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An important additional facility to the above general mammalian pattern occurs in herbivores, where special provision is made for the breakdown of cellulose by bacterial fermentation, and in these species one portion of the digestive tract has structural modifications for this purpose; these modifications include the additional compartments of the stomach of the ruminant and the greatly enlarged caecum and colon of the horse.

Following the physical and chemical treatment of the food and the breakdown of the major constituents into simple sugars, amino acids, fatty acids, etc., these relatively simple substances are absorbed by the columnar cells that line the small intestines and, to a lesser extent, the large intestines.

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ssss1 The general arrangement of the intestines of the domestic mammals. The diagrams of the mammalian gastrointestinal tracts are drawn from the dorsal view although the ruminant intestinal spiral and the abomasum are displaced to the right by the rumen. The duodenum is V‐shaped with the ascending limb lying dorsally on the right at the level of the base of the caecum (mauve). The stomach and the duodenum are both coloured green on the diagram. The jejunoileum is not shown on the diagram but occupies a large space between the duodenum and the colon mainly on the right. The transverse and descending colon retain their basic position. The transverse colon passes cranial to the root of the mesentery which contains the cranial mesenteric artery (red circle).