The large intestine comprises the cecum, colon, rectum, and anal canal. In the carnivores such as the dog and cat, the colon is relatively small and the cecum is only a vestigial component.

In the ventrodorsal plane, the colon has the shape of a question mark and is anatomically subdivided into a short proximal ascending colon with ileocolic and cecocolic junctions or sphincters, a middle transverse colon, and a long descending colon that is continuous with the rectum and anal canal.

The two principal functions of the colon are (1) the absorption of electrolytes and water from the luminal content and (2) the temporary storage and periodic elimination of the resulting feces. Absorption is mostly a function of the proximal colon, whereas storage is mostly a function of the rectum and distal colon. The principal function of the anus is to maintain fecal continence between defecations.

Colonic mucosal cells actively absorb sodium and chloride; water follows passively. Within the colonic mucosa, tubular glands called the crypts of Lieberkuhn contain numerous mucus-secreting goblet cells. The circular and longitudinal muscle layers of the colon are responsible for the normal motility and “tone” of the colon, under the influ­ence of intrinsic and extrinsic innervation and gas­trointestinal (GI) hormones. Retrograde peristaltic contractions (cats only) and phasic segmentation contractions (both dogs and cats) mix and delay passage of the bowel contents, thereby promoting optimal absorption, whereas peristaltic contrac­tions (mass movements) propel the bowel contents downstream, eventually resulting in defecation. Defecation is a well-controlled act involving the colon, rectum, and anus under nervous system control.

In general, most diseases of the colon are man­ifested as either diarrhea or constipation, and thus colonic diseases are categorized as such for the purposes of discussion in this chapter. Perforation and volvulus of the colon are rare causes of an acute abdominal distress presentation. Diseases in or near the anus generally cause dyschezia, often accompanied by constipation and sometimes fecal incontinence.