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 scapule. 
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. 
Structuwre-—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 em.) 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 
hes chiefly on the triceps, under cover of the deitoid 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 fibers 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 —Superficially, the deltoid and infraspinatus muscles; deeply, the 
scapula, the shoulder joint, and the triceps muscle. 
Blood-supply.—Subscapular artery. 
Nerve-swpply.—Axillary nerve. 
1Giinther states that this muscle assists in extension or flexion according to the position 
jof the head of the humerus relative to the glenoid cavity. 
