12,12 



HUMAN ANATOMY. 



Supraacromial brs. cervical plexus 



ing the posterior aspect of the forearm. Here it takes a position between the two 

 layers of the extensor muscles and rapidly decreases in size by giving ofT in quick 

 succession branches to the neighboring muscles. As a much attenuated nerve it 

 reaches the posterior surface of the interosseous membrane at the junction of the 

 middle and lower thirds of the forearm. From the interval between the extensores 



longus and brevis pol- 

 FiG. 1 103. licis it courses along 



the membrane, cov- 

 ered in turn by the ex- 

 tensor longus poUicis, 

 the extensor indicis 

 and the tendons of 

 the extensor longus 

 digitorum, fi n a 1 1 y 

 reaching the dorsum 

 of the wrist, where 

 it presents a small 

 gangliform swelling. 

 In the lower fourth of 

 its course it is some- 

 times called the ex- 

 t e rn al interosseous 

 nerve. 



Bran che s of 

 the posterior interos- 

 seous nerve comprise 

 two sets: those given 

 off before and after 

 traversing the supina- 

 tor brevis. 



Cutaneous brs. 

 circumflex nerve 



Sup. ext. cutaneous 



br. of musculo- 



spiral nerve 



Inf. ext. cutaneous 

 br. of musculo- ■{ 

 spiral nerve I 



Musculo-cutaneou.s 

 nerve, post, cutaneous 

 brancli 

 M uscu io-cutaneou.= 

 nerve, ant. 

 cutaneous brancb 

 Musculo-cutaneous, 

 post, cutaneous br. 



Lesser internal 

 cutaneous nerve 



Internal 

 cutaneous ner\-e 



Those arising be- 

 fore the nerve enters 

 the muscle comprise the 

 nerves for the extensor 

 carpi radialis brevior 

 and the supinator brevis. 

 The latter receives two 

 filaments, which supply 

 the two strata of muscle 

 consequent upon the de- 

 lamination of the supin- 

 ator brevis by the pos- 

 terior interosseous 

 nerve. Quite frequently 

 the nerve to the exten- 

 sor carpi radialis long- 

 tor arises from this por- 

 tion of the posterior 

 interosseous. 



The branches giv- 

 en of? after leaving the 

 muscle include the sup- 

 ply of the extensor car- 

 pi ulnaris, the extensor 

 communis digitorutn, 

 the extensor minimi digiti, the three extensors of the thumb and the extensor indicis. 



The first three of these muscles are supplied by a branch which lea^'es the posterior inter- 

 osseous soon after its emergence from the supinator brevis. This nerve divides into two 

 branches, one of which is distributed to the extensor carpi ulnaris and the other to the remain- 

 ing two muscles. The extensor communis digitorum receives additional innervation from a twig 

 which arises from the posterior interosseous further down the forearm. 



Superficial dissection of right arm, showing cutaneous nerves of 

 anterior surface ; cephalic vein is seen passing up to delto-pectoral 

 interval ; basilic vein pierces deep fascia at lower inner aspect of arm. 



