BRACHIO-RADIA LIS— TRICEPS 309 



Origin. — The whole of the lower three-fifths of the front of the humerus, with 

 the exception of the small space on its inner aspect occupied l^y the coraco- 

 hrachialis and the impression upon the outer aspect for the deltoid muscle, upon 

 either side of which it sends up two small pointed processes; (2) the front of the 

 internal intermuscuhir septum; (3) a small i)ortion of the ui:>per part of the front 

 of the external intermuscular septum above the point where it is i)ierced Ijy the 

 musculo-spiral nerve. 



Insertion. — The inner and lower part of the rough triangular impression upon 

 the front of the coronoid process of the ulna. 



Structure. — The greater part of its origin is by fleshy fil^res which converge 

 upon a tendinous sheet which makes its appearance at first u2:)on the anterior 

 surface of the muscle, just above the level of the elbow-joint. This tendinous sheet 

 receives the converging fleshy fibres upon its posterior aspect, and becomes gradually 

 thicker until it forms a very strong tendon of insertion which is closely connected 

 with the front of the anterior ligament of the elbow-joint. The lower and outer 

 portion of the miiscle is deeply grooved by the brachio-radialis (supinator longus), 

 so that it often looks like a separate muscle. 



Nerve-supply. — (1) From the external cord of the brachial plexus (through 

 the sixtli cer\'ical nerve) by branches of the musculo-cutaneous nerve which enter 

 the anterior surface of the muscle near the inner border of its upper third; (2) 

 from the posterior cord of the brachial plexus (through the seventh cervical nerve) 

 l)y a small filament from the musculo-spiral nerve Avhich enters the front of that 

 l)art of the muscle which is concealed by the origin of the brachio-radialis (supin- 

 ator longus). 



Action. — To flex the ulna. Like the biceps, it will form a lever of the third 

 order, but with much mechanical disadvantage on account of the proximity of its 

 insertion to the axis of the elbow-joint. There will be, however, a proportional 

 gain in speed and range of movement. 



Relations. — In front, the deep fascia on the outer side of the arm which 

 separates it from the cephalic vein, the biceps, coraco-brachialis, deltoid, brachio- 

 radialis (supinator longus), and extensor carpi radialis longior, the brachial vessels, 

 median and musculo-spiral nerves; behind, the triceps and elbow-joint. 



Variations. — The brachialis anticus is sometimes divided into two heads by a continuation of 

 the cleft between the two pointed processes above mentioned, or by a separation of a part of the 

 outer half Occasionally it gives oif muscular slips t<i the radius or the fascia of the forearm, 

 also to adjacent muscles such as the pronator teres, the brachio-radialis, and the extensor carpi 

 radialis longior. 



3. BRACHIO-RADIALIS 



The l)rachio-radialis (supinator longus) is described with the radial group of 

 muscles (page 321). 



EXTENSORS OF THE FOREARM 



The extensor muscles are two in number — the triceps and the anconeus. 



1. TRICEPS 



The triceps extensor cubiti — named from its three heads and its action upon 

 the elliow or cubitus — forms a thick fusiform sheet wrapping round the posterior 

 surface of the humerus in its whole length. 



Origin. — Tlie long head arises from the lower edge of the glenoid cavity and 

 the axillary border of the scapula for one inch (2'5 cm.) below it. 



The external head arises (1) from the posterior surface of the humerus above 

 the musculo-spiral groove, reaching as far up as the base of the greater tuberosity 

 and the insertion of the teres minor; (2) from the back of the external inter- 

 muscular septum above the jioint where it is pierced b}' the muscXdo-si)iral nerve. 



