661
CHAPTER SEVENTEEN
Digestive System
S
ticky foods, such as caramel, lodge between
the teeth and in the crevices of molars,
feeding bacteria such as
Actinomyces,
Streptococcus mutans,
and
Lactobacillus.
These
microbes metabolize carbohydrates in the food,
producing acid by-products that destroy tooth
enamel and dentin (F
g. 17A). The bacteria also pro-
duce sticky substances that hold them in place.
If a person eats a candy bar, for example,
but does not brush the teeth soon afterward, the
actions of the acid-forming bacteria will produce
decay, called dental caries. Unless a dentist cleans
and fills the resulting cavity that forms where
enamel is destroyed, the damage will spread
to the underlying dentin. As a result, the tooth
becomes sensitive.
Dental caries can be prevented in several
ways:
1. Brush and ±
oss teeth regularly.
2. Have regular dental exams and cleanings.
3. Drink ±
uoridated water or receive a ±
uoride
treatment. ²luoride is incorporated into the
enamel’s chemical structure, strengthening it.
4. Have a dentist apply a sealant to children’s
and adolescents’ teeth where crevices might
hold onto decay-causing bacteria. The
sealant is a coating that keeps acids from
eating away at tooth enamel.
17.1
CLINICAL APPLICATION
Dental Caries
mouth inferior to the tongue. Their cells are primarily the
mucous type, so their secretions, which enter the mouth
through many separate ducts (Rivinus’s ducts), are thick and
stringy (see F gs. 17.11 and 17.12
c
).
Table 17.4
summarizes
the characteristics of the major salivary glands.
PRACTICE
14
What is the function of saliva?
15
What stimulates the salivary glands to secrete saliva?
16
Where are the major salivary glands?
17.5
PHARYNX AND ESOPHAGUS
The pharynx is a cavity posterior to the mouth from which the
tubular esophagus leads to the stomach. The pharynx and the
esophagus do not digest food, but both are important passage-
ways, and their muscular walls function in swallowing.
Major Salivary Glands
The
parotid
(pah-rot
id)
glands
are the largest of the major
salivary glands. Each gland lies anterior to and somewhat
inferior to each ear, between the skin of the cheek and the
masseter muscle. A
parotid duct
(Stensen’s duct) passes
from the gland inward through the buccinator muscle, enter-
ing the mouth just opposite the upper second molar on either
side of the jaw. The parotid glands secrete a clear, watery
fl uid rich in salivary amylase
(f
gs. 17.11
and
17.12
a
)
.
The
submandibular
(sub
man-dib
u-lar)
glands
are in
the fl
oor of the mouth on the inside surface of the lower jaw.
The secretory cells of these glands are about equally serous
and mucous. Consequently, the submandibular glands
secrete a more viscous fl uid than the parotid glands (see F
gs.
17.11 and 17.12
b
). The ducts of the submandibular glands
(Wharton’s ducts) open inferior to the tongue, near the lin-
gual frenulum.
The
sublingual
(sub-ling
gwal)
glands
are the smallest
of the major salivary glands. They are on the fl oor of the
FIGURE 17A
Actinomyces
bacteria (falsely colored) clinging to teeth release acids that decay
tooth enamel (1,250×).
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