336
UNIT 2
Covering, Support, and Movement of the Body
10
MUSCLE GALLERY
Table 10.4
Muscles of the Neck and Vertebral Column:
Head Movements and Trunk Extension
(Figure 10.10)
MUSCLE
DESCRIPTION
ORIGIN (O) AND
INSERTION (I)
ACTION
NERVE
SUPPLY
ANTEROLATERAL NECK MUSCLES
(Figure 10.10a and c)
Sternocleidomastoid
(ster
0
no-kli
0
do-mas
9
toid)
(
sterno
5
breastbone;
cleido
5
clavicle;
mastoid
5
mastoid process)
Two-headed muscle located
deep to platysma on
anterolateral surface of
neck; fleshy parts on either
side of neck delineate limits
of anterior and posterior
triangles; key muscular
landmark in neck; spasms
of one of these muscles
may cause torticollis
(wryneck)
O—manubrium of
sternum and medial
portion of clavicle
I—mastoid process of
temporal bone and
superior nuchal line of
occipital bone
Flexes and laterally
rotates the head
;
simultaneous contraction
of both muscles flexes
neck, generally against
resistance as when raising
head when lying on back;
acting alone, each muscle
rotates head toward
shoulder on opposite side
and tilts or laterally flexes
head to its own side
Accessory nerve
(cranial nerve XI) and
branches of cervical
spinal nerves C
2
and C
3
(ventral rami)
Scalenes
(ska
9
le
-
nz)—
anterior, middle, and
posterior
(
scalene
5
uneven)
Located more laterally
than anteriorly on neck;
deep to platysma and
sternocleidomastoid
O—transverse processes
of cervical vertebrae
I—anterolaterally on first
two ribs
Elevate first two ribs
(aid
in inspiration); flex and
rotate neck
Cervical spinal nerves
INTRINSIC MUSCLES OF THE BACK
(Figure 10.10b, d, e)
Splenius
(sple
9
ne-us)—
capitis and cervicis portions
(ka
˘
9
pı˘-tis; ser-vis
9
us)
(
splenion
5
bandage;
caput
5
head;
cervi
5
neck)
(Figures 10.10b and 10.7)
Broad bipartite superficial
muscle (capitis and cervicis
parts) extending from
upper thoracic vertebrae to
skull; capitis portion known
as “bandage muscle”
because it covers and holds
down deeper neck muscles
O—ligamentum
nuchae,* spinous
processes of vertebrae
C
7
–T
6
I—mastoid process of
temporal bone and
occipital bone (capitis);
transverse processes of
C
2
–C
4
vertebrae (cervicis)
Extend or hyperextend
head
; when splenius
muscles on one side are
activated, head rotates
and bends laterally
toward same side
Cervical spinal nerves
(dorsal rami)
*The ligamentum nuchae (lig
0
ah-men
9
tum noo
9
ke) is a strong, elastic ligament extending from the occipital bone of the skull along the tips of the spinous
processes of the cervical vertebrae. It binds the cervical vertebrae together and inhibits excessive head and neck flexion, thus preventing damage to the
spinal cord in the vertebral canal.
Head movements
Muscles originating from the axial skeleton move the head. The major
head flexors are the
sternocleidomastoid muscles
(Figure 10.10a, c),
with help from the suprahyoid and infrahyoid muscles described in
Table 10.3. Lateral head movements (rotating or tilting the head)
result when the muscles on only one side of the neck contract. These
actions are produced by the sternocleidomastoids and a number of
deeper neck muscles, considered in this table. Head extension is aided
by the trapezius muscles of the back, but the main extensors of the
head are the
splenius
muscles deep to the trapezius muscles
(Figure 10.10b).
Trunk extension
Trunk extension is brought about by the
deep
or
intrinsic back
muscles
associated with the bony vertebral column. These muscles
also maintain the normal curvatures of the spine, acting as postural
muscles. As you consider these back muscles, keep in mind that
they are deep. The superficial back muscles that cover them are
concerned primarily with moving the shoulder girdle and upper
limbs (see Tables 10.8 and 10.9).
The deep muscles of the back form a broad, thick column
extending from the sacrum to the skull. Many muscles of varying
length contribute to this mass. Think of each muscle as a string
that when pulled causes one or several vertebrae to extend or to
rotate on the vertebrae below. The largest deep back muscle group
is the
erector spinae
group (Figure 10.10d). Because the origins
and insertions of the different muscle groups overlap extensively,
and many of these muscles are long, large regions of the vertebral
column can be moved simultaneously and smoothly. Acting together,
the deep back muscles extend (or hyperextend) the spine, but muscle
contraction on only one side causes the spine to bend laterally (flex).
Lateral flexion is automatically accompanied by some degree of
rotation of the vertebral column. During vertebral movements, the
articular facets of the vertebrae glide on each other.
In addition to the long back muscles, there are a number of short
muscles that extend from one vertebra to the next. These small
muscles act primarily as synergists in extending and rotating the
spine and as spine stabilizers. They are not described in the table
but you can deduce their actions by examining their origins and
insertions in Figure 10.10e.
The trunk
muscles
that we consider in this table are extensors. The
more superficial muscles, which have other functions, are considered in
subsequent tables. For example, the anterior muscles of the abdominal
wall that cause trunk
flexion
are described in Table 10.6.
previous page 370 Human Anatomy and Physiology (9th ed ) 2012 read online next page 372 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off