332
UNIT 2
Covering, Support, and Movement of the Body
10
Four pairs of muscles are involved in mastication (chewing). All are
innervated by the
mandibular branch
of
cranial nerve V
.
The prime movers of jaw closure (and biting) are the powerful
masseter
and
temporalis
muscles, which are easily palpated when
the teeth are clenched (Figure 10.8a). The
pterygoid
muscles
produce side-to-side grinding (Figure 10.8b). The
buccinator
muscles (see Table 10.1) also play a role in chewing. Normally,
gravity is sufficient to depress the mandible, but if there is
resistance to jaw opening, neck muscles such as the digastric and
mylohyoid muscles (see Table 10.3) are activated.
The tongue is composed of muscle fibers that curl, squeeze,
and fold the tongue during speaking and chewing. These
intrinsic
tongue muscles
change the shape of the tongue and contribute to
its exceptional nimbleness, but they do not really move the tongue.
We discuss them in Chapter 23 with the digestive system.
In this table, we consider only the
extrinsic tongue muscles
,
which anchor and move the tongue (Figure 10.8c).
Cranial nerve
XII,
the
hypoglossal nerve,
innervates all extrinsic tongue muscles
(see Table 13.2).
MUSCLE GALLERY
Table 10.2
Muscles of the Head, Part II: Mastication and Tongue Movement
(Figure 10.8)
MUSCLE
DESCRIPTION
ORIGIN (O) AND
INSERTION (I)
ACTION
NERVE
SUPPLY
MUSCLES OF MASTICATION
Masseter
(mah-se
9
ter)
(
maseter
5
chewer)
Powerful muscle that
covers lateral aspect of
mandibular ramus
O—zygomatic arch and
zygomatic bone
I—angle and ramus of
mandible
Prime mover of jaw
closure
; elevates mandible
Trigeminal nerve
(cranial V)
Temporalis
(tem
0
por-a
˘
9
lis)
(
tempora
5
time; pertaining
to the temporal bone)
Fan-shaped muscle
that covers parts of the
temporal, frontal, and
parietal bones
O—temporal fossa
I—coronoid process of
mandible via a tendon
that passes deep to
zygomatic arch
Closes jaw
; elevates
and retracts mandible;
maintains position of the
mandible at rest; deep
anterior part may help
protract mandible
Trigeminal nerve
Medial pterygoid
(me
9
de-ul ter
9
ı˘-goid)
(
medial
5
toward median
plane;
pterygoid
5
winglike)
Deep two-headed muscle
that runs along internal
surface of mandible and is
largely concealed by that
bone
O—medial surface of
lateral pterygoid plate of
sphenoid bone, maxilla,
and palatine bone
I—medial surface of
mandible near its angle
Acts with the lateral
pterygoid muscle to
protrude (pull anteriorly)
the mandible and
promote side-to-side
(grinding) movements
;
synergist of temporalis
and masseter muscles in
elevation of the mandible
Trigeminal nerve
Lateral pterygoid
(
lateral
5
away from
median plane)
Deep two-headed muscle;
lies superior to medial
pterygoid muscle
O—greater wing and
lateral pterygoid plate of
sphenoid bone
I—condylar process of
mandible and capsule
of temporomandibular
joint
Provides forward sliding
and side-to-side grinding
movements of the
lower teeth
; protrudes
mandible
Trigeminal nerve
Buccinator
See Table 10.1
See Table 10.1
Compresses the cheek
;
keeps food between
grinding surfaces of teeth
during chewing
Facial nerve (cranial
VII)
MUSCLES PROMOTING TONGUE MOVEMENTS (EXTRINSIC TONGUE MUSCLES)
Genioglossus
(je
0
ne-o-glah
9
sus)
(
geni
5
chin;
glossus
5
tongue)
Fan-shaped muscle; forms
bulk of inferior part of
tongue; its attachment to
mandible prevents tongue
from falling backward and
obstructing breathing
O—internal surface of
mandible near symphysis
I—inferior aspect of the
tongue and body of
hyoid bone
Protracts tongue
; can
depress or act in concert
with other extrinsic
muscles to retract tongue
Hypoglossal nerve
(cranial XII)
Hyoglossus
(hi
9
o-glos
0
us)
(
hyo
5
pertaining to hyoid
bone)
Flat, quadrilateral muscle
O—body and greater
horn of hyoid bone
I—inferolateral tongue
Depresses tongue
and
draws its sides inferiorly
Hypoglossal nerve
Styloglossus
(sti-lo-glah
9
sus)
(
stylo
5
pertaining to styloid
process)
Slender muscle running
superiorly to and at right
angles to hyoglossus
O—styloid process of
temporal bone
I—inferolateral tongue
Retracts and elevates
tongue
Hypoglossal nerve
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