Despite LargeAnatomicaI Differences in the Colons of Herbivores Compared to Omnivores and Carnivores, There Are Similarities in Motility

There are important similarities in motility among various species, even those with extensive anatomical differences. The purpose of this discussion is to describe the similarities in motility between species with simple colons and those with complex colons.

The equine hindgut, as an example of an herbivore hindgut, is complex and highly developed (see Figure 28-10). The cecum is large and separated into haustra. The equine cecum is unique among ceca of most species, even other herbivores, because a distinct, sphincterlike orifice joins it to the colon. The colon is divided into a large and a small por­tion, and the large colon is folded on itself so that three dis­tinct flexures exist. 'Γhe longitudinal muscles of the cecum and most areas of the colon are not evenly dispersed around the circumference of the gut. Instead, they form discrete bands, or teniae, that course along the longitudinal axis of the gut. The teniae divide the haustra longitudinally, giving the equine cecum and large colon a sacculated appearance.

Motility in the equine cecum consists of active segmenta­tion and mixing, along with occasional mass movements that appear to transfer large amounts of ingesta to the colon. Motility in the colon consists of segmentation, antiperistalsis, and peristalsis. A colonic pacemaker appears to exist at the pelvic flexure and creates an area of high resistance to flow that results in prolonged retention of material in the ventral portions of the large colon. The pelvic flexure pacemaker in the equine colon is similar in function to the colonic pace­maker in the transverse colon of the dog and cat. Little is known about regulation of motility in the equine small colon. The characteristic ball-shaped form of equine feces probably represents intense segmentation-type motility in the small colon, where the feces are formed (see Chapter 31).

In ruminants and swine, the hindgut consists of a cecum of intermediate complexity, a spiral colon, and a straight colon. Compared with other species, less is known about the hindgut motility of animals with spiral colons. There seems to be an area of high flow resistance at the flexure, or central point, of the spiral colon. This site of flow resistance may represent a pacemaker that generates antiperistaltic motility in the centripetal portion of the colon.