Nine muscles cross the humeroscapular (shoulder) joint and insert on the humerus (table 10.10). Two are called axial muscles because they originate primarily on the axial skeleton—the pectoralis major and latissimus dorsi44 (see figs. 10.15, 10.22, and 10.23). The pectoralis major is the thick, fleshy muscle of the mammary region, and the latissimus dorsi is a broad muscle of the back that extends from the waist to the axilla. These muscles bear the primary responsibility for attachment of the arm to the trunk, and they are the prime movers of the shoulder joint. The pectoralis major flexes the shoulder as in pointing at something in front of you, and the latissimus dorsi extends it as in pointing at something behind you—thus, they are antagonists.
The other seven muscles of the shoulder are called scapular muscles because they originate on the scapula. Among these, the prime mover is the deltoid—the thick muscle that caps the shoulder. It acts like three different muscles. Its anterior fibers flex the shoulder, its posterior fibers extend it, and its lateral fibers abduct it. Abduction by the deltoid is antagonized by the combined action of the pectoralis major and latissimus dorsi. The teres major assists extension of the shoulder and the coracobrachialis assists flexion and adduction.
Tendons of the other four scapular muscles form the rotator cuff—the supraspinatus, infraspinatus, teres minor, and subscapularis (fig. 10.24), nicknamed the "SITS muscles" for their initial letters. The subscapularis fills most of the subscapular fossa on the anterior surface of the scapula. The other three originate on the posterior surface. The supraspinatus and infraspinatus occupy the corresponding fossae above and below the scapular spine, and the teres minor lies inferior to the infraspinatus. The tendons of these muscles merge with the joint capsule of the shoulder as they pass it en route to the humerus. They insert on the proximal end of the humerus, forming a partial sleeve around it. The rotator cuff reinforces the joint capsule and holds the head of the humerus in the glenoid cavity. These muscles act as synergists in shoulder movements. The rotator cuff, especially the tendon of the supraspinatus, is easily damaged by strenuous circumduction (see insight 10.6).
latissimus = broadest + dorsi = of the back
Saladin: Anatomy & I 10. The Muscular System I Text I © The McGraw-Hill
Physiology: The Unity of Companies, 2003 Form and Function, Third Edition
Chapter 10 The Muscular System 357
Table 10.9 Muscles Acting on the Scapula (see figs.10.15,10.17,and 10.21)
O = origin, I = insertion, N = innervation (n. = nerve, nn. = nerves)
Pectoralis (PECK-toe-RAY-liss) Minor
Protracts and depresses scapula when ribs are fixed; elevates ribs when scapula is fixed
O: ribs 3-5 I: coracoid process N: medial and lateral pectoral nn.
Serratus (serr-AY-tus) Anterior
Holds scapula against rib cage; elevates ribs; abducts and rotates scapula to tilt glenoid cavity upward; forcefully depresses scapula; abducts and elevates arm; prime mover in forward thrusting, throwing, and pushing ("boxer's muscle") O: ribs 1-9 I: medial border of scapula N: long thoracic n.
Superior fibers elevate scapula or rotate it to tilt glenoid cavity upward; middle fibers retract scapula; inferior fibers depress scapula. When scapula is fixed, one trapezius acting alone flexes neck laterally and both trapezius muscles working together extend neck O: external occipital protuberance, nuchal ligament, spinous I: clavicle, acromion, scapular spine N: accessory n. (XI), C3-C4
processes of C7-T12
Levator Scapulae (leh-VAY-tur SCAP-you-lee)
Rotates scapula to tilt glenoid cavity downward; flexes neck when scapula is fixed; elevates scapula when acting with superior fibers of trapezius O: transverse processes of vertebrae C1-C4 I: superior angle to medial border of scapula N: C3-C4, dorsal scapular n.
Rhomboideus (rom-BOY-dee-us) Major and Rhomboideus Minor
Retract and elevate scapula; rhomboideus major also fixes scapula and rotates it to tilt glenoid cavity downward
O: spinous processes of vertebrae C7-T1 (r. minor) I: medial border of scapula N: dorsal scapular n.
Since the humeroscapular joint is capable of such a wide range of movements and is acted upon by so many muscles, its actions are summarized in table 10.11.
_Think About It_
Since a muscle can only pull, and not push, antagonistic muscles are needed to produce opposite actions at a joint. Reconcile this fact with the observation that the deltoid muscle both flexes and extends the shoulder.
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