Descending colon [Small colon]

The descending colon is the final and caudal-most segment of the large intestine in domestic animals. It continues from the transverse colon and extends caudally along the dorsal abdominal wall to terminate at the rectum. Its primary functions include water absorption, electrolyte regulation, and fecal storage prior to defecation.

Species-Specific Characteristics

• Ruminants (e.g., cattle, sheep): The descending colon is relatively long and coiled in ruminants. It begins at the end of the centrifugal gyri of the spiral colon and is suspended by the mesocolon descending toward the pelvic inlet.

• Equines (e.g., horses): In horses, the descending colon is notably long (also called the small colon) and characterized by its sacculations (haustra) and bands (taeniae). It functions not only in fecal formation but also in water reabsorption.

• Canines and Felines (dogs and cats): The descending colon is shorter and runs caudally along the left abdominal wall. It is relatively simple and lacks sacculations.

• Swine (pigs): Similar to ruminants, pigs have a long descending colon, but with less coiling.

Anatomical Relations

The descending colon typically lies on the left side of the abdomen, dorsal to the small intestine, and ventral to the lumbar vertebrae. It is supported by the mesocolon and closely associated with the dorsal body wall, ureters, and in females, the uterus.

Functions

• Reabsorption of water and electrolytes

• Formation and transport of feces

• Microbial fermentation (to a minor degree compared to ruminants’ forestomach or horse cecum)