Essentials of Anatomy and Physiology

the roots of the second molars. In such cases, extrac-
tion of a wisdom tooth may be necessary to prevent
damage to other teeth.
The structure of a tooth is shown in Fig. 16–2. The
crown is visible above the gum (gingiva). The root is
enclosed in a socket in the mandible or maxillae. The
periodontal membranelines the socket and produces
a bone-like cement that anchors the tooth. The outer-
most layer of the crown is enamel, which is made by
cells called ameloblasts. Enamel provides a hard chew-
ing surface and is more resistant to decay than are
other parts of the tooth. Within the enamel is dentin,
which is very similar to bone and is produced by cells
called odontoblasts. Dentin also forms the roots of a
tooth. The innermost portion of a tooth is the pulp
cavity, which contains blood vessels and nerve end-
ings of the trigeminal nerve (5th cranial). Erosion of
the enamel and dentin layers by bacterial acids (dental
caries or cavities) may result in bacterial invasion of
the pulp cavity and a very painful toothache.

TONGUE

The tongueis made of skeletal muscle that is inner-
vated by the hypoglossal nerves (12th cranial). On the
upper surface of the tongue are small projections

called papillae, many of which contain taste buds (see

also Chapter 9). The sensory nerves for taste are also

cranial nerves: the facial (7th) and glossopharyngeal

(9th). As you know, the sense of taste is important

because it makes eating enjoyable, but the tongue has

other functions as well.

Chewing is efficient because of the action of the

tongue in keeping the food between the teeth and

mixing it with saliva. Elevation of the tongue is the

first step in swallowing. This is a voluntary action, in

which the tongue contracts and meets the resistance of

the hard palate. The mass of food, called a bolus, is

thus pushed backward toward the pharynx. The

remainder of swallowing is a reflex, which is described

in the section on the pharynx.

SALIVARY GLANDS

The digestive secretion in the oral cavity is saliva,

produced by three pairs of salivary glands, which are

shown in Fig. 16–3. The parotid glands are just

below and in front of the ears. The submandibular

(also called submaxillary) glands are at the posterior

corners of the mandible, and the sublingualglands

are below the floor of the mouth. Each gland has at

least one duct that takes saliva to the oral cavity.

Secretion of saliva is continuous, but the amount

varies in different situations. The presence of food

(or anything else) in the mouth increases saliva secre-

tion. This is a parasympathetic response mediated

by the facial and glossopharyngeal nerves. The sight

or smell of food also increases secretion of saliva.

Sympathetic stimulation in stress situations decreases

secretion, making the mouth dry and swallowing

difficult.

Saliva is mostly water, which is important to dis-

solve food for tasting and to moisten food for swal-

lowing. The digestive enzyme in saliva is salivary

amylase, which breaks down starch molecules to

shorter chains of glucose molecules, or to maltose, a

disaccharide. Most of us, however, do not chew our

food long enough for the action of salivary amylase to

be truly effective. As you will see, another amylase

from the pancreas is also available to digest starch.

Table 16–1 summarizes the functions of digestive

secretions.

Saliva is made from blood plasma and thus contains

many of the chemicals that are found in plasma.

Considerable research is focused on detecting in saliva

chemical markers for diseases such as cancer, with the

372 The Digestive System

Enamel

Dentin

Pulp

cavity

Gingiva

(gum)

Cementum

Periodontal

membrane

Blood vessels

Crown

Neck

Root

Nerve

Figure 16–2. Tooth structure. Longitudinal section of a
tooth showing internal structure.
QUESTION:Which parts of a tooth are living? How do
you know?