346
UNIT 2
Covering, Support, and Movement of the Body
10
MUSCLE GALLERY
Table 10.8
Superficial Muscles of the Anterior and Posterior Thorax:
Movements of the Scapula and Arm
(Figure 10.14)
MUSCLE
DESCRIPTION
ORIGIN (O) AND
INSERTION (I)
ACTION
NERVE
SUPPLY
MUSCLES OF THE ANTERIOR THORAX
(Figure 10.14a)
Pectoralis minor
(pek
0
to-ra
9
lis mi
9
nor)
(
pectus
5
chest, breast;
minor
5
lesser)
Flat, thin muscle directly
beneath and obscured by
pectoralis major
O—anterior surfaces of
ribs 3–5 (or 2–4)
I—coracoid process of
scapula
With ribs fixed, draws
scapula forward and
downward
; with scapula
fixed, draws rib cage
superiorly
Medial and lateral
pectoral nerves (C
6
–C
8
)
Serratus anterior
(ser-a
9
tus)
(
serratus
5
saw)
Fan-shaped muscle; lies
deep to scapula, deep
and inferior to pectoral
muscles on lateral rib
cage; forms medial wall of
axilla; origins have serrated
(sawtooth) appearance;
paralysis results in
“winging” of vertebral
border of scapula away
from chest wall, making
arm elevation impossible
O—by a series of muscle
slips from ribs 1–8 (or 9)
I—entire anterior surface
of vertebral border of
scapula
Rotates scapula so its
inferior angle moves
laterally and upward
;
prime mover to protract
and hold scapula against
chest wall; raises point of
shoulder; important role
in abducting and raising
arm and in horizontal
arm movements (pushing,
punching); called “boxer’s
muscle”
Long thoracic nerve
(C
5
–C
7
)
Subclavius
(sub-kla
9
ve-us)
(
sub
5
under, beneath;
clav
5
clavicle)
Small cylindrical muscle
extending from rib 1 to
clavicle
O—costal cartilage of
rib 1
I—groove on inferior
surface of clavicle
Helps stabilize and
depress pectoral girdle
Nerve to subclavius
(C
5
and C
6
)
Most superficial thorax muscles are
extrinsic shoulder muscles
, which
run from the ribs and vertebral column to the shoulder girdle. They
both fix the scapula in place and move it to increase the range of
arm movements.
The anterior muscles of this group include the
pectoralis major
,
pectoralis minor
,
serratus anterior
, and
subclavius
(Figure 10.14a).
Except for the pectoralis major, which inserts into the humerus, all
muscles of the anterior group insert into the pectoral girdle.
The posterior muscles include the
latissimus dorsi
and
trapezius
muscles
superficially and the underlying
levator scapulae
and
rhomboids
(Figure 10.14c). The latissimus dorsi, like the pectoralis
major muscles anteriorly, insert into the humerus and are more
concerned with moving the arm than the scapula, so we defer their
consideration to Table 10.9 (arm-moving muscles).
The important movements of the pectoral girdle involve
displacing the scapula, i.e., its elevation and depression, rotation,
lateral (forward) movements, and medial (backward) movements.
The clavicles rotate around their own axes to provide stability and
precision to scapular movements.
Except for the serratus anterior, the anterior muscles stabilize
and depress the shoulder girdle. Thus, most scapular movements
are promoted by the serratus anterior muscles anteriorly and by
posterior thoracic muscles. The arrangement of muscle attachments
to the scapula is such that one muscle cannot bring about a simple
(linear) movement on its own. To effect scapular movements,
several muscles must act in combination.
The prime movers of shoulder (scapular) elevation are the
superior trapezius fibers and the levator scapulae. When acting
together to shrug the shoulder, their opposite rotational effects
counterbalance each other. The scapula is depressed largely by
gravity (weight of the arm), but when it is depressed against
resistance, the inferior part of the trapezius, the pectoralis minor,
and the serratus anterior (along with the latissimus dorsi, Table
10.9) are active. Anterolateral movements (abduction) of the
scapula on the thorax wall, as in pushing or punching, mainly
reflect serratus anterior activity. Posteromedial movement
(adduction) of the scapula is effected mainly by the trapezius
(midpart) and the rhomboids. Although the serratus anterior and
trapezius muscles are antagonists in forward/backward movements
of the scapulae, they act together to coordinate
rotational
scapular
movements.
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