392 Chapter 16
Hepatic (^) Splenic
flexure^ flexure^
(^) Transverse (^)
colon (^)
Ascending
colon^
Descending
colon
Ileocecal valve^
Ileum
Sigmoid
colon
Cecum (^)
Vermiform Rectum (^)
appendix (^)
Anal canal
Internal anal sphincter^
External anal sphincter
(^)
Anus
(^)
Figure 16- 14 The anatomy of the large intestine.
(^) ®
Learning
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Three mechanical movements occur in the large
intestine:
■■ Haustral churning
■■ Peristalsis at the rate of 3 to 12 contractions per minute
■■ Mass peristalsis
Food in the stomach initiates mass peristalsis, which is
a strong peristaltic wave that begins in the middle of the
transverse colon and drives its contents into the rectum.
The rectum is the last 7 to 8 inches of the gastroin-
testinal tract. It is situated anterior to the sacrum and the
coccyx (review Chapter 7). The terminal 1 inch of the
rectum is called the anal canal. The mucous membrane of
the anal canal is arranged in a series of longitudinal folds
called the anal columns that contain a network of arteries
and veins.
The opening of the anal canal to the exterior is called
the anus. It is guarded by an internal sphincter of smooth
muscle and an external sphincter of skeletal muscle.
The absorption of water is an important function of the
large intestine. In addition, bacteria in the colon man-
ufacture three important vitamins that are also absorbed in
the colon: vitamin K needed for clotting, biotin needed
for glucose metabolism, and vitamin B 5 needed to make
certain hormones and neurotransmitters. Mucus is also
produced by glands in the intestine. Intestinal water ab-
sorption is greatest in the cecum and ascending colon.
Media Link
Watch an animation about digestion on the
Student Companion
Website..
The Formation of The Feces
By the time the chyme has remained in the large intes-tine
for 3 to 10 hours, it is a semisolid mass of material as a
result of the absorption of water and is now known as the
feces. The feces consist of water, inorganic salts, and
epithelial cells from the mucosa of the tract that were
scraped away as the chyme moved through the tract. In
addition, the feces have bacteria, in particular
Escherichia coli, a normal inhabitant of our intestine that
feeds on undigested materials. The products of bacterial