688
UNIT FIVE
Structure of the Large Intestinal Wall
The wall of the large intestine includes the same types of tis-
sues found in other parts of the alimentary canal but also has
some unique features
(f g. 17.46)
. The large intestinal wall
lacks the villi and plicae circularis characteristic of the small
intestine. Also, the layer of longitudinal muscle F bers is not
uniformly distributed throughout the large intestine wall; the
F bers are in three distinct bands (teniae coli) that extend the
entire length of the colon. These bands exert tension length-
wise on the wall, creating a series of pouches (haustra). The
large intestinal wall also has small collections of fat (epiploic
appendages) in the serosa on its outer surface (see F g. 17.43).
Functions of the Large Intestine
The large intestine has little or no digestive function. This
is in contrast to the small intestine, which secretes digestive
enzymes and absorbs the products of digestion. However,
the mucous membrane that forms the inner lining of the
large intestine includes many tubular glands. Structurally,
these glands are similar to those of the small intestine,
but they are composed almost entirely of goblet cells
(f
g.
17.47)
. Consequently, mucus is the only signiF
cant secre-
tion of this part of the alimentary canal.
Mechanical stimulation from chyme and parasympa-
thetic impulses control the rate of mucous secretion. In both
cases, the goblet cells respond by increasing mucous pro-
duction, which, in turn, protects the intestinal wall against
the abrasive action of materials passing through it. Mucus
also holds particles of fecal matter together, and, because it
is alkaline, mucus helps control the pH of the large intesti-
nal contents. This is important because acids are sometimes
released from the feces as a result of bacterial activity.
Hemorrhoids
are enlarged and inf
amed branches oF the rectal vein
in the anal columns that cause intense itching, sharp pain, and
sometimes bright red bleeding. The hemorrhoids may be internal
(which do not produce symptoms) or bulge out oF the anus. Causes
oF hemorrhoids include anything that puts prolonged pressure on
the delicate rectal tissue, including obesity, pregnancy, constipation,
diarrhea, and liver disease.
Eating more ±
ber-rich Foods and drinking lots oF water can usu-
ally prevent or cure hemorrhoids. Warm soaks in the tub, cold packs,
and careFul wiping oF painFul areas also help, as do external creams
and ointments. Surgery—with a scalpel or a laser—can remove
severe hemorrhoids.
The
anal canal
is formed by the last 2.5 to 4.0 centi-
meters of the large intestine. The mucous membrane in the
canal is folded into a series of six to eight longitudinal
anal
columns.
At its distal end, the canal opens to the outside as
the
anus.
Two sphincter muscles guard the anus—an
inter-
nal anal sphincter muscle,
composed of smooth muscle under
involuntary control, and an
external anal sphincter muscle,
composed of skeletal muscle under voluntary control.
PRACTICE
58
What is the general Function oF the large intestine?
59
Describe the parts oF the large intestine.
60
Distinguish between the internal sphincter muscle and the
external sphincter muscle oF the anus.
FIGURE 17.44
Radiograph oF the large intestine containing a
radiopaque substance that the patient ingested.
External
anal
sphincter
Rectum
Levator ani
muscle
Anal canal
Anal columns
Internal anal
sphincter
Anus
FIGURE 17.45
The rectum and the anal canal are at the distal end
oF the alimentary canal.
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