THE BRACHIAL PLEXUS 129 



The fifth and sixth cervical nerves unite to form the upper 

 trunk ; the seventh cervical nerve constitutes the middle trunk, 

 and the eighth cervical and first thoracic nerves unite to form 

 the lower trunk. Each trunk divides into an anterior and a 

 posterior division, and the three posterior divisions unite with 

 one another, forming the posterior cord. The anterior divis- 

 ions of the upper and middle trunks unite to form the lateral 

 cord, while the anterior division of the lower trunk constitutes 

 the medial cord. 



Prior to the formation of the cords, certain nerves arise 

 from the plexus. They are termed the supra-clavicular 

 branches and they include (i) the supra-scapular nerve, (2) the 

 nerve to the subclavius, (3) the long thoracic nerve (of Bell), 

 and (4) the dorsalis scapulae nerve (to the rhomboids). 



The remaining branches of the brachial plexus arise from 

 the three cords. The lateral cord gives off (i) the lateral 

 anterior thoracic nerve, (2) the musculo-cutaneous nerve, and 

 (3) the lateral head of the median nerve. 



The posterior cord gives off (i) the upper subscapular nerve, 

 (2) the thoraco-dorsal (middle or long subscapular nerve), (3) 

 the lower subscapular nerve, (4) the axillary (circumflex) nerve, 

 and (5) the radial (musculo-spiral) nerve. 



The medial cord gives off (i) the medial anterior thoracic 

 nerve, (2) the medial cutaneous nerve of the arm (lesser in- 

 ternal cutaneous), (3) the medial cutaneous nerve of the 

 forearm (internal cutaneous), (4) the medial head of the 

 median nerve, and (5) the ulnar nerve. 



It is necessary to describe not only the distribution of each 

 individual branch of the plexus but also the destination of the 

 individual spinal nerves which form the plexus, because lesions 

 of the spinal medulla or of the spinal nerves produce effects 

 which may involve several nerves, some of them only partially, 

 whereas lesions of individual branches are necessarily confined 

 to those branches, although affecting the areas supplied by 

 more than one spinal nerve (vide infra). 



