687
CHAPTER SEVENTEEN
Digestive System
The
colon
is divided into four parts—the ascending,
transverse, descending, and sigmoid colons. The
ascend-
ing colon
begins at the cecum and extends upward against
the posterior abdominal wall to a point just inferior to the
liver. There it turns sharply to the left (as the right colic, or
hepatic, flexure) and becomes the
transverse colon.
The
transverse colon is the longest and most movable part of
the large intestine. It is suspended by a fold of peritoneum
and sags in the middle below the stomach. As the transverse
colon approaches the spleen, it turns abruptly downward (as
the left colic, or splenic, fl exure) and becomes the
descend-
ing colon.
At the brim of the pelvis, the descending colon
makes an S-shaped curve, called the
sigmoid colon,
and
then becomes the rectum.
The
rectum
lies next to the sacrum and generally fol-
lows its curvature. The peritoneum F rmly attaches it to the
sacrum, and it ends about 5 centimeters inferior to the tip of
the coccyx, where it becomes the anal canal
(f g. 17.45)
.
Parts of the Large Intestine
The large intestine consists of the cecum, the colon, the rec-
tum, and the anal canal.
Figures 17.43
and
17.44
and refer-
ence plates 11, 12, 18, and 25 depict the large intestine.
The
cecum,
at the beginning of the large intestine, is a
dilated, pouchlike structure that hangs slightly inferior to
the ileocecal opening. Projecting downward from it is a nar-
row tube with a closed end called the
appendix.
The human
appendix has no known digestive function. However, it con-
tains lymphatic tissue. One suggested function of the appen-
dix is that it sequesters useful bacteria when a person has
diarrhea and vomits due to an infection.
In
appendicitis,
the appendix becomes inf
amed and inFected. Surgery
is required to prevent the appendix From rupturing. IF it breaks open,
the contents oF the large intestine may enter the abdominal cavity
and cause a serious inFection oF the peritoneum called
peritonitis.
Anal canal
Sigmoid colon
Epiploic
appendage
Haustra
Descending colon
Tenia coli
Rectum
Appendix
Cecum
Orifice of appendix
Ileocecal sphincter
Ascending colon
Serous layer
Mucous membrane
Transverse colon
Splenic
flexure
Muscular layer
Ileum
Hepatic flexure
FIGURE 17.43
Parts oF the large intestine (anterior view).
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