Equine large and small colon
This
is a dorsal view of the cecum, large colon and small colon of a horse. The ileum
(A), which is the terminal portion of the small intestine, travels from left to
right to enter the base of the cecum (B). The contents of the enlarged
cecum must pass through an approximately 3-4 cm hole, the cecocolic orifice,
to enter the colon. Does that sound like a likely place for an obstruction to occur? Well,
it is. The cecal contents pass through the cecocolic orifice into the right
ventral colon (C). The right ventral colon travels cranially to the level of the
xiphoid, turns and heads caudally as the left ventral colon (D).
Cellulose digestion and fermentation occurs primarily in the cecum and the ventral colon
in these "hindgut-fermentors." As the left ventral
colon reaches the pelvic area, it narrows at the pelvic flexure (E) and
turns dorsally to become the left dorsal colon (F). Don't you just love
these descriptive names? The pelvic flexure, because of the narrowing, is another good
place for blockage to occur. The left dorsal colon travels cranially, turns at the
diaphragm, and then continues caudally on the right side as the right dorsal colon
(G). Now we are back on the side we started. Cranial to the root of the
mesentery, the colon then travels from right to left as the transverse colon (H).
Because of the narrowing of the transverse colon, this is another prime spot for blockage
to occur. Remember the short ascending colon in the dog, which was present between the
cecum and the transverse colon? Well, all of that between the cecum and transverse colon
(right ventral colon, left ventral colon, left dorsal colon, and right dorsal colon),
which we refer to as the large colon, is a modification of that short
ascending colon. The transverse colon then turns caudally to become the descending
colon (I), which is also known as the small colon in the horse.