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2a Cheeks, Lips, and Palate

2a Cheeks, Lips, and Palate

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Chapter Twenty-Six  Digestive System

Superior lip

Superior labial frenulum


palatine folds

Hard palate

Soft palate




Hard palate

Oral cavity

Soft palate

Palatoglossal arch

Palatopharyngeal arch

Palatine tonsil





Palatine tonsil



Lingual tonsil



Salivary duct orifices


Lingual frenulum



Inferior labial frenulum






Inferior lip

(a) Oral cavity, anterior view

(b) Sagittal section

Figure 26.3

Oral Cavity. (a) Ingested food and drink enter the GI tract through the oral cavity, shown here in anterior view. (b) A diagrammatic sagittal section shows the

structures of the oral cavity and the pharynx.

palate), whereas the posterior one-third is soft and muscular (called the

soft palate). The hard palate is formed by the palatine processes of the

maxillae and the horizontal plates of the palatine bones. It is covered

with dense connective tissue and nonkeratinized stratified squamous

epithelium and exhibits prominent transverse palatine folds, or friction ridges, that assist the tongue in manipulating ingested materials

prior to swallowing. The arching soft palate is primarily composed of

skeletal muscle and covered with nonkeratinized stratified squamous

epithelium. Extending inferiorly from the posterior part of the soft palate is a conical median projection called the uvula (yū′vyū-lă; uva =

grape). When you swallow, the soft palate and the uvula elevate to close

off the posterior entrance into the nasopharynx and prevent ingested

materials from entering the nasal region.

The fauces represent the opening between the oral cavity and

the oropharynx. The fauces are bounded by paired muscular folds:

the palatoglossal arch (anterior fold containing the palatoglossus

muscle) and the palatopharyngeal arch (posterior fold containing

the palatopharyngeal muscle). The palatine tonsils are housed between the arches (see section 24.4a). These tonsils serve as an “early

line of defense” as they monitor ingested food and drink for antigens,

and initiate an immune response when necessary.

26.2b  Tongue

The tongue (tŭng) is an accessory digestive organ that is formed

primarily from skeletal muscle and covered with stratified squamous

epithelium. Numerous small projections, termed papillae (pă-pil′ē;

sing., papilla; papula = pimple), cover the superior (dorsal) surface of

the tongue (see section 19.3a). The tongue’s stratified squamous epithelium is keratinized over the filiform papillae but nonkeratinized over

the rest of the tongue. Sections 25.1a and 25.3a described the tongue as

a participant in sound production. In addition, the tongue manipulates

and mixes ingested materials during chewing and helps compress the

materials against the palate to turn them into a bolus. The tongue also

performs important functions in swallowing. The inferior surface of

the tongue attaches to the floor of the oral cavity by a thin, vertical mucous membrane, the lingual frenulum (figure 26.3a). In addition, the

posteroinferior surface of the tongue contains lingual tonsils. Extrinsic

and intrinsic skeletal muscles move the tongue (see section 11.1d).

26.2c  Salivary Glands

The salivary glands collectively produce and secrete saliva ­(să-lı ′̄ vă),

a fluid that assists in the initial activities of digestion. The volume of

saliva secreted daily ranges between 1.0 and 1.5 liters. Most saliva is

produced during mealtime, but smaller amounts are produced continuously to ensure that the oral cavity mucous membrane remains moist.

Water makes up 99.5% of the volume of saliva and is its primary ingredient. Saliva also contains a mixture of other components (table 26.1).

Saliva has various functions. It moistens ingested food and helps

it become a slick, semisolid bolus that is more easily swallowed. Saliva

also moistens, cleanses, and lubricates the oral cavity structures. The first

step in chemical digestion occurs when amylase in saliva begins to break

down carbohydrates. Saliva contains antibodies and an antibacterial

substance called lysosyme that help inhibit bacterial growth in the oral

cavity. Finally, saliva is the watery medium into which food molecules

are dissolved so that taste receptors on the tongue can be stimulated.

A small amount of saliva is produced by unicellular glands

within the oral cavity, collectively known as intrinsic salivary

glands. These intrinsic salivary glands are responsible for secreting

lingual lipase, an enzyme that is activated by the low pH levels in the

stomach to break down lipids. Most saliva, however, is produced from

multicellular exocrine glands outside the oral cavity called extrinsic

salivary glands. Three pairs of multicellular salivary glands are

located external to the oral cavity: the parotid, submandibular, and

sublingual glands (figure 26.4a).

The parotid (pă-rot′id; para = beside, ot = ear) salivary

glands are the largest salivary glands. Each parotid gland is located

anterior and slightly inferior to the ear, partially overlying the masseter muscle. The parotid salivary glands produce about 25–30% of the

saliva, which is conducted through the parotid duct to the oral cavity. The parotid duct extends from the gland, parallel to the zygomatic

arch, before penetrating the buccinator muscle and opening into the

vestibule of the oral cavity near the second upper molar.


Chapter Twenty-Six  Digestive System

Table 26.1

Saliva Characteristics

Production Rate

pH Range

Composition of Saliva

Solute Components

Neural Control of Saliva


1–1.5 L/day

Slightly acidic (pH 6.4 to 6.8)

99.5% water; 0.5% solutes

Ions (e.g., Na+, K+, chloride,


Immunoglobulin A (helps decrease

bacterial infections)

Lysozyme (antibacterial enzyme)


Lingual lipase (from intrinsic

salivary glands)

Salivary amylase (enzyme that

breaks down carbohydrates)

Parasympathetic axons in CN IX

stimulate parotid salivary gland


Parasympathetic axons in CN VII

stimulate submandibular and

sublingual salivary gland secretions

Sympathetic activation from cervical

ganglia stimulates mucin secretion

Parotid salivary gland

Figure 26.4

Parotid duct

Masseter muscle

Salivary Glands. Saliva is produced primarily by three pairs of

extrinsic salivary glands. (a) The relative locations of the parotid,

submandibular, and sublingual salivary glands are shown in a

diagrammatic sagittal section. (b) Serous and mucous alveoli

are shown in a diagrammatic representation of salivary gland

histology. (c) A photomicrograph reveals the histologic detail of the

submandibular salivary gland.

(c) © McGraw-Hill Education/Al Telser, photographer

Mucosa (cut)

Sublingual ducts

Submandibular duct

Sublingual salivary gland

Mylohyoid muscle (cut)

Submandibular salivary gland

(a) Salivary glands

Salivary duct

Mucous cell

Mucous cells

Salivary duct

Mucous alveolus

Duct epithelium

Mixed alveoli

Serous alveolus

LM 200x

Serous cell

(b) Salivary gland histology

(c) Submandibular salivary gland

Serous cells


Chapter Twenty-Six  Digestive System

Table 26.2

Extrinsic Salivary Gland Characteristics

Salivary Gland

Structure and Location

Types of Secretion

Percentage of Saliva Produced


Largest of the salivary glands; located

anterior and slightly inferior to ears;

parotid duct conducts secretions into the

vestibule near upper 2nd molar

Only serous secretions



Located inferior to mandibular body;

submandibular duct opens lateral to

lingual frenulum

Both mucus and serous secretions



Smallest of the salivary glands; located

inferior to tongue; tiny sublingual ducts

open into floor of oral cavity

Both mucus and serous secretions


As their name suggests, the submandibular (sŭb′man-dib′

yū-lăr) salivary glands are inferior to the body of the mandible. They

produce most of the saliva (about 60–70%). A submandibular duct

transports saliva from each gland through a papilla in the floor of the

mouth on the lateral sides of the lingual frenulum.

The sublingual (sŭb-ling′gwăl) salivary glands are inferior to the

tongue and internal to the oral cavity mucosa. Each sublingual salivary

gland extends multiple tiny sublingual ducts that open onto the inferior

surface of the oral cavity, posterior to the submandibular duct papilla.

These tiny glands contribute only about 3–5% of the total saliva.

Two types of secretory cells are housed in the salivary glands:

mucous cells and serous cells (figure 26.4b, c). Mucous cells secrete

mucin, which forms mucus upon hydration, whereas serous cells secrete a watery fluid containing ions, lysozyme, and salivary amylase.

The proportion of mucous cells to serous cells varies among the three

types of salivary glands. The submandibular and sublingual glands

produce both serous and mucus secretions, whereas the parotid

glands produce only serous secretions. Table 26.2 summarizes the

structure of the salivary glands and their secretions.

The salivary glands are primarily innervated by the parasympathetic division of the autonomic nervous system. In particular, the

facial nerve (CN VII) innervates the submandibular and sublingual

glands, whereas the glossopharyngeal nerve (CN IX) innervates the

parotid gland (see section 15.8). Parasympathetic innervation stimulates salivary gland secretion, which is why your mouth “waters”

when you see a delicious dinner in front of you. Your salivary glands

are preparing the body for the start of the digestion process. In contrast, sympathetic stimulation inhibits normal secretion from these

glands, which is why you may experience a dry mouth after a fightor-flight response.



Research suggests that a “dry mouth” (inadequate production

of saliva) is correlated with increased dental problems, such

as caries (cavities). What are the possible reasons for this


26.2d  Teeth

The teeth are collectively known as the dentition (den-tish′ŭn; dentition = teething). Teeth are responsible for mastication, the first part

of the mechanical digestion process. A tooth has an exposed crown,

a constricted neck, and one or more roots that anchor it to the jaw

(figure 26.5). The roots of the teeth fit tightly into dental alveoli,

which are sockets within the alveolar processes of both the maxillae

and the mandible. Collectively, the roots, the dental alveoli, and the

periodontal ligaments that bind the roots to the alveolar processes

form a gomphosis joint (described in section 9.2a).






Pulp cavity

Root canal





Dental alveolus

Blood vessels

and nerves in

apical foramen

Figure 26.5

Anatomy of a Molar. Ingested material is chewed by the teeth in the oral


Each root of a tooth is ensheathed within hardened material

called cementum (sē-men′tŭm; rough quarry stone). A tough, d­ urable

layer of enamel (ē-nam′ĕl) forms the crown of the tooth. Enamel,

the hardest substance in the body, is composed primarily of calcium

phosphate crystals. Dentin (den′tin) forms the primary mass of a

tooth. Dentin is comparable to bone but harder, and is deep to the

cementum and the enamel. The center of the tooth is a pulp ­cavity

that contains a connective tissue called pulp. A root canal opens into

the connective tissue through an opening called the apical foramen

and is continuous with the pulp cavity. Blood vessels and nerves pass

through the apical foramen and are housed in the pulp.

The mesial (mē′zē-ăl; mesos = middle) surface of the tooth

is the surface closest to the midline of the mouth, while the distal

surface of the tooth is farthest from the mouth midline. Other tooth

surfaces include: the buccal surface, adjacent to the internal surface

of the cheek; the labial surface, adjacent to the internal surface of

the lip; the ­lingual surface, facing the tongue; and the occlusal

(ŏ-klū′zăl) surface, where the teeth from the opposing superior and

inferior arches meet.

Two sets of teeth develop and erupt during a normal lifetime. In

an infant, 20 deciduous (dē-sid′yū-ŭs; deciduus = falling off) teeth,

Chapter Twenty-Six  Digestive System

Table 26.3


Oral Cavity Structures and Their Functions





Composed of dense irregular connective tissue and nonkeratinized

stratified squamous epithelium

Surround necks of teeth and cover alveolar processes

Hard palate

Anterior roof of mouth; bony shelf covered by dense connective

tissue and nonkeratinized stratified squamous epithelium

Forms anterior two-thirds of roof of mouth; separates oral cavity

from nasal cavity


Form part of anterior walls of oral cavity; covered with keratinized

stratified squamous epithelium

Close oral cavity during chewing

Extrinsic salivary glands

Three pairs of large multicellular glands: parotid glands, sublingual

glands, and submandibular glands

Produce saliva

Soft palate

Posterior roof of mouth formed from skeletal muscle and covered

with nonkeratinized stratified squamous epithelium; the uvula hangs

from it

Forms posterior one-third of roof of mouth; helps close off entryway

to nasopharynx when swallowing


Hard structures projecting from the alveolar processes of the

maxillae and mandible: incisors, canines, premolars, and molars

Mastication (chewing food)


Composed primarily of skeletal muscle and covered by stratified

squamous epithelium; surface covered by papillae

Manipulates ingested material during chewing; pushes material

against palate to turn it into a bolus; detects tastes (via taste buds)


Aggregates of partially encapsulated lymphatic tissue

Detect antigens in swallowed food and drink and initiate immune

response if necessary


Conical, median, muscular projection extending from the soft palate

Assists soft palate in closing off entryway to nasopharynx when



Space between cheek and gums

Space between lips/cheeks and gums where ingested materials are

mixed with saliva and mechanically digested

also called milk teeth erupt between 6 months and 30 months after

birth. These teeth are eventually lost and replaced by 32 permanent

teeth. As figure 26.6 shows, the more anteriorly placed permanent

teeth tend to appear first, followed by the posteriorly placed teeth.

(The exceptions to this rule are the first molars, which appear at

about age 6 and are sometimes referred to as the 6-year molars.) The

last teeth to erupt are the third molars, often called wisdom teeth in

the late teens or early 20s. Often the jaw lacks space to accommodate these final molars, and they may either emerge only partially

or grow at an angle and become impacted (wedged against another

structure). Impacted teeth cannot erupt properly because of the angle

of their growth.



If the deciduous teeth are eventually replaced by permanent

teeth, why do humans have deciduous teeth in the first place?

The most anteriorly placed permanent teeth are called incisors

(in-sı ′̄ zŏr; incido = to cut into). They are shaped like a chisel and

have a single root. They are designed for slicing or cutting into

food. Immediately posterolateral to the incisors are the canines

(kā′nīn; canis = dog), also called cuspids, which have a pointed tip

for puncturing and tearing food. Premolars, also called bicuspids,

are located posterolateral to the canines and anterior to the molars.

They have flat crowns with prominent ridges called cusps that are

used to crush and grind ingested materials. Premolars may have

one or two roots. The molars (mō′lăr; molaris = millstone) are the

thickest and most posteriorly placed teeth. They have large, broad,

flat crowns with distinctive cusps, and three or more roots. Molars

are also adapted for grinding and crushing ingested materials. If the

mouth is divided into quadrants, each quadrant contains the following

number of permanent teeth: two incisors, one canine, two premolars,

and three molars (figure 26.6). In the United States, general dentists

have adopted the Universal Numbering System (recognized by the

American Dental Association) to identify teeth (figure 26.6). Tooth

number 1 is the most posterior tooth on the right side of the maxilla.

Numbering continues anteriorly and around to the most posterior

tooth on the left side of the maxilla (number 16). Number 17 is the

most posterior tooth on the left side of the mandible, and it continues

along the mandibular arch to the most posterior tooth on the right

side of the mandible (number 32). This system is based upon 32 teeth

so—if someone is missing tooth number 1 (wisdom tooth)—the first

number will be 2 instead of 1, acknowledging the missing tooth.

Table 26.3 summarizes the structures of the oral cavity and

their functions.




What are the main components of saliva, and what functions

do they serve?

What are the types of permanent teeth, and what is each

tooth’s main function in mastication?

26.3  Pharynx

✓✓Learning Objectives

7. Describe the structure of the pharynx.

8. Explain the action of the pharyngeal constrictors.

The common space used by both the respiratory and digestive systems is the pharynx (see section 25.2c and figure 26.1). The nonkeratinized stratified squamous epithelial lining of the oropharynx and

the laryngopharynx provides protection against the abrasive activities

associated with swallowing ingested materials.

Three skeletal muscle pairs, called the superior, middle, and

inferior pharyngeal constrictors, form the wall of the pharynx.

Sequential contraction of the pharyngeal constrictors decreases the

diameter of the pharynx, beginning at its superior end and moving

toward its inferior end, thus pushing swallowed material toward the

esophagus. As the pharyngeal constrictors contract, the epiglottis


Chapter Twenty-Six  Digestive System

Central incisor (7–9 mos.)

Lateral incisor (9–11 mos.)

Canine (18–20 mos.)

1st molar (14–16 mos.)



2nd molar (24–30 mos.)

Permanent teeth

2nd molar (20–22 mos.)



Deciduous teeth

1st molar (12–14 mos.)

Canine (16–18 mos.)

Lateral incisor (7–9 mos.)

(a) Child’s skull

Central incisor (6–8 mos.)

(b) Deciduous teeth

Right Upper (Maxillary) Quadrant

Left Upper (Maxillary) Quadrant

Central incisor (7–8 yrs.)

Lateral incisor (8–9 yrs.)

Canine (11–12 yrs.)

1st premolar (10–11 yrs.)

2nd premolar (10–12 yrs.)

Upper teeth


1st molar (6–7 yrs.)







Hard palate










3rd molar (17–25 yrs.)

2nd molar (11–13 yrs.)

1st molar (6–7 yrs.)



2nd molar (12–13 yrs.)

3rd molar (17–25 yrs.)



Lower teeth





26 25 24 23





2nd premolar (11–12 yrs.)

1st premolar (10–12 yrs.)

(d) Teeth numbering

Canine (9–10 yrs.)

Lateral incisor (7–8 yrs.)

Central incisor (6–7 yrs.)

Right Lower (Mandibular) Quadrant

Left Lower (Mandibular) Quadrant

(c) Permanent teeth

Figure 26.6

Teeth. (a) Both deciduous teeth and unerupted permanent teeth are visible in this cutaway section of a child’s skull. (b, c) Comparison of the dentition of deciduous

and permanent teeth, including the approximate age at eruption for each tooth. Also shown in (c) are the dental quadrants, which are formed by a sagittal plane that

passes between the central incisors in the maxilla and mandible. (d) Teeth may be precisely identified using the Universal Numbering System.

(a) © McGraw-Hill Education/Photo by Christine Eckel


Chapter Twenty-Six  Digestive System

of the larynx closes over the laryngeal opening to prevent ingested

materials from entering the larynx and trachea (see section 25.3a).

The vagus nerves (CN X) innervate most pharyngeal muscles

(see section 15.8). The principal arteries supplying the pharynx are

branches of the external carotid arteries. The pharynx is drained by

the internal jugular veins (see section 23.3c).




Lesser omentum




How do pharyngeal constrictors move swallowed material to

the esophagus?




Greater omentum

26.4  General Arrangement

Parietal peritoneum

of Abdominal GI Organs

Mesentery proper

✓✓Learning Objectives





Visceral peritoneum

Identify and describe the peritoneum location and function.

Explain the derivation of specific mesenteries.

Compare and contrast the four tunics in the GI tract wall.

Describe the blood vessels, lymphatic structures, and nerves

that supply the GI tract.

The abdominal organs of the GI tract are supported by serous membranes, and the walls of these organs have specific layers, called tunics.

Transverse colon


Peritoneal cavity


Urinary bladder

26.4a  Peritoneum, Peritoneal Cavity, and Mesentery

The abdominopelvic cavity is lined with moist serous membranes

(figure 26.7). The portion of the serous membrane that lines the

inside surface of the body wall is called the parietal peritoneum

(per′i-tō-nē′ŭm; periteino = to stretch over). The portion of the serous

membrane that folds back (reflects) to cover the surface of internal

organs is called the visceral peritoneum. Between these two layers

is the peritoneal cavity, a potential space where the peritoneal

layers that face each other secrete a lubricating serous fluid. The

thin layer of fluid in the peritoneal cavity lubricates both the body

wall and the internal organ surfaces, allowing the abdominal organs to move freely, and reducing any friction resulting from this

movement (see figure 1.10d and section 1.4e).

Learning Strategy

The peritoneum is similar to the pleura and the serous pericardium. These

membranes have an outer parietal layer that lines the body wall and a

visceral layer that covers the organ. The space between the parietal and

visceral layers is where serous fluid is secreted, and this fluid acts as a

lubricant to prevent friction as the organ moves.

Within the abdomen, organs that are completely surrounded

by visceral peritoneum are called intraperitoneal (in′tră-per′i-tōnē′ăl) organs. They include the stomach, part of the duodenum of

the small intestine, the jejunum and the ileum of the small intestine,

the cecum, the appendix, and the transverse and sigmoid colon of the

large intestine. Retroperitoneal (re′trō-per′i-tō-nē′ăl) organs typically lie directly against the posterior abdominal wall, so only their

anterolateral portions are covered with peritoneum. Retroperitoneal

organs include most of the duodenum, the pancreas, the ascending

and descending colon of the large intestine, and the rectum.

Figure 26.7

Peritoneum and Mesenteries. Many abdominal organs are held in

place by double-layered serous membrane folds called mesenteries. The

peritoneum is the serous membrane lining the internal abdominal wall

(parietal layer) and covering the outer surface of the abdominal organs

(visceral layer). This sagittal view through the abdominopelvic cavity shows

the relationship between the peritoneal membranes and the abdominal

organs they ensheathe.

The mesenteries (mes′en-ter-ē; mesos = middle, enteron =

intestine) are folds of peritoneum that support and stabilize the intraperitoneal GI tract organs. Blood vessels, lymph vessels, and nerves

are sandwiched between the two folds and supply the digestive organs. There are several different types of mesenteries. The greater

omentum (ō-men′tŭm) extends inferiorly like an apron from the

greater curvature of the stomach and covers most of the abdominal

organs (figure 26.8a). It often accumulates large amounts of adipose

connective tissue. The lesser omentum connects the lesser curvature

of the stomach and the proximal end of the duodenum to the liver.

The lesser omentum may be subdivided into a hepatogastric ligament,

which runs from the liver to the stomach, and a hepatoduodenal ligament, which runs from the liver to the duodenum.

The mesentery proper is a fan-shaped fold of peritoneum

that suspends most of the small intestine from the internal surface

of the posterior abdominal wall (figure 26.8b). The peritoneal fold

that attaches parts of the large intestine to the internal surface of the

posterior abdominal wall is called the mesocolon (mez′ō-kō′lon). Essentially, a mesocolon is a mesentery for parts of the large intestine.

There are several distinct sections of the mesocolon, each named

for the portion of the colon it suspends. For example, transverse


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.

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