306 



THE SPINAL NERVES. 



Occasionally the radial nerve supplies the whole of the back of the hand and fingers. Its 

 outer division often gives a branch to the palm. Absence of the radial nerve was met with 

 by Hepburn, the musculo-cutaneous supplying the thumb, and the ulnar nerve all the four 

 fingers (Journ. Anat., xxi, 511). 



A. 



RADIAL- 



Fig. 201. DORSAL CUTANEOUS NERVES OP THE HAND. (A, after Brooks ; B, after Hedon.) 



The radial nerve is shown in blue, the branches of the ulnar in red. offsets of the musculo-cutaneous, 

 musculo-spiral and median in black. The two figures illustrate variations in the cutaneous supply. In 

 A, the radial and ulnar nerves overlap to a considerable extent, and the lower external cutaneous of the 

 musculo-spiral extends to the back of the hand. In B, the overlapping of the radial and ulnar nerves 

 is slight, and the musculo-cutaneous is prolonged independently into the hand. 



Posterior interosseous nerve. This nerve (r. profundus n. radialis), the 

 larger of the two divisions of the musculo-spiral, is composed of fibres derived from 

 the sixth and seventh, sometimes also the eighth, cervical nerves. It winds to the 

 back of the forearm round the outer side of the radius, traversing the cleft in the 

 supinator brevis muscle, and is prolonged between the deep and superficial layers of 

 the extensor muscles to somewhat below the middle of the forearm, where it sinks 

 beneath the long extensor of the thumb, and reaches the lower part of the 

 interosseous membrane. 



Much diminished in size by the separation of numerous branches for the 

 muscles, the nerve lies at the back of the wrist beneath the tendons of the extensor 

 indicis and the common extensor of the fingers, and forms here a small enlargement, 

 from which filaments are given to the adjoining ligaments arid articulations. 



Branches. (a) Muscular branches. Before the nerve passes to the back of the 

 forearm it gives offsets to the extensor carpi radialis brevior (6, 7 c) and the supinator 

 brevs (6 c) muscles. After perforating the supinator brevis, it supplies branches (7 c) 



