THE MUSCLES OF THE SHOULDER 299 



biceps emerges. These branches, fleshy superficially, tendinous deeply, are united 

 by a fibrous membrane already mentioned in connection with the deep pectoral 

 muscle; some fibers are attached to this membrane and the capsule of the shoulder 

 joint. A bursa is often present under the muscle at the tuber scapuliE. 



Relations. — Superficially, the skin, fascia, cutaneus, trapezius, and brachio- 

 cephalicus; deeply, the scapula and its cartilage, the subscapularis muscle, and the 

 suprascapular vessels and nerve; in front, the anterior deep pectoral muscle; be- 

 hind, the spine of the scapula and infraspinatus muscle. 



Blood-supply. — Suprascapular and posterior circumflex arteries. 



Nerve-supply. — Suprascapular nerve. 



3. Infraspinatus. — This muscle occupies the greater part of the infraspinous 

 fossa and extends beyond it posteriorly. 



Origin. — The infraspinous fossa and the scapular cartilage. 



Insertion. — (1) The lateral tuberosity of the humerus, distal to the lateral 

 insertion of the supraspinatus; (2) the posterior eminence of the lateral tuberosity. 



Action. — To abduct the arm and rotate it outward.^ It also acts as a lateral 

 ligament. 



Structure. — This muscle is also covered by a strong aponeurosis, from which 

 many fibers arise, and by means of which the deltoid is attached to the spine of 

 the scapula. A thick tendinous layer partially divides the muscle into two strata, 

 and, coming to the surface at the shoulder joint, constitutes the chief means of in- 

 sertion. This tendon, an inch or more (3 cm.) in width, passes over the posterior 

 eminence of the lateral tuberosity of the humerus; it is bound down by a fibrous 

 sheet, and a synovial bursa is interposed between the tendon and the bone. The 

 portion of the tendon which crosses the lateral tuberosity is in part cartilagi- 

 nous. When the long insertion is cut and reflected, the short insertion, partly 

 tendinous, partly fleshy, is exposed. 



Relations. — Superficially, the skin, fascia, cutaneus, trapezius, and deltoid; 

 deeply, the scapula and its cartilage, the shoulder joint and capsule, the long head 

 of the triceps, the teres minor, and branches of the posterior circumflex artery of the 

 scapula. 



Blood-supply. — Subscapular artery. 



Nerve-supply. — Suprascapular nerve. 



4. Teres minor. — This is a much smaller muscle than the foregoing. It 

 lies chiefly on the triceps, under cover of the deltoid and infraspinatus. 



Origin. — (1) The rough lines on the distal and posterior part of the infra- 

 spinous fossa; (2) a small part of the posterior border of the scapula, about its 

 middle; (3) a tubercle near the rim of the glenoid cavity. 



Insertion. — The deltoid tuberosity and a small area just above it. 



Action. — To flex the shoulder joint and to abduct the arm; also to assist in 

 outward rotation. 



Structure. — The muscle is not rounded, but flat and triangular in the horse. 

 Its origin from the posterior border of the scapula is by means of a fascicular aponeu- 

 rosis which also gives origin to flbers of the infraspinatus and triceps. The short, 

 deep part of the muscle which lies on the joint capsule behind the lateral tuberosity 

 of the humerus is covered at its origin by the distal edge of the tendon of origin of 

 the long head of the triceps. A bursa is commonly found between the terminal 

 part of the muscle and the capsule of the shoulder joint, and is often continuous 

 with that of the infraspinatus. 



Relations. — Superflcially, the deltoid and infraspinatus muscles; deeply, the 

 scapula, the shoulder joint, and the triceps muscle. 



Blood-supply. — Subscapular artery. 



Nerve-supply. — Axillary nerve. 



1 Giinther states that this muscle assists in extension or flexion according to the position 

 of the head of the humerus relative to the glenoid cavity. 



