Fundamentals of Anatomy and Physiology

382

■■ Bicarbonates and phosphates, which are buffer chem-
icals, keep the saliva at a slightly acidic pH of 6.35 to

6.85.^
■■ Urea and uric acid are waste products.
■■ Mucin forms mucus to lubricate food.
■■ The enzyme lysozyme destroys bacteria, thus protect-ing
the mucous membrane from infection and the teeth from
possible decay.

Teeth

The teeth, also known as the dentes (DEN-teez), are lo-
cated in the sockets of the alveolar processes of the man-
dible and maxillae bones. The teeth break up food by
chewing. Chewing is called mastication (mass-tih-KAY-
shun). There are 20 temporary or deciduous teeth that form
in infants between the ages of 6 months to 2 years. By the
age of 13, there will develop 32 permanent teeth to replace
the deciduous ones (Figure 16-6). The eight front teeth are
called incisors (in-SIGH-zors) and are used to cut food.
The four canine teeth are used to tear food. Be-cause
they have one cusp, they are also called cuspids (KUSS-
pids). The molar teeth grind food. There are two kinds of
molar teeth. The eight premolars have two cusps or
projections and are also called bicuspids; some of the 12
molars have three cusps and are called tricuspids but
many have four cusps and the maxillary first molar actu-
ally has five cusps.
The alveolar processes are covered by the gums or
gingivae (JIN-jih-vee) that extend slightly into each
socket. The sockets are lined by the periodontal ligament
that anchors the teeth in position and acts as a shock
absorber to soften the forces created during chewing. A
tooth can be divided into three principal portions:

Chapter 16

■■ The crown is the portion above the level of the gums

and is covered with enamel, the hardest substance in

the body that protects the tooth from wear and acids.^

■■ The cervix or neck is the constricted junction between

the crown^ and the root.^

■■ The root can consist of one, two, or three projections

embedded in the socket. Larger teeth, like molars, will

have more than one root.

Teeth are made of dentin, a bonelike substance that

encloses the pulp cavity in the crown. The exposed sur-face

of the crown is covered with enamel. Narrow exten-sions of

the pulp cavity project into the root, called root canals. At

the base of each root canal is an opening, the apical

foramen, through which blood vessels and nerves enter the

tooth and become part of the pulp. The dentin of the root is

covered with another substance called cemen-tum, which

attaches the root to the periodontal ligament.

The Pharynx

The pharynx (FAIR-inks) is part of both the digestive and

the respiratory systems. Its function in the digestive system

is to begin the process of swallowing or degluti-tion

(deg-loo-TISH-un). Swallowing moves food from the

mouth to the stomach. Swallowing begins when the tongue,

with the teeth and saliva, forms a soft mass called the food

bolus. Food is forced to the back of the mouth cavity and

into the oropharynx (or-oh-FAIR-inks). This is the

voluntary stage of swallowing. Next, the involuntary stage

begins (Figure 16-7). First, the respiratory passage-ways

close and breathing is temporarily interrupted. The food

bolus stimulates oropharyngeal receptors that send

impulses to the brain. This causes the soft palate and

COMMON DISEASE, DISORDER, OR CONDITION

Mumps

Mumps, caused by a virus that infects the salivary glands and, in particular, the pa-rotid

salivary glands, affects children between the ages of 5 and 9. Initial exposure stimulates

antibody production and results in permanent immunity. In older males, the virus may

infect the testes, causing sterility. If the virus infects the pancreas, diabetes can result.

Today, an effective vaccine (part of an early childhood series) is readily available. Thus,

the incidence of mumps in the United States has been greatly reduced.