232 CLINICAL APPLIED ANATOMY. 



THE SPINAL NERVES. 

 INJURIES AND DISEASES OF NERVE PLEXUSES. 



The brachial plexus is responsible for the whole nerve supply 

 of the arm. If a lesion involves all parts of the plexus the arm 

 will be completely paralysed. The damage is usually incomplete 

 and three types of paralysis may be recognised, corresponding 

 to the upper, middle and lower parts of the plexus respectively. 



The upper arm type is variously known as Duchenne's 

 obstetrical paralysis of the infant, Erb's paralysis, anaesthetic 

 paralysis and climber's paralysis. The muscles paralysed are 

 the deltoid, supraspinatus, infraspinatus, teres minor, biceps, 

 brachialis anticus and supinator longus. The position assumed 

 by the arm is characteristic. It hangs powerless by the side ; the 

 humerus is rotated inwards by its unopposed internal rotators ; 

 the flexors of the elbow being powerless the forearm is extended ; 

 the supinating action of the biceps is absent, so the hand is fully 

 pronated, and the palm directed backwards and outwards. The 

 muscles which are paralysed derive their supply from the fifth 

 and sixth cervical roots. The same muscular group may be put 

 in action in health by faradising Erb's motor point which lies 

 just behind the posterior border of the sterno-mastoid in front 

 of the transverse process of the sixth cervical vertebra. The 

 lesion is due to the shearing action of the clavicle, which under 

 certain circumstances compresses the plexus against the first 

 rib. For this to occur the arm must be abducted and raised 

 vertically above the shoulder, a movement which causes the 

 clavicle to travel directly backwards. The muscular mechanism 

 of the shoulder is not enough to produce this, but force must be 

 applied to the abducted and raised arm. Hence the lesion may 

 occur at birth when the arm is above the head, as the result 

 of forcible artificial respiration, when the weight of the body 

 hangs on the arm in hand over hand climbing, or when a child is 

 lifted by one arm. By some authors it is held that a similar 

 lesion may be produced, owing to traction on the upper cords of 



