_Action._—(1) To tense the fascia of the forearm and to extend the elbow joint. _Structure._—The origin consists of a very thin aponeurosis which blends with those of the caput longum and the latissimus dorsi. The muscular portion is quite thin in its anterior part, somewhat thicker behind, and is narrower than the aponeurotic origin. It is succeeded by an aponeurotic insertion, which ends chiefly by blending with the fascia of the forearm a little below the elbow. There is, however, a small but constant tendinous attachment to the olecranon. _Relations._—Externally, the panniculus, triceps (long and internal heads), the inner and middle flexors of the carpus, and the ulnar vessels and nerve; internally, the latissimus dorsi, serratus magnus, and posterior pectoral muscles. _Blood-supply._—Subscapular, ulnar, and deep brachial arteries. _Nerve-supply._—Radial nerve. 4. =Triceps brachii= (Figs. 178, 179, 187) (Brachial triceps; triceps extensor cubiti).—This, together with the preceding muscle, constitutes the large muscular 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._—(1) To extend the elbow joint; (2) to flex the shoulder joint. _Structure._—The muscle arises by a wide, strong aponeurosis 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 will 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 fasciæ 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 circumflex 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.