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4a Peritoneum, Peritoneal Cavity, and Mesentery

4a Peritoneum, Peritoneal Cavity, and Mesentery

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Chapter Twenty-Six  Digestive System

Greater omentum



Falciform ligament

Round ligament

of the liver

Lesser omentum


Transverse colon



Greater omentum

Mesentery proper

Small intestine

(a) Omenta

(b) Mesentery proper and mesocolon

Figure 26.8

Omenta and Mesentery. Cadaver photos show anterior views of (a) the greater and lesser omenta, and (b) the mesentery proper and some of

the mesocolon.

(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




Lamina propria









Submucosal gland

Blood vessel


Submucosal nerve plexus


Inner circular layer

Myenteric nerve plexus

Outer longitudinal layer


Figure 26.9

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

Learning Strategy

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

inferior region.

■ 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.

Blood Vessels

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

and 26.10a).

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

large intestines.


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

and 18.5e).




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?

26.5  Esophagus

✓✓Learning Objective

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.

26.5b  Histology

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

its function.

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

body wall.

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




Superior esophageal











Inferior esophageal



LM 11x

(b) Esophagus, transverse section

(a) Pharynx and esophagus

Figure 26.10

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

esophageal mucosa.

Stratified squamous



Lamina propria

(b) © Alfred Pasieka/Getty Images; (c) © McGraw-Hill Education/Al Telser,


Muscularis mucosae

LM 65x

(c) Esophageal mucosa

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