THE BRACHIAL PLEXUS 155 



SEGMENTAL SUPPLY OF THE MUSCLES OF THE UPPER LIMB 



Most of the muscles of the upper limb are innervated by 

 more than one segment of the spinal medulla, but the 

 clinical evidence (Kocher, Thorburn, Sherren and others) 

 appears to show that each muscle is dependent on a single 

 segment for its principal action. Thus, the deltoid receives 

 branches from both the fifth and sixth cervical nerves, but 

 it is completely paralysed when the anterior ramus of the 

 fifth is divided. The functions of the additional supply may 

 be sensory or they may be motor, subsidiarily to the main 

 segment, for muscles contract not only when their particular 

 action is desired but also in association with other muscles. 

 For example, when a heavy weight is being carried in the 

 hand, with the arm by the side, the flexors of the fingers 

 apparently do most of the work, but in order to take the 

 strain off the ligaments of the wrist, elbow and shoulder- 

 joints, the flexors and extensors of the wrist and elbow, the 

 deltoid, coraco-brachialis, etc., are all firmly contracted. 



The clinical evidence, unfortunately, is not entirely satis- 

 factory, as witnessed by the fact that the leading authorities 

 do not always agree with one another with reference to the 

 main segments for certain muscles. It seems probable that 

 a group of muscles, possessing a common action (e.g. flexors 

 carpi radialis and ulnaris, digitorum sublimis and profundus, 

 pollicis longus and palmaris longus all assist in flexion of 

 the wrist-joint), should receive its nerve-supply from a single 

 segment. The segmental supply as put forward by Sherren 

 supports this view. Kocher, on the other hand, has come 

 to the conclusion that the segmental supply of a muscle 

 depends rather on the particular joint on which its principal 

 action depends. 



In the present state of existing knowledge, it will perhaps 

 be useful to summarise the different views. 



Fifth Cervical Segment. According to Kocher, this segment 

 is responsible for the supply of (i) the abductors and lateral 



