tubular and apocrine. The secretion is a strong-smelling territory marker and commonly becomes impacted in dogs when it requires manual expression.
6.2