The Digestion and Absorption of Food
535
Just below the epithelium is the lamina propria, which is
a layer of connective tissue through which pass small blood ves-
sels, nerve fi bers, and lymphatic vessels. (These structures do
not appear in Figure 15–6 but are in
Figure 15–7
.) The lamina
propria is separated from underlying tissues by the mucularis
mucosa, which is a thin layer of smooth muscle. The combina-
tion of these three layers—the epithelium, lamina propria, and
muscularis mucosa—is called the
mucosa
(see Figure 15–6).
Beneath the mucosa is the
submucosa,
which is a sec-
ond connective tissue layer. This layer contains a network of
nerve cells, termed the
submucosal plexus,
and blood and
lymphatic vessels whose branches penetrate into both the
overlying mucosa and the underlying layers of smooth muscle
called the
muscularis externa.
Contractions of these muscles
provide the forces for moving and mixing the gastrointestinal
contents. The muscularis externa has two layers: (1) a relatively
thick inner layer of
circular muscle,
whose fi bers are oriented
in a circular pattern around the tube so that contraction pro-
duces a narrowing of the lumen, and (2) a thinner outer layer
of
longitudinal muscle,
whose contraction shortens the
tube. Between these two muscle layers is a second network of
nerve cells known as the
myenteric plexus.
Finally, surrounding the outer surface of the tube is a
thin layer of connective tissue called the
serosa.
Thin sheets
Ducts from external exocrine
glands (liver, pancreas,
salivary glands)
Abdominal cavity
Mucosa
Submucosa
Muscularis
externa
Serosa
Epithelium
Lamina
propria
Muscularis
mucosa
Submucosal
nerve plexus
Circular
muscle
Myenteric
nerve plexus
Longitudinal
muscle
Major blood and
lymphatic vessels
Lumen of gastrointestinal tract
Endocrine cells
Mucous cells
Exocrine cells
Figure 15–6
Structure of the gastrointestinal wall in longitudinal section. Not shown are the smaller blood vessels and lymphatics, neural connections
between the two nerve plexuses, and neural terminations on muscles, glands, and epithelium.
Lumen
Capillaries
Nerve fiber
Venule
Lacteal
Muscularis
mucosa
Villus
Epithelial cells
Microvilli
Arteriole
Vein
Lymphatic
vessels
Artery
Figure 15–7
Structure of villi in small intestine.
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