THE NERVOUS SYSTEM 165 



N. extensorius cranialis (n.ex.cr.). Sieglbaur (1904). 



Musculo-spiral, or radial nerve .... Humphry (187 1). 

 N. radialis profundus , . Fiirbringer (1873); Hoffmann (1873-8); 



Ribbing (1907). 

 N. radialis superior ...... Osawa (1902). 



This is the larger of the two extensor nerves and is the main con- 

 tinuation of N. spinalis 3. Immediately on entering the arm the 

 nerve turns slightly anteriorly and gives a branch to the M. anconeus 

 humeralis lateralis. It then passes obliquely across the limb between 

 the MM. anconeus humeralis lateralis and medialis, remaining 

 lateral to the humerus, and emerges at the surface of the muscle on 

 the flexor side of the elbow. Here it gives a fairly large branch 

 which spreads over the skin on the extensor surface of the fore-arm — 

 the A^. cutaneus antibrachii dorsalis (n.cut.abr.d.). A little farther on 

 another cutaneous branch arises and passes right along the radial 

 side of the fore-arm as far as the wrist, where it becomes known as 

 the N. dorsalis manus radialis (n.d.m.r.). On reaching the carpus it 

 divides, one small twig continuing along the radial side of the first 

 digit, while the rest of the nerve crosses the wrist to the interdigital 

 space between the first and second fingers. Here it divides again 

 and supplies the skin on the adjoining sides of these two digits. 

 Although mainly cutaneous, this nerve also sends a twig — soon 

 after its origin at the elbow — to the M. extensor antibrachii et carpi 

 radialis, and, as it crosses the carpus, it gives very fine twigs to the 

 MM. extensores breves digiti I and II. 



The main extensor cranialis nerve, after giving off these cutaneous 

 branches, turns mesially and describes a half-spiral turn round the 

 head of the radius, passing between this bone and the M. extensor 

 antibr. et carpi rad., so as to lie in the interosseal space between the 

 radius and the ulna. As it passes over the radius it gives two twigs 

 to the M. extensor antibr. et carpi rad. 



About midway between the wrist and the elbow the nerve divides 

 into two branches. 



{a) The N. dorsalis manus intermedins (n.d.m.i.) which remains 

 in the interosseal space, and passes along the fore-arm to the 

 carpus, where it becomes more superficial, and lies directly under 

 the M. extensor digitorum communis. From here it passes to the 

 space between the second and third digits and bifurcates to supply 

 the neighbouring sides of these two fingers. As it passes over the 

 carpus the nerve supplies fine twigs to the MM. extensores breves 

 digitorum II and III, and at the proximal part of the wrist, a larger 



