Overview
The small colon (colon descendens) continues the transverse colon to the left of the root of the mesentery and reaches the pelvic inlet, where it becomes the rectum. It hangs from a long descending mesocolon, so its loops move freely in the left dorsal quadrant of the abdomen, among those of the jejunum. Water is absorbed here and the contents are shaped into the horse's characteristic faecal balls.
Parts
- Descending colon: much narrower than the large colon, with two taeniae, one on the mesenteric border and a free one on the antimesenteric border, and two rows of sacculations.
- Rectum: the cranial part is covered by peritoneum and suspended by the mesorectum; the caudal part is retroperitoneal and widens into the rectal ampulla, surrounded by connective tissue and fat in the pelvic cavity. It ends at the anal canal.
Clinical relevance
- Rectal palpation: the basis of the examination of the colicky horse and of the mare's reproductive tract. On the left it reaches the caudal pole of the left kidney, the nephrosplenic ligament and the edge of the spleen; on the right, the base and ventral taenia of the caecum; and the bladder, uterus and ovaries.
- Rectal tear: the most feared complication of palpation; it usually occurs in the dorsal wall. It is graded I to IV by the layers involved; grades III and IV carry a risk of septic peritonitis. Adequate relaxation (sedation, spasmolytics) and lubrication prevent it.
- Small colon impaction: more common in ponies and miniature horses; enteroliths or faecaliths blocking the lumen also cause it.
- Meconium retention: the most common cause of colic in the newborn foal, especially colts; the meconium is held up in the small colon and rectum and is cleared with enemas.
- Mesocolic tear: in mares at foaling the mesocolon can tear and leave a segment of small colon without blood supply.