664
UNIT FIVE
2. The hyoid bone and the larynx are elevated. A fl
aplike
structure attached to the larynx, called the
epiglottis,
(ep
ı˘-glot
is) closes off the top of the trachea so that
food is less likely to enter the trachea.
3. The tongue is pressed against the soft palate and uvula,
sealing off the oral cavity from the nasal cavity.
4. The longitudinal muscles in the pharyngeal wall
contract, pulling the pharynx upward toward the food.
5. The lower portion of the inferior constrictor muscles
relaxes, opening the esophagus.
6. The superior constrictor muscles contract, stimulating
a peristaltic wave to begin in other pharyngeal muscles.
This wave forces the food into the esophagus.
The swallowing refl
ex momentarily inhibits breathing. Then,
during the third stage of swallowing, peristalsis transports
the food in the esophagus to the stomach.
Although the pharyngeal muscles are skeletal muscles,
they are under voluntary control only in the sense that swal-
lowing (deglutition) can be voluntarily initiated. Complex
refl
exes control the precise actions of these muscles during
swallowing.
Swallowing Mechanism
Swallowing can be divided into three stages. In the first
stage, which is voluntary, food is chewed and mixed with
saliva. Then, the tongue rolls this mixture into a mass, or
bolus,
and forces it into the pharynx.
The second stage of swallowing begins as food reaches the
pharynx and stimulates sensory receptors around the pharyn-
geal opening. This triggers the swallowing refl ex, illustrated in
f gure 17.14
, which includes the following actions:
1. The soft palate (including the uvula) raises, preventing
food from entering the nasal cavity.
Nasopharynx
with mucosa
removed to
show muscles
Pharyngeal
constrictor
muscles:
Superior
Middle
Inferior
Hard
palate
Food
mass
Tongue
Epiglottis
Trachea
Inferior
pharyngeal
constrictor
muscles
Esophagus
Soft palate
Larynx
Epiglottis
Hyoid bone
Food
mass
Soft
palate
Superior
pharyngeal
constrictor
muscles
Food
mass
Esophagus
Larynx
Tongue
Peristaltic wave
Stomach
Esophagus
Food mass
(a)
The tongue forces food into the pharynx.
(b)
The soft palate, hyoid bone, and larynx are raised,
the tongue is pressed against the palate, the epiglottis
closes, and the inferior constrictor muscles relax so
that the esophagus opens.
(c)
Superior constrictor muscles contract and
force food into the esophagus.
(d)
Peristaltic waves move food through the esophagus
to the stomach.
FIGURE 17.14
Steps in the swallowing ref
ex. (The mucosa in (
a
), (
b
), and (
c
) has been removed to reveal the underlying muscles.)
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