30 THE UPPER EXTREMITY 



seen that all the three posterior divisions unite to form the 

 posterior cord of the plexus, and, further, that the most medial 

 of these divisions is much smaller than the other two. Of 

 the three anterior divisions the two lateral join to constitute 

 the lateral cord, whilst the medial passes distally by itself, 

 as the medial cord of the plexus. From the three cords 

 of the plexus are given off the branches which supply the 

 upper extremity. 



From the above description it will be seen that the plexus 

 may be divided into four stages : 



First Stage, . . Five separate nerves (viz. four lower cervical and first 



thoracic). 



Second Stage, . Three nerve-trunks (viz. an upper, middle, and lower). 

 Third Stage, . Three anterior divisions and three posterior divisions. 

 Fourth Stage, . Three nerve -cords (viz. a lateral, medial, and a 



posterior). 



The plexus lies in the lower and medial part of the 

 posterior triangle of the neck, behind the middle third of 

 the clavicle, and in the axilla ; extending from the lateral 

 border of the scalenus anterior to the lower border of the 

 pectoralis minor. As a rule the first two stages are in the 

 neck, the third stage is behind the clavicle, and the last 

 stage is in the axilla. 



It has been customary to divide the branches of the 

 plexus into supraclavicular and infraclavicular groups, but 

 such a division is neither scientifically accurate nor practically 

 important. The branches of the plexus spring either from 

 its roots, or its trunks, or its cords. The parts of the plexus 

 above the clavicle, and the branches given off in the supra- 

 clavicular region, are found and cleaned by the dissector of 

 the head and neck, and the remaining parts of the plexus 

 and its branches are displayed by the dissector of the upper 

 extremity, but the two dissectors must combine to examine 

 thoroughly the general relations and the branches of the 

 plexus. 



The Eelations of the Plexus. Superficial to the cervical 

 part of the plexus lie the skin, the superficial fascia, the 

 platysma, the deep fascia, the external jugular vein, the 

 transverse cervical and transverse scapular veins, the 

 posterior belly of the omo-hyoid muscle, and the transverse 

 cervical artery. Behind it is the scalenus medius muscle. 

 In the interval between the neck and the axilla, the clavicle 



