342 A MANUAL OF ANATOMY 



BACK OF THE BRACHIAL REGION. 



Triceps Extensor Cubiti — Origin. — (i) The long head arises fron 

 the infraglenoid ridge of the scapula, where it is superficially ten 

 dinous. (2) The external head arises from (a) the outer part of th 

 posterior surface of the humerus, reaching as high as the insertioi 

 of the teres minor, and as low as the spiral groove ; and (b) th 

 back of the external intermuscular septum above the point wher 

 it is pierced by the musculo- spiral nerve. (3) The internal heai 

 arises from (a) the whole of the posterior surface of the humeru 

 below the spiral groove, reaching upwards on the inner side of th 

 groove, in a tapering manner, as high as a point about ^ inch above 

 and posterior to, the lower border of the insertion of the tere 

 major ; (&) the back of the internal intermuscular septum ove 

 its whole extent ; and (c) the back of the external inter 

 muscular septum below where it is pierced by the musculo-spira 

 nerve. 



Insertion. — (i) The back part of the upper surface of the olecranoi 

 process of the ulna ; (2) the deep fascia covering the anconeus ; an< 

 (3) slightly into the posterior ligament of the elbow- joint. 



The long and external heads terminate in a broad fiat tendon 

 which occupies about the lower half of the arm, the fibres of tb 

 long head ending on its inner side, and those of the external hea( 

 on its upper and outer parts. Most of the fibres of the interna 

 head terminate on the deep surface of the tendon, but some ar< 

 inserted directly into the olecranon. The deepest and lowest fibre 

 of this head are inserted into the posterior ligament of the elbow 

 joint, and form the so-called subanconeus. There is usually ; 

 bursa over the front part of the upper surface of the olecranon 

 separating the tendon of the muscle from the posterior ligamen 

 of the elbow-joint. 



The long head is related to the lower part of the capsular ligamen 

 of the shoulder- joint. 



Nerve-supply. — The musculo-spiral nerve, which is a branch o 

 the posterior cord of the brachial plexus, its fibres being derivec 

 from the last four cervical, and sometimes from the first thoracic. 



Action. — ^The external and internal heads are simple extensor; 

 of the elbow-joint. The long head also extends the elbow-joint 

 and from its relation to the shoulder- joint it depresses the arir 

 upon the scapula. 



Musculo-spiral Nerve. — ^This nerve at first lies behind the thirc 

 part of the axillary artery, and then behind the upper part of the 

 brachial. Thereafter it passes downwards and backwards, with the 

 superior profunda artery, between the long and internal heads ol 

 the triceps. It next winds round the back of the humerus in th( 

 spiral groove, between the external and internal heads of the triceps. 

 Having pierced the upper part of the external intermusculai 

 septum, it descends in the groove between the brachio-radiali- 

 and brachialis anticus to a point a little above the external epi- 



