_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, latissimus dorsi, and tensor fasciæ antibrachii muscles, the brachial and deep brachial vessels, and the median and ulnar nerves; behind, the long head, branches of the deep brachial vessels, and the radial nerve. _Blood-supply._—Deep brachial and ulnar arteries. _Nerve-supply._—Radial nerve. 5. =Anconeus= (M. anconeus parvus s. subanconeus).—This is a small fleshy muscle which covers the olecranon fossa and is covered by the triceps. It is somewhat difficult to separate from the outer head. _Origin._—The distal third of the posterior surface of the humerus. _Insertion._—The outer surface of the olecranon. _Action._—To extend the elbow joint, and to raise the capsule of the joint and prevent its being pinched during extension. _Structure._—It is almost entirely fleshy, and is adherent by its deep face to the joint capsule. _Relations._—Superficially, the triceps muscle; deeply, the humerus and the elbow joint. _Blood-supply._—Deep brachial artery. _Nerve-supply._—Radial nerve. [Illustration: FIG. 189.—DIGIT OF HORSE, POSTERIOR VIEW.