338
UNIT 2
Covering, Support, and Movement of the Body
10
MUSCLE
DESCRIPTION
ORIGIN (O) AND
INSERTION (I)
ACTION
NERVE
SUPPLY
Erector spinae
(e-rek
9
tor spi
9
ne)
Also called
sacrospinalis
(Figure 10.10d, left side)
Prime mover of back extension. Each side consists of three columns—the iliocostalis, longissimus, and spinalis
muscles—forming intermediate layer of intrinsic back muscles. Erector spinae provide resistance that helps
control action of bending forward at the waist and act as powerful extensors to promote return to erect
position. During full flexion (i.e., when touching fingertips to floor), erector spinae are relaxed and strain
is borne entirely by ligaments of back. On reversing the movement, these muscles are initially inactive, and
extension is initiated by hamstring muscles of thighs and gluteus maximus muscles of buttocks. As a result of
this peculiarity, lifting a load or moving suddenly from a bent-over position can injure muscles and ligaments
of back and intervertebral discs. Erector spinae muscles readily go into painful spasms following injury to back
structures.
Iliocostalis
(il
0
e-o-kos-
ta
˘
9
lis)—lumborum,
thoracis, and cervicis
portions (lum
9
bor-um;
tho-ra
9
sis) (
ilio
5
ilium;
cost
5
rib;
thorac
5
thorax)
Most lateral muscle group
of erector spinae muscles;
extend from pelvis to neck
O—iliac crests
(lumborum); inferior 6
ribs (thoracis); ribs 3 to 6
(cervicis)
I—angles of ribs
(lumborum and thoracis);
transverse processes of
cervical vertebrae C
6
–C
4
(cervicis)
Extend and laterally flex
the vertebral column
;
maintain erect posture;
acting on one side, bend
vertebral column to same
side
Spinal nerves (dorsal
rami)
Longissimus
(lon-jis
9
ı˘-
mus)—thoracis, cervicis,
and capitis parts
(
longissimus
5
longest)
Intermediate tripartite
muscle group of erector
spinae; extend by many
muscle slips from lumbar
region to skull; mainly
pass between transverse
processes of vertebrae
O—transverse processes
of lumbar through
cervical vertebrae
I—transverse processes
of thoracic or cervical
vertebrae and to ribs
superior to origin as
indicated by name;
capitis inserts into
mastoid process of
temporal bone
Thoracis and cervicis act
together to
extend and
laterally flex vertebral
column
; capitis
extends
head and turns the face
toward same side
Spinal nerves (dorsal
rami)
Spinalis
(spi-na
˘
9
lis)—
thoracis and cervicis
parts (
spin
5
vertebral
column, spine)
Most medial muscle column
of erector spinae; cervicis
usually rudimentary and
poorly defined
O—spinous process of
upper lumbar and lower
thoracic vertebrae
I—spinous process of
upper thoracic and
cervical vertebrae
Extends vertebral column
Spinal nerves (dorsal
rami)
Semispinalis
(sem
9
e-spı˘-na
˘
9
lis)—thoracis,
cervicis, and capitis regions
(
semi
5
half)
(Figure 10.10d, right side)
Composite muscle forming
part of deep layer of
intrinsic back muscles;
extends from thoracic
region to head
O—transverse processes
of C
7
–T
12
I—occipital bone (capitis)
and spinous processes
of cervical (cervicis) and
thoracic vertebrae T
1
–T
4
(thoracis)
Extends vertebral column
and head and rotates
them to opposite side
;
acts synergistically with
sternocleidomastoid
muscles of opposite side
Spinal nerves (dorsal
rami)
Quadratus lumborum
(kwod-ra
9
tus lum-bor
9
um)
(
quad
5
four-sided;
lumb
5
lumbar region)
(See also Figure 10.20a)
Fleshy muscle forming part
of posterior abdominal wall
O—iliac crest and lumbar
fascia
I—transverse processes
of lumbar vertebrae L
1
L
4
and lower margin of
12th rib
Flexes vertebral column
laterally
when acting
separately; when pair acts
jointly, lumbar spine is
extended and 12th rib is
fixed; maintains upright
posture; assists in forced
inspiration
T
12
and upper lumbar
spinal nerves (ventral
rami)
MUSCLE GALLERY
Table 10.4
Muscles of the Neck and Vertebral Column:
Head Movements and Trunk Extension
(Figure 10.10)
(continued)
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