THE BRACHIAL PLEXUS. 



1311 



The branch for the long head of the triceps, before its entrance into the muscle, breaks up 

 into four or five filaments. 



The nerve supply of the inner head of the triceps is usually effected by two branches, an 

 upper and a lower. The upper is short and enters the muscle soon after leaving the musculo- 

 spiral. The lower, called the collateral ulnar branch, is longer and extends for a considerable 

 distance along the inner surface of the triceps in close association with the ulnar nerve. 

 Posterior to the internal intermuscular 



septum it enters its muscle. Tiny 

 filaments accompany the collateral 

 ulnar artery to the capsular ligament 

 of the elbow. 



The nerves to the outer head of the 

 triceps and to the anconeus take their 

 origin as a single trunk. The former 

 passes directly to the inner surface of 

 the outer head, while the latter leaves 

 the musculo-spiral groove and tra- 

 verses the outer portion of the internal 

 head of the triceps until the anconeus 

 is reached. 



bb. After leaving the groove and 

 while lying in the cleft between the 

 brachialis anticus and the brachio- 

 radialis, twigs are given ofT for the 

 supply of the brachio-radialis, the 

 extensor carpi radialis longior and the 

 brachialis anticus. 



The nerve to the brachio-radialis 

 enters the mesial surface of that muscle 

 and usually supplies a filament to the 

 capsule of the elbow. 



The nerve to the extensor carpi 

 radialis longior may arise either from 

 the posterior interosseous or directly 

 from the musculo-spiral. 



The nerve to the brachialis anti- 

 cus, while usually present, is not con- 

 stant. It enters and supplies the lateral 

 portion of that muscle. 



c. The humeral branches com- 

 prise one which is supplied to the 

 periosteum of the extensor surface of 

 the humerus and one which enters the 

 shaft of the humerus with the nutrient 

 artery, when the latter arises as a 

 branch of the superior profunda. 



d. The articular branches are des- 

 tined for the elbow. They arise from 

 the musculo-spiral as it lies between 

 the brachialis anticus and the brachio- 

 radialis, from the ulnar collateral nerve 

 and from the nerve to the anconeus. 



e. The terminal branches of the 

 musculo-spiral arises at about the level 

 of the external condyle and in the fis- 

 sure between the brachialis anticus 

 and the brachio-radialis. They com- 

 prise («a) the posterior interosseous 

 and {bb) the radial. 



Fig. 



1102. 



Int. cvitaneous 

 branch of mus- 

 culo-spiral nerve 



Lesser int. cu- 

 taneous nerve 



Inf. ext. cutaneous 

 branch of musculo- 

 spiral nerve 



Int. cutaneous 

 nerve, po.st. ^ 

 branch ^ 



Post, cutaneous 

 br. of musculo- 

 cutaneous nerve 



Inf. ext. cutaneous 

 branch musculo-spiral 



Dor.sal branch 

 of ulnar nerve 



Radial nerve 



From ulnar nerve 



From 



median 



nerve 



Superficial dissection of right forearm, showing cutaneous nerves 

 of posterior surface. 



aa. The posterior interosseous nerve (r. profundus n. radialis) ( Fig. 

 1 1 00) is the larger of the terminal branches and is mainly motor in function. Its 

 fibres can be traced back to the sixth, seventh and sometimes the eighth cervical 

 nerve. Shortly after its origin it approaches the supinator brevis, through a fissure 

 in whose substance it makes its way to the lateral side of the radius, in this way reach- 



