Equine Alimentary System - Anatomy & Physiology

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Caecum

• On the right side of the abdomen.
• Very large, roughly 1m in length with a 30L capacity.
• Consists of a base, body and apex (blind ending).
• The base lies in the right dorsal part of the abdomen in contact with the abdominal roof.
• Apex lies on the ventral abdominal wall, and terminates at the level of the xiphoid cartilage.
• Exists at the junction with the ileum and colon.
  • The caecocolic orifice is where the caecum opens into the ascending colon. This exists as a transverse slit formed by a constriction of the ascending colon.
  • The ileum opens into the caecum at the ileal papilla. This is a small projection into the caecum housing the ileal sphincter and venous plexus that, together, control the ileal orifice.
• Taenia are present.
  • Taenia are formed by concentration of the longitudinal muscle layer.
  • Between the taenia are sacculations, or haustra.
  • Haustra appear as folds on the interior surface.
    • There are four taenia over the caecum: -dorsal - ventral - lateral -medial.
    • The dorsal taenia provides the attachment site for the ileocaecal fold, which joins the caecum to the ileum.
    • The lateral taenia provides the attachment site for the caecocolic fold, which joins the caecum to the ascending colon.
    • The ventral taenia is free.
    • The medial and lateral taenia are where the caecal vessels and lymph nodes are located.

In the horse, the caecum is responsible for the digestion of complex carbohydrates such as cellulose. Ingesta is regularly transported from the ileum to the caecum, this movement can be heard upon auscultation of the right dorsal quadrant of the caudal abdomen. Ausculatation of this area is carried out in assesment of colic.

Colon

Ascending colon

• The ascending colon is very large and takes up most of the ventral abdomen.
• It is the shape of a double "U", where one U is on top of the other.
• There are four limbs that lie parallel to each other, and three flexures that change the direction of the limbs.
• Sequence of the limbs and flexures of the ascending colon:
  • Right Ventral Colon (for those with an RVC bias remember, "the RVC comes first!")
  • Sternal Flexure
  • Left Ventral Colon
  • Pelvic Flexure
  • Left Dorsal Colon
  • Diaphragmatic Flexure
  • Right Dorsal Colon
    • The right ventral colon passes out of the caecocolic orifice and continues cranially to the xiphoid region.
    • The sternal flexure passes across the midline.
    • The left ventral colon runs caudally on the ventral abdominal floor.
    • Cranial to the pelvic inlet it turns dorsally and then cranially as the pelvic flexure.
    • The left dorsal colon runs cranially, parallel and dorsal to the left ventral colon.
    • At the diaphragm it turns caudally as the diaphragmatic flexure.
    • The right dorsal colon continues caudally. It is the shortest limb of the ascending colon.
• The transverse colon continues as the right dorsal colon turns medially.
• Taenia are present.
  • Different parts of the colon can be distinguished by the number of taenia present:
    • The right and left ventral colon and the sternal flexure have four taenia.
    • The left dorsal colon and pelvic flexure have one taenia.
    • The right dorsal colon and diaphragmatic flexure have three taenia.
    • The descending colon has two taenia.