Human Physiology, 14th edition (2016)

The Digestive System 623

Mastication (chewing) of food mixes it with saliva, secreted

by the salivary glands. In addition to mucus and various anti-

microbial agents, saliva contains salivary amylase, an enzyme

that can catalyze the partial digestion of starch. Deglutition, or

swallowing, is divided into three phases: oral, pharyngeal, and

esophageal. Swallowing is a complex activity that requires the

coordinated contractions of 25 pairs of muscles in the mouth,

pharynx, larynx, and esophagus. The muscles of the mouth,

pharynx, and upper esophagus are striated and innervated by

somatic motor neurons, whereas the muscles of the middle

and lower esophagus are smooth and innervated by autonomic

Figure 18.3 The layers of the
digestive tract. ( a ) An illustration of the
major tunics, or layers, of the small intestine.
The inset shows how folds of mucosa form
projections called villi in the small intestine.
( b ) An illustration of a cross section of the small
intestine showing layers and glands.

Myenteric

plexus

Submucosal

plexus

Lymph nodule

Lacteal

Epithelium

Muscularis

externa

Lamina propria

Intestinal crypt

(of Lieberkühn)

Muscularis mucosae

Gland in submucosa

Inner circular

Outer

longitudinal

Artery

Serosa

Mucosa

Submucosa

Circular muscle

Longitudinal muscle

Villi

Vein Mesentery

Plica circularis

Serosa

(a)

(b)

| CHECKPOINT

1. Define the terms digestion and absorption, describe
   how molecules are digested, and indicate which
   molecules are absorbed.
   2a. Describe the structure and function of the mucosa,
   submucosa, and muscularis.
   2b. Describe the location and composition of the
   submucosal and myenteric plexuses and explain the
   actions of autonomic nerves on the gastrointestinal
   tract.

18.2 FROM MOUTH TO STOMACH

Peristaltic contractions of the esophagus deliver food to the stom-
ach, which secretes very acidic gastric juice that is mixed with
the food by gastric contractions. Proteins in the resulting mixture,
called chyme, are partially digested by the enzyme pepsin.

LEARNING OUTCOMES

After studying this section, you should be able to:

3. Describe the structure and functions of the
   esophagus and stomach.


4. Describe the composition and actions of gastric
   juice and explain how gastric secretion is regulated.