Colon - Anatomy & Physiology

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Introduction

The colon can be divided into the following portions:

• Ascending
• Transverse
• Descending

Structure

The following anatomocial arrangement is found only in cats and dogs, see species differences.

• The ascending colon continues from the iluem at the ileocolic junction.
  • It runs to the right of the cranial mesenteric artery in a caudal to cranial direction.
  • At the cranial border of the mesentry it turns medially to become the transverse colon.
• The transverse colon runs from the right side of the abdomen to the left side of the abdomen.
  • Cranial to the transverse colon is the stomach, and caudal to it is the small intestine and cranial mesenteric artery.
• The descending colon continues on from the transverse colon running caudally on the left from the level of (???).
  • It then passes more medially as it enters the pelvic cavity
  • Upon entering the pelvic cavity it is continued as the rectum.

Species Differences

Ruminant

• The ascending colon is the longest part of the colon and is composed of three parts:
  • Ansa proximalis
    • Has a sigmoid flexure that passes around the caudal border of the mesentry to the left side of the root of the mesentry.
  • Ansa spiralis
    • Consists of two centripetal turns and two centrifugal turns in the ox.
      • There are three turns in the sheep and four in the goat.
      • In the ox, the ansa spiralis is a flat disc, whilst in the small ruminants it takes the form of a cone.
  • Ansa distalis
    • Goes back around the caudal border of the mesentry, to the right side of the root of mesentry.
    • It then passes cranially adjacent to the mesentry until it reaches the cranial border of the mesentry.
• The transverse colon crosses the midline of the abdomen, from right to left at the cranial border of the mesentry.
• The descending colon continues caudally to the rectum and anus.
  • It has a sigmoid flexure before it enters the pelvic cavity.

Pig

• The arrangement of the transverse and descending colon is similar to that of the dog and cat, but the ascending colon is different.
  • The ascending colon is elongated and coiled to form a cone-shaped organ.
  • The base of the cone is attached to the dorsal abdominal wall and the apex points ventrally.
  • The position of the ascending colon varies with filling of the stomach.
  • From the caecum, there are clockwise centripetal turns to the apex of the cone.
  • Then the centrifugal turns run anti-clockwise on the inside of the cone.
    • Centripetal turns have two taenia, whilst centrifugal turns have none.