THE BRACHIAL PLEXUS 131 



THE SUPRA-CLAVICULAR BRANCHES OF THE BRACHIAL 

 PLEXUS 



1. The Supra-scapular nerve (C. 5 and 6) supplies the supra- 

 and the infra-spinatus muscles. 



The supra-spinatus arises from the supra-spinous fossa of the scapula 

 and runs laterally above the capsule of the shoulder -joint to be inserted 

 into the greater tubercle of the humerus. It initiates the movement of 

 abduction at the shoulder-joint and helps the deltoid to maintain the limb 

 in that position. The bulk of the muscle is hidden by the insertion of the 

 trapezius, but when the supra-spinatus becomes atrophied there is some 

 hollowing out above the spine of the scapula. 



The infra-spinatus arises from the large infra-spinous fossa of the scapula 

 and runs laterally, posterior to the capsule of the shoulder-joint, to be 

 inserted into the greater tubercle of the humerus. It is a powerful lateral 

 rotator, and it assists in the movements of adduction and extension. The 

 infra-spinatus is partly overlapped by the deltoid, the trapezius and the 

 latissimus dorsi, but, when it is atrophied, the dorsal surface of the scapula 

 can readily be palpated through the skin above the inferior angle, as the 

 muscle is only covered by fasciae in that situation. 



Lesions of the supra-scapular nerve are by no means 

 common. They result in weakening of the movements of 

 abduction and lateral rotation at the shoulder, but as the 

 deltoid and the teres minor are not involved, the disability 

 is not very marked. In many cases the movement of abduc- 

 tion cannot be initiated, but, if passively commenced, it can be 

 continued and maintained. No sensory changes accompany 

 complete paralysis of the supra-scapular nerve. 



2. The Nerve to the Subclavius (C. 5 and 6) is of little 

 practical importance. The subclavius, which extends from 

 the inferior aspect of the clavicle to the sternal end of the 

 first rib, helps to steady the clavicle during movements at 

 the shoulder-joint. Paralysis of this muscle produces little 

 disability. 



3. The Dorsalis Scapulae Nerve (C. 5) arises from the 

 anterior ramus of C. 5 before the latter joins C. 6 to form the 

 upper trunk of the plexus. The nerve crosses the floor of 



