THE MUSCLES OF THE ARM 261 



mass which fills the angle between the posterior border of the. scapula and the 

 humerus. It is clearly divisible into three heads, as described below. 



(a) Long head (Caput longum tricipitis; anconeus longus; caput magnum). 

 — This, the largest and longest of the three heads, is a powerful, thick, triangular 

 muscle, which extends from the posterior border of the scapula to the olecranon. 



Origin. — The posterior border of the scapula. 



Insertion. — The outer and posterior part of the summit of the olecranon. 



Action.— (!) To extend the elbow joint; (2) to flex the shoulder joint. 



Structure. — The muscle arises by a wide, strong ai:)oneurosis from the posterior 

 border of the scapula. From this the bundles of the fleshy portion converge to 

 the short, strong tendon of insertion. A careful examination Avill show that the 

 muscle is penetrated by a tendinous intersection from which many fibers take origin 

 obliquely. The superficial face is covered by an aponeurosis which is specially 

 developed at its distal part. A small bursa occurs under the tendon of insertion. 



Relations. — Externally, the panniculus, deltoid, infraspinatus, teres minor, 

 and the external head; internally, the tensor fasciae antibrachii, teres major, 

 latissimus dorsi, and posterior deep pectoral muscles, and the subscapular vessels; 

 in front, the brachialis, and the inner head, the deep brachial and posterior circum- 

 flex vessels, and the axillary and radial nerves; behind, the skin and fascia. 

 . Blood-supply. — Subscapular and deep brachial arteries. 



Nerve-supply. — -Radial nerve. 



(b) External head (Caput laterale tricipitis s. anconeus lateralis s. externus; 

 caput medium). — This is a strong, quadrilateral muscle, which lies on the outer 

 surface of the arm. Its proximal third is covered by the deltoid and teres minor 

 muscles; the remainder only by the thin panniculus and the skin. 



Origin. — The deltoid tuberosity and the curved rough line which extends from 

 it to the neck of the humerus. 



Insertion. — (1) A small prominent area on the outer surface of the olecranon; 

 (2) the tendon of the long head. 



Action. — To extend the elbow joint. 



Structure. — The origin consists of short tendinous fibers. The belly is thick, 

 and is composed of parallel bundles which are directed obliquely downward and 

 backward. They are inserted partly into the tendon of the long head and partly 

 into the olecranon below and in front of that tendon. 



Relations. — Externally, the deltoid, teres minor, and panniculus muscles; 

 internally, the long and inner heads and the brachialis muscle. Branches of the 

 circumflex vessels and axillary nerve emerge between the posterior edge of the 

 muscle and the long head. The deep face of the muscle is related to the branches 

 of the deep brachial artery and of the radial nerve. 



Blood-supply. — Posterior circumflex and deep brachial arteries. 



Nerve-supply. — Radial nerve. 



(c) Internal head (Fig. 193) (Caput mediale tricipitis; anconeus medialis s. 

 internus; caput parvum). — This is much the smallest of the three heads. It is 

 situated on the inner surface of the arm, and extends from the middle third of the 

 humerus to the olecranon. 



Origin. — The middle third of the inner surface of the shaft of the humerus, 

 behind and below the internal tubercle. 



Insertion. — The inner and fore part of the summit of the olecranon, between the 

 insertion of the long head and the origin of the ulnar head of the flexor perforans. 



Action. — To extend the elbow joint. 



Structure. — The muscle is fleshy except at its insertion, where it has a flat 

 tendon, under which a small bursa may be found. 



Relations. — Externally, the humerus, brachialis, anconeus, and the external 

 head; internally, the posterior deep pectoral, coraco-brachialis, teres major, 



