AXILLARY SPACE 41 



smaller than the other two. Of the three anterior divisions 

 the two upper join to constitute the lateral cord, whilst the 

 lower passes distally by itself, as the medial cord of the plexus. 

 From the three cords of the plexus the branches are given 

 off which supply the superior extremity (Figs. 19, 20, 22). 



From the above description it will be seen that the plexus 

 may be divided into four stages : 



First Stage . . Five separate nerves (viz. , lower four 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, a medial, and a 



posterior). 



The plexus extends from the lateral border of the scalenus 

 anterior to the lower border of the pectoralis minor, and it 

 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. 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 (Figs. 21, 22). 



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 of practical 

 importance. 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 supraclavicular region, must be found and cleaned by the dissector of 

 the head and neck ; the remaining parts of the plexus and its branches 

 should be 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 Relations 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 trans- 

 verse 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 (Fig. 21). 



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

 and the transverse scapular artery and vein are in front of 

 it ; and the third part of the subclavian artery is anterior to 

 its lowest trunk. The scalenus medius is still behind it. 



In the axilla it has in front of it the integument, the fasciae, 



