350
UNIT 2
Covering, Support, and Movement of the Body
10
Recall that the ball-and-socket shoulder joint is the most flexible
joint in the body, but pays the price of instability. Several muscles
cross each shoulder joint to insert on the humerus. All muscles acting
on the humerus originate from the pectoral girdle. However, two of
these—the latissimus dorsi and pectoralis major—primarily originate
on the axial skeleton.
Of the nine muscles covered here, only the
pectoralis major
,
latissimus dorsi
, and
deltoid muscles
are prime movers of arm
movements (Figure 10.15a, b). The remaining six are synergists
and fixators. Four of these, the
supraspinatus
,
infraspinatus
,
teres minor
, and
subscapularis
(marked with an asterisk* in
Figure 10.15b, d), are
rotator cuff muscles.
They originate on
the scapula, and their tendons blend with the fibrous capsule
of the shoulder joint en route to the humerus. Although the
rotator cuff muscles act as synergists in angular and rotational
movements of the arm, their main function is to reinforce the
capsule of the shoulder joint to prevent dislocation of the
humerus. The remaining two muscles, the small
teres major
and
coracobrachialis
, cross the shoulder joint but do not reinforce it.
Generally speaking, muscles that originate
anterior
to the
shoulder joint (pectoralis major, coracobrachialis, and anterior
fibers of the deltoid)
flex
the arm, i.e., lift it anteriorly. The prime
mover of arm flexion is the pectoralis major. The biceps brachii of
the arm assists in this action (Figure 10.15a, c; see Table 10.10).
Muscles originating
posterior
to the shoulder joint extend the
arm. These include the latissimus dorsi and posterior fibers of the
deltoid muscles (both prime movers of arm extension) and the
teres major. Note that the pectoralis and latissimus dorsi muscles
are
antagonists
of one another in flexing and extending the arm.
The middle region of the fleshy deltoid muscle of the shoulder,
which extends over the superolateral side of the humerus, is the
prime mover of arm abduction. The main arm adductors are the
pectoralis major anteriorly and latissimus dorsi posteriorly. The
small muscles acting on the humerus promote lateral and medial
rotation of the arm. The interactions among these nine muscles are
complex and each contributes to several movements. Table 10.12
(Part I) summarizes their actions.
MUSCLE GALLERY
Table 10.9
Muscles Crossing the Shoulder Joint: Movements of the Arm (Humerus)
(Figure 10.15)
MUSCLE
DESCRIPTION
ORIGIN (O) AND
INSERTION (I)
ACTION
NERVE
SUPPLY
Pectoralis major
(pek
0
to-ra
9
lis ma
9
jer)
(
pectus
5
breast, chest;
major
5
larger)
Large, fan-shaped muscle
covering superior portion
of chest; forms anterior
axillary fold; divided into
clavicular and sternal parts
O—sternal end of
clavicle, sternum,
cartilage of ribs 1–6 (or
7), and aponeurosis of
external oblique muscle
I—fibers converge to
insert by a short tendon
into intertubercular
sulcus and greater
tubercle of humerus
Prime mover of arm
flexion; rotates arm
medially; adducts arm
against resistance; with
scapula (and arm) fixed,
pulls rib cage upward,
thus can help in climbing,
throwing, pushing, and
forced inspiration
Lateral and medial
pectoral nerves (C
5
–C
8
and T
1
)
Deltoid
(del
9
toid)
(
delta
5
triangular)
Thick, multipennate muscle
forming rounded shoulder
muscle mass; a common site
for intramuscular injection,
particularly in males, where
it tends to be quite fleshy
O—embraces insertion
of the trapezius;
lateral third of clavicle;
acromion and spine of
scapula
I—deltoid tuberosity of
humerus
Prime mover of arm
abduction when all
its fibers contract
simultaneously
;
antagonist of pectoralis
major and latissimus
dorsi, which adduct the
arm; if only anterior
fibers are active, can act
powerfully in flexing
and rotating humerus
medially, therefore
synergist of pectoralis
major; if only posterior
fibers are active, causes
extension and lateral
rotation of arm; active
during rhythmic arm-
swinging movements
while walking
Axillary nerve
(C
5
and C
6
)
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