190 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



teres major (see page 187), and becomes aponeurotic upon its inner surface. It is frequently 

 connected with the latissimus dorsi by a tendinous slip. 



The outer head {anconeus lateralis) (Figs. 270 and 271) arises from the postero-external 

 surface of the humerus, commencing immediately below the greater tubercle, and from the upper 

 half or the upper two-thirds of the external intermuscular septum. The origin of this outer 

 head is fibrous above but fleshy below; its fibers run downward and inward to the common 

 tendon of the triceps. 



The inner head (anconeus medialis) (Figs. 271 to 273) arises more deeply than the outer one, 

 and its fibers are shorter and extend further downward; as a rule, however, it is not as strong 

 as the outer head. It arises from the entire length of the internal intermuscular septum, opposite 

 to the brachialis and partly covered by the biceps, from the posterior surface of the humerus 

 below the groove for the radial nerve, and from the external intermuscular septum as low down 

 as the external epicondyle. The portion of it which comes from the internal intermuscular septum 

 and which is not concealed by the biceps, lies immediately beneath the brachial fascia, as does 

 also that portion arising from the external intermuscular septum, which is not covered by the outer 

 head. A large part of the inner head is concealed by the outer one, and between the origins of 

 the inner and outer heads the radial nerve runs in its groove. 



The fibers of the inner and outer heads unite with those of the long head and pass into the 

 common extensor tendon, which commences upon the posterior surface of the muscle at about 

 the middle of the arm and is inserted chiefly into the olecranon process of the ulna. The insertion 

 not only completely surrounds the olecranon, but also radiates to the ulna and the antibrachial 

 fascia. 



The triceps occupies the entire extensor surface of the arm between the intermuscular septa, 

 and is consequently situated in the postero-external and postero-internal brachial regions. The 

 relations of the long head have already been described (see page 187). The inner head is in 

 relation with the brachialis at the internal intermuscular septum, and at the internal epicondyle 

 its fibers are continued directly into the anconeus; the outer head, at the external intermuscular 

 septum, is in relation successively with the brachialis, the brachioradialis, and sometimes also 

 with the extensor carpi radialis longus. 



The triceps is supplied by the radial nerve. It extends the forearm. 



From a functional and topographic standpoint the triceps is associated with the anconeus 

 (anconeus quartus) (Figs. 271, 278, and 280), which is situated in the upper part of the forearm, 

 and is a flat triangular muscle lying beneath but not adherent to the antibrachial fascia. The 

 lower angle of the muscle is placed between the flexor carpi ulnaris and the superficial group of 

 extensors. It arises by a short tendon from the external epicondyle, passes over the articular 

 capsule of the elbow- joint, to which it is adherent, and is inserted into the upper portion of the 

 posterior surface of the ulna immediately below the olecranon. The upper fibers of the muscle 

 are usually directly continuous with the lower portion of the inner head of the triceps. 



The anconeus has a function and innervation similar to that of the triceps, and in addition it increases the tension 

 of the articular capsule of the elbow-joint. There is frequently a small muscular fasciculus passing between the internal 

 epicondyle and the olecranon over the ulnar nerve; it is known as the epitrochleo-anconeus. 



