256 FASCIA AND MUSCLES OF THE HORSE 



triceps. The belly of the muscle lies for the most part in a cavity formed in 

 the triceps. It is widest about its middle. 



Relations. — Superficially, the skin, fascia, panniculus, and mastoido-humeralis; 

 deeply, the infraspinatus, teres minor, triceps, and brachialis muscles, and 

 branches of the posterior circumflex artery and axillary nerve. 



Blood-supply. — Subscapular artery (chiefly through the posterior circumflex). 



Nerve-supply. — Suprascapular and axillary nerves. 



2. Supraspinatus. — This muscle occupies the supraspinous fossa, which it 

 fills, and beyond which it extends, thus coming in contact with the subscapularis. 



Origin. — The supraspinous fossa, the spine, and the lower part of the car- 

 tilage of the scapula. 



Insertion. — The inner and outer lips of the l)icipital groove. 



Action. — To extend the shoulder joint. It also assists in preventing dis- 

 location. 



Structure. — The surface of the muscle is covered by a strong aponeurosis, 

 from the deep face of which many fibers arise. The muscle is thin at its origin 

 from the cartilage, but becomes considerably thicker below. At the neck of the 

 scapula it divides into two branches, between which the tendon of origin of the 

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

 inserted into the lips of the Ijicipital groove. They are united by a fibrous mem- 

 brane already mentioned in connection with the deep pectoral muscle; some 

 fibers are attached to this membrane and the capsule of the shoulder joint. 



Relations. — Superficially, the skin, fascia, panniculus, trapezius, and mas- 

 toido-humeralis; deeply, the scapula and its cartilage, the sul)scapularis muscle, 

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

 muscle; l^ehind, 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. 



Origin. — The infraspinous fossa and the scapular cartilage. 



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

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



Action. — To abduct the arm and rotate it outward.' It also plays the part 

 of 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 

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

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

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

 the long insertion is cut and reflected, the short insertion, partly tendinous, partly 

 fleshy, is exposed. 



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

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

 head of the triceps, the teres minor, and the nutrient artery of the scapula. 



Blood-supply. — Subscapular artery. 



Nerve-supply. — Suprascapular and axillary nerves. 



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- 



' Giinther states that this muscle assists in extension or fie.xion according to the position 

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



