681
CHAPTER SEVENTEEN
Digestive System
A f lmy, double Fold oF peritoneal membrane called the
greater omentum
drapes like an apron From the stomach over
the transverse colon and the Folds oF the small intestine
(f g.
17.34)
. IF the wall oF the alimentary canal becomes inFected,
cells From the omentum may adhere to the infl
amed region
and help seal it oFF, lowering the risk that the inFection will
spread to the peritoneal cavity.
Structure of the Small Intestinal Wall
The inner wall oF the small intestine has a velvety appear-
ance throughout its length, due to many tiny projections oF
mucous membrane called
intestinal villi
(
f
gs. 17.35
and
17.36
; see f g. 17.3). These structures are most numerous in
the duodenum and the proximal jejunum. They project into
the lumen oF the alimentary canal, contacting the intestinal
contents. Villi greatly increase the surFace area oF the intesti-
nal lining, aiding absorption oF digestive products.
Each villus consists oF a layer oF simple columnar epi-
thelium and a core oF connective tissue containing blood
capillaries, a lymphatic capillary called a
lacteal,
and nerve
f bers (see f
g. 17.35). At their Free surFaces, the epithelial
cells have many f ne extensions called
microvilli
that Form a
brushlike border and greatly increase the surFace area oF the
intestinal cells, Further enhancing absorption (see
f
gs.
17.3
and
17.37
). The blood capillaries and lacteals carry away
absorbed nutrients, and impulses transmitted by the nerve
f bers can stimulate or inhibit activities oF the villus.
Between the bases oF adjacent villi are tubular
intesti-
nal glands
(crypts oF Lieberkühn) that dip into the mucous
Duodenum
Visceral peritoneum
Pancreas
Rectum
Greater omentum
Transverse colon
Stomach
Liver
Lesser omentum
Small intestine
Peritoneal cavity
Urinary bladder
Parietal peritoneum
Mesentery
FIGURE 17.33
Mesentery formed by folds of the peritoneal
membrane suspends parts of the small intestine from the posterior
abdominal wall.
Liver
Stomach
Gallbladder
Transverse colon
underneath
Greater
omentum
Simple columnar
epithelium
Lacteal
Blood capillary network
Intestinal gland
Goblet cells
Arteriole
Venule
Lymph vessel
Villus
FIGURE 17.34
The greater omentum hangs like an apron over the
abdominal organs.
FIGURE 17.35
Structure of a single intestinal villus.
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