MUSCLES OF SHOULDER
DELTOID MUSCLE
The deltoid muscle is triangular with a semicircular origin along the
lateral third of the clavicle, the lateral border of the acromion, and the
lower lip of the crest of the spine of the scapula. All fasciculi converge to
be inserted on the deltoid tuberosity of the humerus. The deltoid muscle is a
principal abductor of the humerus, an action produced primarily by its powerful
central portion. Because of their position and greater fiber length, the
clavicular and scapular portions of the deltoid muscle have different actions
from those of the central portion of the muscle. The clavicular portion assists
in flexion and internal rotation of
the arm, while the scapular portion assists in extension and external rotation.
The axillary nerve (C5, C6) from the posterior cord of the brachial
plexus supplies the deltoid muscle. An upper branch curves around the posterior
surface of the humerus and courses from behind forward on the deep surface of
the muscle, sending offshoots into the muscle. A lower branch supplies the
teres minor muscle by ascending onto its lateral and superficial surface. It
then becomes the superior lateral brachial cutaneous nerve. The posterior
circumflex humeral artery serves this muscle.
PECTORALIS MUSCLE
The pectoralis major muscle originates from the medial half of the
clavicle on its anterior surface and the anterior
surface of the manubrium and body of the sternum. Additional fascicles arise
from the cartilages of the second to sixth ribs as well as from the anterior
layer of the sheath of the rectus abdominis muscle. The muscular fibers
converge to insert on the crest immediately distal to the greater
tubercle, lateral to the bicipital groove. The tendon folds on itself to
form a bilaminar U-shaped tendon with the fold of the tendon below. Thus, the
fibers of the clavicular part insert as the upper part of the anterior lamina;
the lower sternal and abdominal fibers reach up into the superior part of the
posterior limb; and the sternal fibers distribute into the anterior lamina, the
fold, and the lower part of the posterior lamina.
The pectoralis major muscle flexes and adducts the humerus; it is also
capable of medial rotation of the arm but usually becomes active only when this
action is resisted. The clavicular portion of the pectoralis major muscle
elevates the shoulder and flexes the arm, while the sternocostal portion draws
the shoulder downward. The muscle is innervated by the lateral and medial
pectoral nerves from both the lateral and medial cords of the brachial plexus,
involving all the roots (C5 to T1). The
pectoral branches of the thoracoacromial artery accompany the nerves to the
muscle.
The deltopectoral triangle is a separation just below the clavicle of the
upper and adjacent fibers of the deltoid
and pectoralis major muscles. Distally, the separation of these adjacent fibers
is made by the cephalic vein and the deltoid branch of the thoracoacromial
artery.
The pectoralis minor muscle arises from the outer surfaces of the third,
fourth, and fifth ribs near their costal cartilages, with a slip from the
second rib a frequent addition. The muscle fibers converge to an insertion on
the medial border and upper surface of the coracoid process. The pectoralis
minor muscle draws the scapula forward, medially, and strongly downward. With
the scapula fixed, the muscle assists in forced inspiration. The muscle is
innervated by the medial pectoral nerve (C8, T1), which completely penetrates
the muscle to pass across the interpectoral space into the pectoralis major
muscle. Pectoral branches of the thoracoacromial
artery are distributed with the nerve. Deep to the tendon of the pectoralis
minor muscle pass the axillary artery and the cords of the brachial plexus.
SERRATUS MUSCLE
The serratus anterior muscle originates laterally from the first eight
ribs. The muscle fibers converge to insert on the deep surface of the lateral
border of the scapular body. Contraction of the muscle protracts the scapula
and participates in upward rotation of the scapula. Weakness results in scapula
winging (see Plates 1-20 and 1-52). Innervation is supplied by the long
thoracic nerve (C5 to C8), which can easily be injured during axillary lymph
node dissection. The blood supply is primarily
through the lateral thoracic artery.
The subclavius muscle is a small, pencil-like muscle that arises from the
junction of the first rib and its cartilage. It lies parallel to the underside
of the clavicle and inserts in a groove on the underside of the clavicle,
between the attachments of the conoid ligament laterally and the
costoclavicular ligament medially. The muscle assists by its traction on the
clavicle in drawing the shoulder forward and downward. The nerve to the
subclavius muscle is a branch of the superior trunk of the brachial plexus, with fibers from the fifth
cervical nerve, which reaches the upper posterior border of the muscle. There
is a small, special clavicular branch of the thoracoacromial artery to the
muscle.
TRAPEZIUS MUSCLE
The trapezius muscle is divided into upper, middle, and lower divisions
with a broad origin from the occipital protuberance superiorly to the spinous
process of the T12 vertebrae inferiorly. It inserts onto the posterior border
of the lateral third of the clavicle, the medial border of the acromion, and
the upper border of the crest of the spine of the scapula. The directionality
of the upper and lower divisions allows it to rotate the scapula so the glenoid
faces superiorly, which allows full elevation
of the upper extremity. The middle division serves to retract the scapula. When
the function of the trapezius is absent, the scapula wings laterally owing to unopposed
contraction of the serratus anterior (see Plate 1-52). The nerves reaching the
trapezius muscle are the spinal accessory (cranial nerve XI) and direct
branches of ventral rami of the second, third, and fourth cervical nerves. The
accessory nerve perforates and supplies the sternocleidomastoid muscle and then
crosses the posterior triangle of the neck directly under its fascial covering,
coursing diagonally downward to reach the underside of the trapezius muscle.
The trans- verse cervical artery of the subclavian system supplies the
trapezius muscle; it is supplemented in the lower third of the muscle by a
muscular perforating branch of the
dorsal scapular artery.
The levator scapulae originates from the transverse processes of the
first three or four cervical vertebrae. It inserts into the medial border of
the scapula from the superior angle to the spine. It is overlapped and
partially obscured by the sternocleidomastoid and trapezius muscles. It
functions to elevate and adduct the scapula. Innervation is provided by the
dorsal scapular nerve (C3 to C5), and blood supply is from the dorsal scapular
artery.
RHOMBOIDEUS MUSCLE
The rhomboideus minor muscle originates from the lower part of the
ligamentum nuchae and the spinous processes of C7 to T1. It lies parallel to
the rhomboi- deus major muscle, directed downward and lateralward, and it is
inserted on the medial border of the scapula at the root of the scapular spine.
The rhomboideus major muscle arises from the spinous processes of T2 to
T5 and inserts on the medial border of the scapula below its spine. Both
rhomboideus muscles draw the scapula upward and medially and assist the
serratus anterior muscle in holding it firmly to the chest wall. Their oblique
traction aids in depressing the point of the shoulder. The innervation and
blood supply is the same as for the levator scapulae.
LATISSIMUS DORSI MUSCLE
The latissimus dorsi muscle originates from the inferior thoracic
vertebrae, the thoracolumbar fascia, the iliac crest, and the lower third to
fourth ribs. It inserts onto the floor of the intertubercular groove of the humerus.
Contraction of this muscle extends the humerus, drawing the arm downward and
backward and rotating it internally. The muscle is innervated by the thoracodorsal
nerve from the posterior cord of the brachial plexus, with fibers from the
seventh and eighth cervical nerves. The thoracodorsal artery, a branch of the
sub- scapular artery, and a vein of the same name accompany the nerve.
