_Origin._—By three portions: by the internal and external heads, and by the long scapular head. The internal and the external heads are blended except at their proximal extremities. Each head arises from the anconal surface of the shaft of the humerus. The internal head being located toward the deltoid attachment and the external head into the pneumatic fossa. The long head extends from a circumscribed area just posterior to the glenoid cavity of the scapula. The fibers pass around the shoulder-joint, beneath the deltoid. _Insertion._—The broad tendon passes over the elbow-joint and is inserted to the entire under surface of the olecranon of the ulna. _Shape._—Long, large, fleshy. _Relations._—The supraspinatus passes between the internal and the external heads. It is bounded by the biceps and the deltoid. _Action._—An extensor of the antibrachial region directly antagonizing the biceps. =Brachialis Anticus= (Fig. 15, No. 9; Fig. 14, No. 5). _Location._—In the flexure of the humerus and the ulna. _Origin._—It arises fleshy from a circumscribed area on the inner side of the anconal part of the distal extremity of the humerus. _Insertion._—The fibers passing directly over the elbow-joint become inserted to the lateral surface of the proximal end of the ulna close to the margin of the sigmoid cavity. _Shape._—Small, fleshy. _Relations._—With the joint. _Action._—Assists in flexing the forearm upon the arm. Protects the structures in the anterior part of the elbow-joint. THE MUSCLES OF THE FOREARM AND THE HAND Extensor metacarpi radialis longior Supinator brevis Extensor digitorum communis Pronator brevis Flexor metacarpi radialis Extensor ossis metacarpi pollicis Pronator longus Extensor indicis longus Flexor digitorum sublimis Flexor digitorum profundus Flexor carpi ulnaris Flexor carpi ulnaris brevior =Extensor Metacarpi Radialis Longior= (Fig. 13, No. 17). _Location._—Superior to the radius. _Origin._—By two strong tendinous heads; the outer from the tubercle of the external condyle of the humerus, just above the origin of the tendon of the tensor patagii brevis; the inner and stronger portion from the tubercle found above the oblique trochlear facet of the distal end of the radius. _Insertion._—Becoming a flat, broad, and strong tendon at about the middle of the forearm, finally becomes inserted to the apex of the anchylosed first metacarpal of the carpo-metacarpus. _Shape._—A thick, fusiform belly, tendinous at both extremities. _Relations._—By the side of the pronator brevis. _Action._—Raises the hand, and draws it forward toward the radial margin of the forearm and retains it on the same side. =Extensor Digitorum Communis= (Fig. 15, No. 2). _Location._—The upper metacarpal region.