The Minute Anatomy of the Brachial Plexus. 439 



in the internal cutaneous which runs down to the wrist. The little 

 finger is supplied by the 9th in front, and by the 8th behind ; the 

 front of the three middle fingers by the 7th and 8th, the back by the 

 6th and 7th. 



In the sensory system, therefore, as in the motor, the upper nerves 

 can be traced down the pre-axial, and the lowest down the post-axial 

 border ; but a remarkable difference appears between the sensory and 

 the motor system. Whereas in the latter it seemed the tendency for 

 the lowest nerves to supply the lowest muscles, in the sensory nerves 

 the extreme are nearest the upper, the middle nerves nearest the lower 

 extremity of the limb. 



Thus, if the limb be seen from the front the two highest nerves on 

 the outer and inner sides respectively are the 4th and the 10th. 

 Lower than these the 5th and 6th take the outer, the 9th and 10th 

 the inner side. Below the elbow the 6th alone takes the outer, and 

 the 9th alone the inner. In the hand, while the 6th and 9th continue 

 their positions, the 7th and 8th for the first time begin to join in the 

 supply. 



Below the elbow a distinction begins to appear between the dorsal 

 and palmar surfaces. If a line be drawn round the lower third of the 

 forearm from the middle of the front surface to the radial border, 

 across the dorsum, round the ulna, and so to the middle line again, it 

 will cut in order cutaneous branches of the 6th, 7th, 8th, and 9th. 

 Thus on the outer side behind the 6th in the musculo-cutaneous 

 comes the musculo-spiral cutaneous from the 7th, and on the inner 

 side behind the 9th in the internal cutaneous comes the dorsal 

 cutaneous formed by the 8th. When the 8th was dissected in the 

 internal cutaneous it was found going to the dorsum, and where 

 the 7th and 8th joined to form the musculo-spiral branch the 7th lay 

 nearest of the two to the radial border. 



In the fingers, again, the 8th is the lowest nerve supplying the back. 

 The 9th the lowest in front. The extremes therefore are on the palmar, 

 the dorsal area occupies an intermediate position. Above the elbow 

 the 8th in the internal cutaneous of the musculo-spiral, holding a 

 position intermediate between the 10th in the intercosto-humeral, 

 and the 5th and 6th in the circumflex, is an example of the same 

 law. 



The sensory nerves therefore obey the following rules : — 



A. Of two spots on the shin that which is nearer the pre-axial border 



tends to be supplied by the higher nerve. 



B. Of two spots in the pre-axial area the lower tends to be supplied by 



the lower nerve, and of two spots in the post-axial area the lower 

 tends to be supplied by the higher nerve. 



This, however unexpected, is not very difficult to understand. The 



