152 THE NERVOUS SYSTEM 



of the first metacarpal at the carpo-metacarpal joint, and this rotation, com- 

 bined with flexion, enables the thumb to be opposed to the little finger. 



The muscles of the thenar eminence are usually the first to 

 be involved in progressive muscular atrophy, and they are 

 affected with great constancy in those cases of cervical rib 

 which give rise to symptoms (see also p. 156). 



The digital nerve to the radial side of the index finger gives 

 a branch of supply to the first lumbrical muscle (p. 145). 



The medial division of the median nerve breaks up into two 

 branches, which pass to the clefts between the index and middle, 

 and middle and ring fingers, respectively. The former supplies 

 the second lumbrical muscle. At the cleft, each breaks up 

 into digital nerves for the volar aspects of the adjacent 

 borders of the two fingers. These digital nerves also supply 

 the distal part of the dorsal surface of the fingers (cf. Ulnar 

 Nerve, p. 145). 



Injuries of the median nerve may be divided into two 

 groups : (a) Those occurring distal to the origin of the 

 motor branches to the flexor muscles of the forearm but 

 proximal to the origin of the motor branches to the muscles of 

 the thenar eminence, (b) Those occurring proximal to the 

 origin of the motor branches to the flexor muscles of the 

 forearm. 



(a) When the median nerve is divided near the wrist, the 

 motor symptoms are not very striking. True abduction and 

 true opposition of the thumb are impossible, but these move- 

 ments are simulated by the abductor pollicis longus (extensor 

 ossis metacarpi pollicis) and the extensors, and the flexor 

 pollicis longus, respectively. Examination reveals the fact 

 that the abduction obtained does not take place in an antero- 

 posterior plane (p. 151), while in the simulated opposition the 

 metacarpal bone of the thumb is not rotated. 



The paralysis of the first and second lumbricals upsets the 

 muscular balance of the hand, and the grasping power is much 

 less than might have been expected. The index and middle 

 fingers tend to be extended at the metacarpo-phalangeal and 



