_Relations._--Lateral surface with the integument, and at the proximal end with the spinodeltoid and acromiodeltoid (Fig. 75, _e_ and _f_). Medial surface with the brachialis (Fig. 75, _i_), the caput longum (_g_), the caput mediale, the brachioradialis (_k_), the anconeus (Fig. 80, _l_), and the origins of the extensor carpi radialis longus (Fig. 75, _l_), the extensor communis digitorum (_m_), and the extensor digitorum lateralis (_n_). =Caput longum= (anconeus longus) (Fig. 75, _g_; Fig. 68, _k_; Fig. 77, _i_).--A prismatic, fusiform mass connecting the scapula with the olecranon. _Origin_ (Fig. 78, _i_) by a thick, flat tendon from a triangular area one or two centimeters long at the glenoid end of the scapula on the glenoid border. The base of the area is about one millimeter from the margin of the glenoid fossa. _Insertion_ (Fig. 86, _b_).--The muscle ends in a thick tendon which passes over the bifurcated ventral end of the olecranon and is inserted upon the rounded tuberosity which forms the dorsal angle of the olecranon. _Relations._--Lateral surface with the caput laterale (Fig. 75, _h_), a portion of the caput mediale (Fig. 77, _j_ and _k_), the spinodeltoid (Fig. 75, _e_), the infraspinatus (Fig. 75, _c_), and the integument. Medial surface with the epitrochlearis (Fig. 65, _r_) and the conjoined portions of the latissimus dorsi and teres major (Fig. 79, _d′_). Ventral border with the caput mediale (Fig. 77, _j_ and _k_). =Caput mediale.=--The medial head consists of three portions (Fig. 79, _g_, _h_, _j_). (1) The long portion (anconeus posterior) (Fig. 77, _j_; Fig. 79, _g_; Fig. 80, _f_). _Origin_ (Fig. 83, _e_) from a triangular area on the dorsal surface of the humeral shaft. The base of the area is against the articular head, and its apex about one-sixth the length of the humerus from the head. It is between the coracobrachialis and the lateral head of the triceps. [Illustration: FIG. 80.--DEEP MUSCLES ON THE LATERAL SURFACE OF THE HUMERUS. _a_, part of M. supraspinatus; _b_, part of M. infraspinatus; _c_, M. teres minor; _d_, M. teres major; _e_, part of M. latissimus dorsi joining M. teres major; _f_, long part of caput mediale of M. triceps brachii; _g_, cut insertion of M. acromiodeltoideus; _h_, M. brachialis; _i_, M. brachioradialis; _j_, intermediate part of caput mediale of M. triceps brachii; _k_, M. extensor carpi radialis longus; _l_, M. anconeus; _m_, M. extensor communis digitorum. 1, great tuberosity of humerus; 2, 3, superficial radial and dorsal interosseous branches, respectively, of the radial nerve.] _Insertion_ (Fig. 86, _a_).--The muscle ends in a long, slender tendon which passes through the furrow on the ventral angle of the olecranon and is inserted into an oblique ridge which forms the dorsal limit of the furrow. The tendon is separated from the floor of the furrow by a synovial bursa.