THE BRACHIAL PLEXUS 103 



prone position, the brachio-radialis and the extensor carpi 

 radialis longus assist in its production. 



Epicritic and protopathic insensibility occurs over the 

 radial half of the forearm. In front, the boundary line is very 

 constant and runs from the line of the ring finger at the wrist 

 to the tendon of the biceps. Dorsally the boundary line is not 

 so definite and the insensitive area varies inversely with the 

 size of the dorsal cutaneous nerve of the forearm (lower external 

 cutaneous branch of the musculo-spiral nerve). 



Section of the cutaneous portion of the musculo-cutaneous 

 nerve produces the same sensory symptoms as those which 

 result from injury of the main trunk. Section of its volar or 

 of its dorsal branch produces no change in sensibility owing to 

 the overlapping by adjoining nerves. 



Medial Cutaneous Nerve of the Forearm (Internal 

 Cutaneous Nerve). When the main trunk of this nerve is cut, 

 epicritic and protopathic sensibility are lost over the ulnar side 

 of the forearm, but if one of the terminal branches is injured, 

 epicritic sensibility alone is lost over the area involved. 



Ulnar Nerve. The ulnar nerve is liable to injury at the 

 wrist, where it is exposed to cuts and stabs, and at the elbow, 

 where it may be involved in fractures, dislocations, or operations 

 on the joint. 



i. At the Wrist. The injury may occur either proximal or 

 distal to the origin of the dorsal cutaneous branch, and, though 

 the motor symptoms are exactly similar, the sensory changes 

 are slightly different in the two cases. 



All the intrinsic muscles of the hand, save those supplied 

 by the median nerve (p. 86), are paralysed, and a characteristic 

 deformity (partial main en griff e) is produced. In cases seen 

 immediately after the accident, the injury to the nerve may be 

 overlooked when no tendons are cut, as the patient appears, on 

 superficial examination, to be able to perform most of the 

 movements of the fingers. It is always essential in these cases 

 to ask the patient to separate the fingers, and, if he is unable to 

 do so, paralysis of the dorsal interossei and of the abductor 

 digiti quinti is at once discovered and a more thorough examina- 

 tion should then be made. 



In old-standing cases, the paralysed muscles atrophy and 

 the fingers are extended at the metacarpo-phalangeal joints, as 

 the balance between the flexors and extensors is lost, owing 

 to paralysis of the interossei. In the little and ring fingers the 



