Essentials of Anatomy and Physiology

goal of using saliva rather than blood for diagnostic
tests.

PHARYNX

As described in the preceding chapter, the oropharynx
and laryngopharynx are food passageways connecting
the oral cavity to the esophagus. No digestion takes
place in the pharynx. Its only related function is swal-
lowing, the mechanical movement of food. When the
bolus of food is pushed backward by the tongue, the
constrictor muscles of the pharynx contract as part of
the swallowing reflex. The reflex center for swallow-
ing is in the medulla, which coordinates the many
actions that take place: constriction of the pharynx,
cessation of breathing, elevation of the soft palate to
block the nasopharynx, elevation of the larynx and clo-

sure of the epiglottis, and peristalsis of the esophagus.

As you can see, swallowing is rather complicated, but

because it is a reflex we don’t have to think about mak-

ing it happen correctly. Talking or laughing while eat-

ing, however, may interfere with the reflex and cause

food to go into the “wrong pipe,” the larynx. When

that happens, the cough reflex is usually effective in

clearing the airway.

ESOPHAGUS

The esophagusis a muscular tube that takes food

from the pharynx to the stomach; no digestion takes

place here. Peristalsis of the esophagus propels food in

one direction and ensures that food gets to the stom-

ach even if the body is horizontal or upside down. At

the junction with the stomach, the lumen (cavity) of

the esophagus is surrounded by the lower esophageal

sphincter (LES or cardiac sphincter), a circular

smooth muscle. The LES relaxes to permit food to

enter the stomach, then contracts to prevent the

backup of stomach contents. If the LES does not close

completely, gastric juice may splash up into the esoph-

agus; this is a painful condition we call heartburn, or

gastroesophageal reflux disease (GERD). Most people

experience heartburn once in a while, and it is merely

uncomfortable, but chronic GERD is more serious.

The lining of the esophagus cannot withstand the cor-

rosive action of gastric acid and will be damaged, per-

haps resulting in bleeding or even perforation.

Medications are available to treat this condition.

STRUCTURAL LAYERS

OF THE ALIMENTARY TUBE

Before we continue with our discussion of the organs

of digestion, we will first examine the typical structure

of the alimentary tube. When viewed in cross-section,

the alimentary tube has four layers (Fig. 16–4): the

mucosa, submucosa, external muscle layer, and serosa.

Each layer has a specific structure, and its functions

contribute to the functioning of the organs of which it

is a part.

MUCOSA

The mucosa, or lining, of the alimentary tube is made

of epithelial tissue, areolar connective tissue, and two

The Digestive System 373

Parotid duct

Sublingual

ducts

Parotid

gland

Submandibular

gland

Sublingual gland

Submandibular duct

Figure 16–3. The salivary glands shown in left lateral
view.
QUESTION:Why are these exocrine glands? What is
saliva made from?