CHAPTER 15
Digestive Tract
291
Small Intestine
Figure 15-9.
Overview of the small intestine.
The
small intestine
is a very long, tubular organ, about 6 to 7 m long, with a relatively small diameter. It connects the stomach to the
large intestine and can be divided into three regions based on anatomy and function. (1) The
duodenum
, the most proximal region
of the small intestine, is a short, C-shaped segment about 20 to 25 cm long. Mucous glands called
Brunner glands
are present only
in the duodenum. Bile and pancreatic secretions enter the duodenum through their duct systems. (2) The
jejunum
makes up about
two F fths of the rest of the small intestine. It has a larger diameter and thicker wall than the ileum. The jejunum has long villi and
has neither Brunner glands nor Peyer patches. (3) The
ileum
is the most distal portion of the intestine, and it makes up about three
F fths of the small intestine. The ileum has a thinner wall and fewer villi than the jejunum, and it has clusters of lymphatic nodules,
called
Peyer patches
, in the lamina propria. This illustration shows the general tunics (layers) of the small intestine. Like the other
parts of the digestive tract, the small intestine consists of a mucosa (epithelium, lamina propria, and muscularis mucosae), submu-
cosa, muscularis externa, and serosa/adventitia. Several
myenteric
(
Auerbach
)
plexuses
are illustrated between the two layers of the
muscularis externa; submucosal (Meissner) plexuses are located in the submucosal layer.
Mucosa
Myenteric (Auerbach) plexus
Jejunum
Submucosa
Epithelium
Lamina
propria
Muscularis
mucosae
Vein
Submucosal
(Meissner) plexus
Artery
Nerve
Muscularis
externa
Serosa/adventitia
Esophagus
Stomach
Inner circular
muscle
Outer
longitudinal
muscle
Duodenum
Small Intestine
I. Duodenum
A. Mucosa
B. Submucosa (Brunner glands)
C. Muscularis externa
D. Serosa/adventitia
II. Jejunum
A. Mucosa
B. Submucosa
C. Muscularis externa
D. Serosa
III. Ileum
A. Mucosa (Peyer patches)
B.
Submucosa (Peyer patches may extend into this layer)
C. Muscularis externa
D. Serosa
Cell Types in the Small Intestine
Villi: columnar absorptive cells and goblet cells
Glands (crypts) of Lieberkühn: absorptive cells, goblet cells,
Paneth cells, enteroendocrine cells, and stem cells
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