THE RADIAL NERVE. 305 



(b) The internal portion communicates with the posterior branch of the 

 musculo-cutaneous nerve in the forearm, sends an offset to the back of the wrist, 

 which usually joins in an arch with the dorsal branch of the ulnar nerve, and 

 then divides into four branches for the thumb and the outer two fingers. One 



Fig. 2'JO. VlKW OK THE RADIAL SIDE OF THE FOREARM, SHOWING THE 



FINAL DISTRIBUTION OF THE MUSCULO-SPIRAL NERVE. (Hirschfeld and 



Leveille.) J 



The supinator longus and the radial extensoi-s have been divided, and 

 their upper parts removed ; the extensor coimnunis digitorum is pulled 

 backwards, and the supinator bi-evis has been partially dissected to show 

 the posterior interosseous nerve passing through it. 



1, on the tendon of the biceps muscle, the musculo-cutaneous nerve ; 

 1', near the wrist, the lower part of this nerve and its plexus of union 

 with the radial nerve ; 2, trunk of the musculo-spiral nerve in the 

 interval between the. brachialis anticus, on which the number is placed, and 

 the supinator longus muscles ; 2', its muscular twigs to the long supinator 

 and long radial extensor ; 2", the posterior interosseous nerve passing 

 through the substance of the supinator brevis ; 3, placed upon the cut 

 lower portion of the supinator longus, and lower down, the radial nerve ; 

 4, the external digital nerve of the thumb ; 5, digital nerves of the 

 forefinger and thumb arising in common ; 6, the same of the fore and 

 middle fingers ; 7, twig of union with the dorsal branch of the ulnar 

 nerve ; 8, placed upon the common extensor of the fingers, the muscular 

 branches of the posterior interosseous nerve to the long extensor muscles ; 

 9, upon the extensor longus pollicifc, the branches to the short extensoi 

 muscles. 



of these supplies the ulnar side of the thumb, the second 

 passes to the radial side of the index finger, the third 

 divides for the adjacent sides of the index and middle 

 fingers, and the fourth is directed to the interspace 

 between the middle and ring fingers. In their course these 

 branches supply twigs to the skin of the outer part of the 

 back of the hand ; and the innermost one crosses, and 

 usually communicates with the outer branches of the dorsal 

 uluar nerve. 



The distance to which the branches of the radial nerve 

 extend varies in the several digits. On the thumb they 

 reach to the nail, on the index finger to the second 



phalanx, and on the middle finger only to the first phalanx, while from the inner- 

 most branch filaments pass to the root of the ring finger. 



From the dorsal digital nerves twigs are said to be given to the carpo-metacarpal, 

 metacarpo-phalangeal, and first interphalangeal articulations ; and according to Hedon 

 filaments pass forwards through the interosseous spaces to the skin of the palm. Lejars 

 describes an offset from the outer division of the radial nerve to the abductor pollicis muscle 

 (Bull. Soc. Anat. Paris, 1890, 433) 



Varieties. The area of distribution of the radial and ulnar nerves, and the degree to 

 which they overlap on the back of the hand, as well as the extent of the dorsal nerves on the 

 three middle fingers, are subject to much variability. From observations in man and 

 mammals Hedon and Zander conclude that the primitive arrangement is that in which the 

 radial nerve supplies three and a half digits and the ulnar one and a half, all the digital 

 branches extending to the nail. The reduction of the dorsal nerves on the three middle 

 fingers appears to be an arrangement peculiar to man, in whom however the nerves are not 

 unfrequently continued to the nail in the index and ring fingers, but very rarely in the 

 middle finger. (H. St. John Brooks, ' ; On the distribution of the cutaneous nerves on the 

 dorsum of the human hand/ 1 Internat. Monatschr. f. Anat. u. Phys., v. 1888 : E. Hedon. 

 ' Etude critique sur rinnervation de la face dorsale de la main," Internat. Monatschr. f. Anat. 

 u. Phys.. vi. 1889 ; R. Zander. Ueber die sensibeln Xerven auf der Riickenflache der Hand 

 bei Saugetieren und beim Menschen.'' Anat. Anz.. iv. 1889.) 



