4. Triceps extensor cubiti (brachkeus posti cus, tri-scapulo-humero-olecranien.)—The tri ceps muscle of the arm is situated on the poste rior surface of the humerus, and, as its name implies, has its origin by three heads. The long head arises by a short, flat, thick tendon from a rough portion of the inferior costa of the scapula, immediately below the glenoid cavity, and passing downwards in front of the inser tion of the teres minor, and behind the teres major it forms a large belly, which covers the posterior surface of the os humeri. The se cond or short head arises from the outer and back part of the os humeri, beginning by a pointed origin immediately below the insertion of the teres minor ; it continues to arise from the external ridge of the humerus as low down as the external condyle ; from the surface of the bone behind this ridge, and from the back part of the external intermuscular ligament. The third head, which is the shortest, called brachi(cus externas, arises by an acute point from the internal ridge of the os humeri, be ginning immediately below the insertion of the teres major ; it also arises from the internal ridge as far down as the internal condyle, from the surface of the humerus behind this ridge, and from the posterior surface of the internal in termuscular ligament. The three heads unite
above the middle of the os humeri, and cover the whole of the back part of that bone ; they form a thick broad tendon, which is inserted into the rough surface on the superior part of the olecranon process of the ulna, adhering closely to the ligamentous fibres covering the posterior surface of the synovial membrane. of the elbow joint ; the lowest fibres of the second and third heads of this muscle, which arise from the back of the condyles, run nearly horizontally into the tendon.
The triceps is covered posteriorly by the teres minor, deltoid,-fascia of the arm and in teguments; in front it is in contact with the posterior surface of the humerus, the inter muscular ligaments, and the back part of the capsule of the elbow-joint. This muscle ex tends the elbow ; when the long head contracts, it draws the scapula towards the humerus, and, if the scapula be fixed, it draws the humerus backwards.