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the horizontal position. In this movement it is aided by the supraspinatus and infraspinatus. The anterior (clavicular) portion of the muscle assists the pectoralis major in drawing the arm forwards, while the posterior portion draws it backwards.
M. Supraspinatus.-The supraspinatus arises by fleshy fibres from the supraspinous fossa (except near the neck of the bone) and from the deep fascia over it (Fig. 329, p. 368).
It is directed laterally under the trapezius muscle, the acromion and coracoacromial ligament, to be inserted by a broad thick tendon into the most proximal facet on the larger tubercle of the humerus, and into the capsule of the shoulderjoint (Fig. 336, p. 376).
M. Infraspinatus.-The infraspinatus arises from the infra-spinous fossa of the scapula (excepting near the neck, of the bone and the flat surface along the axillary margin) and from the thick fascia over it (Fig. 337, p. 376).
The fibres of the muscle converge to the neck of the scapula; and are inserted by tendon into the middle facet on the larger tubercle of the humerus, and into the capsule of the shoulder-joint (Fig. 336, p. 376). A bursa separates the muscle from the neck of the scapula, and in a minority of cases communicates with the synovial cavity of the shoulder-joint.
The supraspinatus and the upper part of the infraspinatus muscles are concealed by the trapezius, acromion, and deltoid. They cover the neck of the scapula, the transverse scapular artery, and suprascapular nerve, and the capsule of the shoulder-joint.
Action. The muscle assists the deltoid in abducting and drawing back the arm at the shoulder-joint.
M. Teres Minor.-The teres minor is a small muscle, arising by fleshy fibres from the proximal two-thirds of the flat surface on the dorsal aspect of the axillary margin of the scapula, and from fascial septa separating it from the infraspinatus and teres major muscles (Fig. 337, p. 376).
Lying alongside the lateral border of the infraspinatus, it is inserted, under cover of the deltoid, by a thick flat tendon, into the most distal of the three facets on the
FIG. 335. MUSCLES OF POSTERIOR WALL OF
larger tubercle of the humerus and into the capsule of the shoulder-joint, and, by fleshy fibres, into the posterior aspect of the surgical neck and body of the humerus distal to the tubercle for about an inch (Fig. 341, p. 380).
It is separated from the teres major by the long or scapular head of the triceps brachii, and by the posterior circumflex vessels of the humerus and the axillary nerve. Its origin is pierced by the circumflex scapular artery. The muscle is invested by the deep fascia enclosing the infraspinatus, and is sometimes inseparable from that muscle.
teres and flexor
FIG. 336.- MUSCLE
ATTACHMENTS TO THE ANTERIOR ASPECT OF THE RIGHT HUMERUS.
margin (except for a small area at the inferior angle), and from fascial septa, which separate it on the one side from the subscapularis, and on the other from the infraspinatus and teres minor (Fig. 337).
The muscle is directed along the axillary margin of the scapula to the front of the body of the humerus, where it is inserted, by a broad flat tendon, into the medial border of the sulcus intertubercularis medial to the latissimus dorsi muscle (Fig. 337). Just before its insertion it is closely adherent to the tendon of the latissimus dorsi.
The teres major lies below the subscapularis muscle in the posterior wall of
the axilla. The latissimus dorsi muscle, sweeping round from the back, covers its axillary surface on its way to its insertion. The muscle forms the inferior boundary of a triangular space in the posterior wall of the axilla, of which the other boundaries are, above, the borders of the subscapularis and teres minor muscles, and laterally the surgical neck of the humerus. This space is subdivided by the long head of the triceps brachii, which passes behind the teres major muscle, into (a) a quadrilateral space above, for the passage of the axillary nerve and posterior circumflex artery of the humerus; and (b) a smaller triangular space below, for the circumflex scapulæ artery.
Nerve-Supply. The teres major is supplied, along with part of the subscapularis muscle, by the lower subscapular nerve (C. 5. 6.),
Action.-A medial rotator of the humerus.
M. Subscapularis.-The subscapularis is a large triangular muscle which covers the costal surface of the scapula. It arises by fleshy fibres from the whole of the subscapular fossa and the groove along the axillary margin, excepting the surfaces at the angles of the bone (Fig. 333, p. 372). Springing from several ridges in the fossa are fibrous septa projecting into the substance of the muscle, which increase the extent of its attachment.
Converging to the head of the humerus, the muscular fibres are inserted by a broad, thick tendon into the smaller tubercle of the humerus and into the capsule of the shoulder-joint, and by fleshy fibres into the surgical neck and the body of the humerus distal to the tubercle for about an inch, under cover of the coracobrachialis and short head of the biceps (Fig. 336, p. 376).
This muscle forms the greater part of the posterior wall of the axilla. Its medial or anterior surface is in contact with the serratus anterior and the axillary vessels and nerves. It is separated from the neck of the scapula by a bursa, which is in direct communication with the synovial cavity of the shoulder-joint.
The subscapularis minor is an occasional muscle situated below the capsule of the shoulderjoint. It arises from the axillary border of the scapula below the subscapularis, and is inserted into the capsule of the joint or the proximal part of the body of the humerus.
Nerve-Supply. There are two and often three nerves supplying the subscapularis, viz., the short subscapular (C. 5. 6.), which is often double; and the lower subscapular (C. 5. 6.), which, after supplying its lateral (lower) portion, ends in the teres major.
Actions. The muscle aids in drawing the arm forward and medially rotating the humerus.
The principal action of the above group of muscles is on the shoulder-joint. They have also secondary actions in relation to movements of the trunk and limbs.
1. Movements at the Shoulder-Joint.
b. Flexion (Forwards).
Deltoid (anterior fibres)
Deltoid (posterior fibres)
c. Rotation Laterally.
Deltoid (posterior fibres)
Deltoid (anterior fibres)
d. Circumduction-combination of previous muscles.
The various movements at the shoulder-joint are greatly aided by the muscles acting on the shoulder girdle. In raising the arm above the head, for instance, the humerus is brought to the horizontal position by the deltoid and supraspinatus, and the movement is continued by the
elevators of the shoulder girdle. Again, in forward and backward movements at the shoulderjoint, great assistance is derived from muscles acting directly on the shoulder girdle pectoralis minor and serratus anterior; trapezius and rhomboidei.
2. In relation to the trunk and limbs, the shoulder muscles, by fixing the humerus, have auxiliary power on the one hand in movements of the trunk, such as forced inspiration; on the other hand, acting along with muscles fixing the elbow-joint, they stiffen the limb so as to permit of the more refined movements of the wrist and fingers.
FASCIÆ AND MUSCLES OF THE ARM.
The superficial fascia presents no features of importance. There is a bursa beneath it over the olecranon, and occasionally another over the medial epicondyle of the humerus.
The deep fascia forms a strong tubular investment for the muscles on the anterior and posterior aspects of the humerus. It is continuous above with the deep fascia of the shoulder and axilla, and is further strengthened by fibres derived from the insertions of the pectoralis major, latissimus dorsi, and deltoid muscles. At the elbow it becomes continuous with the deep fascia of the forearm, and gains attachment to the epicondyles of the humerus and the olecranon of the ulna; it is strengthened also by important bands associated with the insertions of the biceps anteriorly and the triceps posteriorly, to which reference will be made in the account of these muscles.
About the middle of the arm on the medial side, the deep fascia is perforated for the passage of the basilic vein and the medial cutaneous nerve of the forearm.
The intermuscular septa are processes of the deep fascia attached to the epicondylic ridges of the humerus. The medial and stronger septum is placed between the brachialis muscle anteriorly and the medial head of the triceps posteriorly, and gives origin to both. It extends proximally to the insertion of the coracobrachialis (which is often continued into it), and the ulnar nerve and superior ulnar collateral vessels pass distally over its medial edge. The lateral septum is thinner. It separates the brachialis muscle and brachioradialis in front from the medial and lateral heads of the triceps behind, and gives origin to those muscles. It extends proximally to the insertion of the deltoid, and is pierced by the radial nerve and profunda brachii vessels..
MUSCLES OF THE ARM.
The muscles of the arm comprise the biceps, coracobrachialis, and brachialis || on the anterior aspect, and the triceps brachii on the posterior aspect of the humerus. Except at its extremities, the biceps brachii is superficial, and forms a rounded fleshy mass on the anterior aspect of the arm. The coracobrachialis is visible on its medial side in the proximal half of the arm, particularly when the arm is raised. The brachialis is concealed by the biceps. The triceps brachii forms the thick mass of muscle covering the posterior surface of the humerus.
M. Coracobrachialis.-The coracobrachialis is a rudimentary muscle. It arises under cover of the deltoid from the tip of the coracoid process, by fleshy fibres, in common with the short head of the biceps, and also frequently from the tendon of insertion of the pectoralis minor muscle.
The fleshy belly is pierced by the musculo-cutaneous nerve, and ends in a flat tendon which is inserted into a faint linear impression about an inch in length on the middle of the medial border of the body of the humerus (Fig. 336, p. 376). It is often continued into the medial intermuscular septum.
The coracobrachialis is the remains of a threefold muscle, of which only two elements are usually present in man, but of which in anomalous cases all the parts may be more or less fully developed. The passage of the musculo-cutaneous nerve through the muscle is an indication of its natural separation into two parts, which represent the persistent middle and distal elements.
The commonest variety is one in which the more superficial (distal) part of the muscle extends more distally than usual, so as to be inserted into the medial intermuscular septum, or even into the medial epicondyle of the humerus. A third slip (coracobrachialis superior or brevis,
rotator humeri) may more rarely be present, forming a short muscle arising from the root of the coracoid process, and inserted into the medial side of the humerus just distal to the capsule of the shoulder-joint.