Page 956 - Fundamentals of anatomy physiology

(2) the colon, the largest portion; and (3) the rectum, the last
15 cm (6 in.) of the large intestine and the end of the digestive
tract (Figure 24–24a).
The Cecum
Material arriving from the ileum first enters an expanded
pouch called the cecum (SE. -kum). The ileum attaches to
the medial surface of the cecum and opens into the cecum at
the ileocecal valve (Figure 24–24a,b). The cecum collects and
stores materials from the ileum and begins the process of
compaction.
The slender, hollow appendix, or vermiform appendix
(vermis, worm), is attached to the posteromedial surface
of the cecum (Figure 24–24a,b). The appendix is normally
about 9 cm (3.6 in.) long, but its size and shape are quite
variable. A small mesentery called the mesoappendix con-
nects the appendix to the ileum and cecum. Lymphoid nod-
ules dominate the mucosa and submucosa of the appendix.
The primary function of the appendix is as an organ of the
lymphatic system. Inflammation of the appendix is known
as appendicitis.
The Colon
The colon has a larger diameter and a thinner wall than the
small intestine. Distinctive features of the colon include the
following (Figure 24–24a):
The wall of the colon forms a series of pouches, or haustra
(HAWS-truh; singular, haustrum). The creases between the
haustra affect the mucosal lining as well, producing a series
of internal folds. Haustra permit the colon to expand and
elongate.
Three separate longitudinal bands of smooth muscle—
called the teniae coli (TE. -ne. -e. KO. -le. )—run along the
outer surfaces of the colon just deep to the serosa. These
bands correspond to the outer layer of the muscularis ex-
terna in other portions of the digestive tract. Muscle tone
within the teniae coli is what creates the haustra.
The serosa of the colon contains numerous teardrop-
shaped sacs of fat called omental appendices, also
called fatty or epiploic (ep-i-PLO. -ik; epiploon, omentum)
appendages.
We can subdivide the colon into four regions: the ascend-
ing colon, transverse colon, descending colon, and sigmoid
colon (Figure 24–24a).
1. The ascending colon begins at the superior border of the
cecum and ascends along the right lateral and posterior
wall of the peritoneal cavity to the inferior surface of the
Chapter 24 The Digestive System 943
liver. There, the colon bends sharply to the left at the right
colic flexure, or hepatic flexure. This bend marks the end
of the ascending colon and the beginning of the transverse
colon.
2. The transverse colon curves anteriorly from the right colic
flexure and crosses the abdomen from right to left. The
transverse colon is supported by the transverse mesocolon.
It is separated from the anterior abdominal wall by the lay-
ers of the greater omentum. At the left side of the body, the
transverse colon passes inferior to the greater curvature of
the stomach. Near the spleen, the colon makes a 90° turn
at the left colic flexure, or splenic flexure, and becomes the
descending colon.
3. The descending colon proceeds inferiorly along the left
side until reaching the iliac fossa formed by the inner sur-
face of the left ilium. The descending colon is retroperito-
neal and firmly attached to the abdominal wall. At the iliac
fossa, the descending colon curves at the sigmoid flexure
and becomes the sigmoid colon.
4. The sigmoid flexure is the start of the sigmoid (SIG-
moyd) colon (sigmeidos, the Greek letter S), an S-shaped
segment that is only about 15 cm (6 in.) long. The sig-
moid colon lies posterior to the urinary bladder, sus-
pended from the sigmoid mesocolon. The sigmoid colon
empties into the rectum.
Clinical Note 24
Colorectal Cancer Colorectal cancer is relatively com-
mon in the United States. Aside from skin cancers, colorec-
tal cancer is the third most common cancer in the United
States, affecting both men and women. At the writing
of this edition, the American Cancer Society’s statistics
showed that there were 102,480 new cases of colon cancer
and 40,340 new cases of rectal cancer. The death rate has
declined over the past 20 years for both men and women.
The best defense appears to be early detection and prompt
treatment. Standard screening involves checking the feces
for blood. This simple procedure can be performed easily
on a stool (fecal) sample as part of a routine physical.
For individuals at increased risk because of family history,
associated disease, or older age, visual inspection of the
lumen by fiber-optic colonoscopy is prudent. Such exami-
nation can find polyps before they develop into cancers.
The 5-year survival rate for people whose cancer is found
at an early stage and treated immediately is greater than
90 percent.