665
CHAPTER SEVENTEEN
Digestive System
PRACTICE
17
Describe the regions of the pharynx.
18
List the major events of swallowing.
19
What is the function of the esophagus?
17.6
STOMACH
The
stomach
(stum
ak) is a J-shaped, pouchlike organ,
about 25–30 centimeters long, which hangs inferior to the
diaphragm in the upper-left portion of the abdominal cavity
(see F gs. 17.1 and 17.15; reference plates 4 and 5). The stom-
ach has a capacity of about one liter or more. Its inner lining
has thick gastric folds (rugae) of the mucosal and submucosal
layers that disappear when the wall is distended. The stomach
receives food from the esophagus, mixes it with gastric juice,
initiates the digestion of proteins, carries on limited absorp-
tion, and moves food into the small intestine.
In addition to the two layers of smooth muscle—an
inner circular layer and an outer longitudinal layer—also
in other regions of the alimentary canal, some parts of the
stomach have an additional inner layer of oblique fibers,
which strengthen the stomach wall and help the mixing and
churning. This third innermost muscular layer is most highly
developed near the opening of the esophagus and in the
body of the stomach (F
g. 17.17).
Parts of the Stomach
The stomach, shown in
f gures
17.17 and
17.18
and reference
plate 11, can be divided into the cardia, fundus, body, and
pylorus. The
cardia
is a small area near the esophageal open-
ing. The
fundus,
which balloons superior to the cardia, is a
temporary storage area and sometimes F lls with swallowed
Esophagus
The
esophagus
(e
˘-sof
ah-gus) is a straight, collapsible tube
about 25 centimeters long. It provides a passageway for
food, and its muscular wall propels food from the pharynx
to the stomach. The esophagus descends through the thorax
posterior to the trachea, passing through the mediastinum.
It penetrates the diaphragm through an opening, the
esopha-
geal hiatus
(e
˘-sof
ah-je
al hi-a
tus), and is continuous with
the stomach on the abdominal side of the diaphragm (
f
g.
17.15
and reference plates 17, 23).
In a
hiatal hernia,
part of the stomach protrudes through a weakened
area of the diaphragm, through the esophageal hiatus and into the
thorax. Regurgitation (reF
u×) of gastric juice into the esophagus may
inF
ame the esophageal mucosa, causing heartburn, di±
culty in swal-
lowing, or ulceration and blood loss. In response to the destructive
action of gastric juice, columnar epithelium may replace the squamous
epithelium that normally lines the esophagus (see chapter 5, pages
147–148). This condition, called
Barrett’s esophagus,
increases the risk
of developing esophageal cancer.
Mucous glands are scattered throughout the submucosa
of the esophagus
(f
g. 17.16)
. Their secretions moisten and
lubricate the inner lining of the tube.
Just superior to the point where the esophagus joins
the stomach, some of the circular muscle fibers have
increased sympathetic muscle tone, forming the
lower
esophageal sphincter
(loh
er e
˘-sof
ah-je
al sF
ngk
ter), or
cardiac sphincter
(f
g. 17.17)
. These F
bers usually remain
contracted, and they close the entrance to the stomach. In
this way, they help prevent regurgitation of the stomach
contents into the esophagus. When peristaltic waves reach
the stomach, the muscle F
bers that guard its entrance relax
briefl
y, and the swallowed food enters.
FIGURE 17.15
The esophagus is a passageway between the
pharynx and the stomach.
Esophagus
Diaphragm
Stomach
Esophageal
hiatus
Mucosa
Lumen
Submucosa
Muscular
layer
FIGURE 17.16
This cross section of the esophagus shows its
muscular wall (10×).
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