BURS^ 369 



fRelatio?is. — The trapezius covers a small portion of the muscle in the mid-thoracic region 

 of the back. Over a large area it is subcutaneous, and its fascial investment is adherent to 

 the skin. As it winds about the teres major its tendon comes to lie behind the coraco-brachiaUs 

 muscle. The main nerves and vessels of the arm here pass across its ventral surface. The 

 muscle covers in part the rhomboideus major, the infraspinatus, teres major, serratus posterior 

 inferior, the lower ribs, the external intercostal muscles, the dorsal border of the external and 

 internal oblique muscles, and the lower dorsal part of the serratus anterior (magnus). 



Variations. — It may show considerable variation in the extent of its fleshy portion and in 

 the attachment of its aponeurosis to the vertebral column, crest of the ilium, the ribs, and the 

 scapula. Its origin may be merely from the ribs. It may be divided into separate fascicuh. 

 Frequently a fasciculus arises from the inferior angle of the scapula. The muscle is often inti- 

 mately united to the teres major. For an account of the muscular slip which extends from the 

 latissimus dorsi across the axillary fossa to the tendon of the pectoralis major near the inter- 

 tubercular (bicipital) groove see the latter muscle (p. 374); and for the slip continued from the 

 tendon of the latissimus dor.si to the olecranon see the Triceps Muscle (p. 379). 



The teres major (figs. 356, 388). — Origin. — Directly from the dorsal surface of the inferior 

 angle of the scapula and from the septa which lie between this muscle and the subscapularis, 

 teres minor, and infraspinatus muscles. 



Insertion. — For about five or six cm. from the lower border of the small tubercle of the 

 humerus, along the medial lip of the intertubercular (bicipital) groove. Proximally the fibre- 

 bundles are attached directly to the tubercle; more distally the attachment is by means of a flat 

 tendon which extends for some distance on the dorsal surface of the muscle. 



Structure. — The nearly parallel fibre-bundles pass upward in a spiral direction so that the 

 muscle undergoes a torsion on its axis. The fibre-bundles which have the highest attachment 

 to the scapula have the lowest humeral attachment, and vice versa. 



Nerve-supply. — By a branch of the lower subscapular nerve which enters the muscle near 

 the middle of its scapular border. The nerve fibres are derived from the fifth, sixth (and seventh) 

 cervical nerves. 



Action. — It aids the latissimus dorsi in adducting the arm, and in some positions of the arm 

 acts as a medial rotator and as an extensor. 



Relations. — Dorsally the muscle is covered by the latissimus dorsi and by the fascia which 

 extends from this muscle to the deltoid and rhomboid muscles. It is also crossed by the long 

 head of the triceps. Its lower border and ventral surface are largely covered by the latissimus 

 dorsi and its tendon. Its upper border helps to bound a triangular space the other sides of 

 which are the borders of the scapula and the humerus. In front lies the subscapularis, and 

 behind, the teres minor. Across this space passes the long head of the triceps. Lateral to this 

 head lie the humeral circumflex vessels and axillary (circumflex) nerve; and medial, the circum- 

 flex (dorsal) scapular artery. 



Variations. — The teres major may be connected with the latissimus dorsi by a fasciculus, 

 or it may be fused with that muscle or its tendon. Slips have also been seen extending to the 

 triceps and into the fascia of the arm. The muscle is rarely absent. 



The subscapularis (figs. 356, 388). — Origin. — The fibre-bundles spring— (1) directly and 

 by means of tendinous bands from the costal surface of the scapula, except near the neck and 

 at the upper and lower angles; and (2) from intermuscular septa between it and the teres major 

 and teres minor muscles. 



Insertion. — The tendon of insertion as it passes over the capsule of the joint is intimately 

 bound to this. It is inserted into the lesser tubercle of the humerus and into the shaft im- 

 mediately below this. 



Structure. — The fibre-bundles arising from the tendinous bands attached to the bone con- 

 verge upon several tendinous laminae which extend into the muscle from the tendon of insertion, 

 thus forming small penniform fasciculi. The fibre-bundles arising directly from the bone con- 

 verge toward the extremities of the tendinous laminae, thus forming triangular bundles inter- 

 digitating with the penniform fascicuh. The fasciculus which arises highest on the axillary 

 border goes directly to the humerus. 



Nerve-supply. — By two or three subscapular branches from the back of the brachial plexus. 

 One or more of these may arise in association with the axillary (circumflex) nerve. From the 

 main nerves rami spread out to enter the ventral surface of the muscle near the junction of the 

 lateral and middle thirds. The nerve fibres come from the fifth and sixth cervical nerves. 



Action. — It is the chief medial rotator of the arm. It strengthens the shoulder-joint by 

 drawing the humerus against the glenoid cavity. It is an extensor when the arm is at the side, 

 a flexor when the arm is abducted. The upper portion of the muscle, however, acts as a 

 flexor in both positions. The upper part acts as an abductor but when the arm is abducted the 

 muscle is an adductor. 



Relations. — Ventrally it forms the greater part of the posterior wall of the axillary fossa, 

 and enters into relation with the serratus anterior (magnus) and the combined tendon of the 

 coraco-brachialis and biceps. On it he the axillary vessels, the brachial plexus, and numerous 

 lymph-vessels and glands. At its lateral border lie the teres major, the humeral circumflex 

 vessels, axillary (circumflex) nerve, and circumflex (dorsal) scapular vessels. Behind it lie the 

 long head of the triceps and the teres minor muscle. 



Variations. — It may be divided into several distinct segments. A fasciculus vaay be sent 

 to the tendon of the latissimus dorsi and another to the brachial fascia. The subscapularis 

 minor arises from the axiUary border of the scapula and is inserted into the articular capsule 

 (capsular ligament) of the shoulder-joint or into the crest of the lesser tubercle of the humerus. 



BURS^ 



B. subacromialis. — A large bursa, nearly constantly found, between the acromion and 

 coraco-acromial ligament and the insertion of the supraspinatus muscle and capsule of the joint, 



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