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4a Peritoneum, Peritoneal Cavity, and Mesentery
Chapter Twenty-Six Digestive System
of the liver
(b) Mesentery proper and mesocolon
Omenta and Mesentery. Cadaver photos show anterior views of (a) the greater and lesser omenta, and (b) the mesentery proper and some of
(a, b) © McGraw-Hill Education/Christine Eckel
mesocolon is associated with the transverse colon, whereas the sigmoid mesocolon is associated with the sigmoid colon.
A peritoneal ligament is a peritoneal fold that attaches one
organ to another organ, or attaches an organ to the anterior or lateral
abdominal wall. Some examples of peritoneal ligaments include the
coronary ligament, a peritoneal fold attaching the superior surface
of the liver to the diaphragm at the margins of the bare area of the
liver; the falciform (fal′si-fōrm; falx = sickle) ligament, a peritoneal
fold that attaches the liver to the anterior internal abdominal wall;
and the lienorenal ligament, a fold of peritoneum between the spleen
and the kidney.
26.4b General Histology of GI Organs
(Esophagus to Large Intestine)
The GI tract from the esophagus through the large intestine is a tube
composed of four concentric layers, called tunics. From deep (the lining of the lumen) to superficial (the external covering), these tunics
are the mucosa, the submucosa, the muscularis, and the adventitia
or serosa (figure 26.9). The general pattern of the tunics is described
next. There are variations in the general pattern, which will be described in detail when we discuss the specific organs in which they
The mucosa (myū-kō ′să), or mucous membrane has three components: (1) an inner (superficial) epithelium lining the lumen of the GI
tract; (2) an underlying areolar connective tissue, called the lamina
propria; and (3) a relatively thin layer of smooth muscle, termed
the muscularis mucosae. The muscularis mucosae is very thin (or
absent) at the level of the laryngopharynx and thickens progressively
as it approaches the stomach.
For most of the abdominal GI tract organs, the lining epithelium is a simple columnar epithelium. The portions of the GI tract
that must withstand abrasion (such as the esophagus) are lined by a
nonkeratinized stratified squamous epithelium (see section 4.1e).
The submucosa is composed of either areolar or dense irregular connective tissue and has far fewer cells than the lamina propria. Submucosa components include: accumulations of lymphatic tissue in some
submucosal regions; mucin-secreting glands that project ducts across
the mucosa and open into the lumen of the tract in the esophagus and
duodenum; many large blood vessels and lymph vessels; and nerves
that extend fine branches into both the mucosa and the muscularis.
These nerve fibers and their associated ganglia are collectively referred
to as the submucosal nerve plexus (or Meissner plexus). It contains
sensory neurons, sympathetic postganglionic axons, and parasympathetic ganglia (see sections 14.2a, 18.4b, and 18.3, respectively).
The muscularis (mŭs′kyū-lā ′ris) typically contains two layers of
smooth muscle. Exceptions to this pattern include the esophagus
(which contains a mixture of skeletal and smooth muscle) and the
stomach (which contains three layers of smooth muscle). The fibers
of the inner layer of smooth muscle are oriented circumferentially
around the GI tract, and are called the inner circular layer. The
fibers of the outer layer are oriented lengthwise along the GI tract,
and are called the outer longitudinal layer.
Chapter Twenty-Six Digestive System
Submucosal nerve plexus
Inner circular layer
Myenteric nerve plexus
Outer longitudinal layer
Tunics of the Abdominal GI Tract. The wall of the abdominal GI tract has four tunics. From the lining of its lumen to the external covering, the tunics are the
mucosa, submucosa, muscularis, and adventitia or serosa.
If you think of the GI tract as a tube, then contractions of
the circular layer constrict the diameter of the tube lumen, whereas
contractions of the longitudinal layer shorten the tube. At specific
locations along the GI tract, the inner circular muscle layer is greatly
thickened to form a sphincter. A sphincter closes off the lumen
opening at some point along the GI tract, and in so doing it can help
control the movement of materials through the GI tract. The nerve
fibers and the associated ganglia located between the two layers of
smooth muscle control its contractions and are collectively referred
to as the myenteric (mı ′̄ en-ter′ik; mys = muscle, enteron = intestine)
nerve plexus (or Auerbach plexus).
Adventitia or Serosa
The outermost tunic may be either an adventitia or a serosa. An
adventitia (ad-ven-tish′ă) is composed of areolar c onnective tissue
with dispersed collagen and elastic fibers. A serosa (se-rō′să) has the
same components as the adventitia, but is covered by a visceral peritoneum. Intraperitoneal organs have a serosa, because they are completely surrounded by visceral peritoneum. Retroperitoneal organs
primarily have an adventitia, because these organs are only partially
covered by visceral peritoneum. For example, the ascending colon
(which is retroperitoneal) has an adventitia, whereas the stomach (an
intraperitoneal organ) has a serosa.
Chapter Twenty-Six Digestive System
You just learned that the “default” pattern of the tunics is:
1. Mucosa (typically lined with simple columnar epithelium)
3. Muscularis (typically formed from two layers of smooth muscle)
4. Adventitia or serosa
Now determine how the digestive organs follow or deviate from this
default pattern. For example:
■ The esophagus has these tunics, but it deviates from the pattern
in two ways: Its mucosa has a stratified squamous epithelium, and
its muscularis has a skeletal muscle in its superior region, skeletal
and smooth muscle in the middle region, and smooth muscle in its
■ The stomach has these tunics, but it deviates from the pattern
in that its muscularis has three layers of smooth muscle, not two.
■ The small intestine follows the basic “default” pattern of tunics.
■ The large intestine has these tunics, but in its muscularis, the
outer longitudinal layer of muscle forms three distinct bands called
teniae coli (described later in section 26.9b).
26.4c Blood Vessels, Lymphatic Structures,
and Nerve Supply
The GI tract has a rich blood and nerve supply. In addition, extensive
lymphatic structures along the length of the GI tract act as sentinels to
monitor for antigens that may have been ingested (see section 24.1). The
blood vessels, lymphatic structures, and nerves enter the GI tract from
either the surrounding structures (e.g., nearby organs, abdominal wall)
or via the supporting mesentery.
Branches of the celiac trunk, superior mesenteric artery, and
inferior mesenteric artery supply the abdominal GI tract (see section 23.3f). These artery branches split into smaller branches that
extend throughout the walls of the GI organs. Branches travel within
the tunics, and the mucosa contains capillaries that have fenestrated
endothelial cells to promote absorption. The veins arising in the mucosa form anastomoses in the submucosa before exiting the wall of
the GI tract adjacent to their companion arteries. Eventually, the veins
merge to form the hepatic portal system of veins (see sections 23.3f
Lymph Vessels and Tissues
Lymphatic capillaries arise as blind tubes in the mucosa of the GI
tract. In the small intestine, each villus usually contains a single,
blind-ended, central lymphatic capillary called a lacteal. Recall from
section 24.2a that lacteals are responsible for absorbing dietary lipids
and lipid-soluble vitamins (vitamins that can be absorbed only if
they are dissolved in lipids first). Outside the organ walls, lymphatic
capillaries merge to form lymphatic vessels. These vessels enter and
exit the many lymph nodes scattered near the organs and within the
mesentery. Eventually, this lymph will be transported to the cisterna
chyli, which drains into the thoracic duct.
The lymphatic structures within the GI tract lie primarily
in the lamina propria of the mucosa. Lymphatic structures called
MALT (mucosa-associated lymphatic tissue) are found in the
small intestine and appendix (see section 24.4a). In the small intestine, these aggregate nodules are called Peyer patches. They appear to the naked eye as oval bodies about the size of a pea, but are
often much larger structures. Less commonly, lymphatic structures
are found external to the simple epithelium throughout the stomach
and intestines where they are known as diffuse lymphatic tissue.
Also, solitary lymphatic nodules may occur in the esophagus, the
pylorus of the stomach, and along the entire length of the small and
The nerves associated with the GI tract consist of both autonomic
motor and visceral sensory axons (see sections 18.1 and 18.5a). There
are three main groups of autonomic plexuses:
The celiac plexus contains sympathetic axons (from the T5–T9
segments of the spinal cord) and parasympathetic axons (from
the vagus nerve). This plexus supplies structures that receive
their blood supply from branches of the celiac trunk.
■The superior mesenteric plexus contains sympathetic
axons (from the T8–T12 segments of the spinal cord) and
parasympathetic axons (from the vagus nerve). This plexus
transmits autonomic innervation to structures that receive
their blood supply from branches of the superior mesenteric
■The inferior mesenteric plexus contains sympathetic
axons (from the L1–L2 segments of the spinal cord) and
parasympathetic axons (from the pelvic splanchnic nerves).
This plexus supplies structures that receive blood from
branches of the inferior mesenteric artery.
In general, parasympathetic innervation promotes digestive
system activity by stimulating GI gland secretions and peristalsis,
and by relaxing GI sphincters. These changes in activity induce
vasodilation and an increase in GI blood flow. Sympathetic innervation opposes parasympathetic innervation by inhibiting peristalsis
and some GI gland secretions. It also closes the GI sphincters and
vasoconstricts the blood vessels to the GI tract (see sections 18.5b
W H AT D I D YO U LE A R N ?
Compare the terms intraperitoneal and retroperitoneal, and
give examples of each type of organ.
What are the four main tunics of the abdominal GI tract, and
what is the “default” pattern in each tunic?
13. Describe the structure and function of the esophagus.
The esophagus (ē-sof′ă-gŭs; gullet) is a tubular passageway for swallowed materials being conducted from the pharynx to the stomach
(see figure 26.1). The inferior region of the esophagus connects to the
stomach by passing through an opening in the diaphragm called the
esophageal hiatus (hı ̄-ā′tŭs; to yawn).
26.5a Gross Anatomy
The esophageal wall is thick and composed of concentric tunics that
are continuous superiorly with those of the pharynx and inferiorly
Chapter Twenty-Six Digestive System
with those of the stomach. In the adult human, the esophagus is about
25 centimeters (about 10 inches) long, and most of its length is within
the thorax, directly anterior to the vertebral bodies. Only the last
1.5 centimeters of the esophagus are located in the abdomen. The
empty esophagus is flattened; the lumen only opening slightly with
the passage of a food bolus.
The esophageal mucosa is different from that in the abdominal GI
tract organs in that it is composed of thick, nonkeratinized stratified squamous epithelium (figure 26.10). This epithelium is better
suited to withstand the abrasions of the bolus as it moves through
the esophagus (see section 4.1e). Because the esophagus does not
absorb any nutrients, this thicker, protective epithelium supports
The submucosa is thick and composed of abundant elastic
fibers that permit distension during swallowing. It houses numerous mucous glands that provide a thick, lubricating mucus for the
The muscularis is composed of an inner circular layer and an
outer longitudinal layer. The muscularis of the esophagus is unique
in that it contains both skeletal and smooth muscle. The two layers of
muscle in the superior one-third of the muscularis layer are skeletal,
rather than smooth, to ensure that the swallowed material moves
rapidly out of the pharynx and into the esophagus before the next
respiratory cycle begins. (Remember that smooth muscle contracts
more slowly than does skeletal muscle; see section 10.9b.) Striated
and smooth muscle fibers intermingle in the middle one-third of the
esophagus, and only smooth muscle is found within the wall of the
inferior one-third. This transition marks the beginning of a continuous smooth muscle muscularis that extends throughout the stomach
and the small and large intestines to the anus. The outermost layer
of the esophagus is an adventitia, which adheres it to the posterior
The superior esophageal sphincter (or pharyngoesophageal
sphincter) is a contracted ring of circular skeletal muscle at the superior end of the esophagus. It is the area where the esophagus and the
pharynx meet. This sphincter is closed during inhalation of air so it
enters the larynx and trachea instead of the esophagus.
The inferior esophageal sphincter (gastroesophageal, or c ardiac
sphincter) is a contracted ring of circular smooth muscle at the inferior end of the esophagus. This sphincter isn’t strong enough alone
(b) Esophagus, transverse section
(a) Pharynx and esophagus
Histology of the Esophagus. (a) The esophagus extends inferiorly
from the pharynx and conducts swallowed materials to the stomach.
(b) A photomicrograph of a transverse section through the esophagus
identifies the tunics in its wall. (c) A photomicrograph shows the
(b) © Alfred Pasieka/Getty Images; (c) © McGraw-Hill Education/Al Telser,
(c) Esophageal mucosa