154 Spinal Nerves. 



R. SUPERF. 



tl. RADIAL 



tfcephalica 



N. DORSAL 

 POLLIC. 



498. The Nerves at the Dorsal Side of the Hand. 



7. Musculo- spiral nerve*, the largest branch of the brachial plexus, 

 has its origin from the posterior cord of the plexus. At first it lies behind the 

 axillary artery, winds round the back of the Immerus, passing from the inner 

 to the outer side of the bone, between the external and internal heads of the 

 M. triceps; it pierces the external intermuscular septum and descends between 

 the M. bracliialis anticus and M. supinator lonyus to the front of the external 

 condyle; its muscular branches supply the MM. triceps, anconaeus, supinator 

 lonyus, extensor carpi radialis lonyior and IracJiiaUs anticus. It sends cutaneous 

 branches to the inner and extensor surfaces of the upper- and forearm. It ends 

 by dividing into two branches: 



a) The radial nerve lies a little to the outer side of the radial artery, 

 concealed beneath the M. supinator lonyus; a few inches above the wrist it 

 pierces the deep fascia and divides into two branches, an external, smaller, sup- 

 plying the integument of the radial side and ball of the thumb, and an inter- 

 nal, larger, whose branches supply the adjoining sides of the thumb, index and 

 middle fingers, and, with the dorsal branch of the ulnar nerve, the contiguous 

 sides of the middle and ring fingers. 



b) The posterior inter osseous nerve, passes through the fibres of 

 the M. supinator brevis to the outer side of the forearm; its muscular branches 

 supply the MM. extensor carpi radialis brevior, supinator brevis, extensor communis 

 ditjitorum, extensor minimi digiti, extensor carpi ulnaris, the 3 extensors of the 

 thumb and the extensor indicis. It also sends the R. interosseous externus down 

 to tho capsule of the wrist-joint. 



* This nerve is sometimes known as the radial nerve ; its terminal branches are then 

 called the superficial (radial) and the deep (posterior interosseous). 



