THE BRACHIAL PLEXUS. 



1293 



ing this plexus emerge in the interval between the scalenus anticus and medius and 

 from the side of the neck pass beneath the clavicle and into the axilla through its apex. 

 The plexus is divided, therefore, into two portions, a cervical ox supraclavicular part 

 (pars supraclavicularis) and an axillary or i7ifraclavicular part (pars infraclavicularis). 

 In the posterior cervical triangle the plexus lies first above and then to the outer side 

 of the subclavian artery and vein, is crossed by the posterior belly of the omo-hyoid 

 muscle and is frequently threaded by the transverse cervical or the posterior scapu- 

 lar artery. After entering the axilla its component parts, while lying mainly to the 

 outer side, form a close fasces around the axillary artery, whose sheath they occupy. 

 In the upper part of the axilla the plexus is overlain by the subclavius and pectoralis 

 major muscles and before dividing into its terminal branches it lies enclosed between 

 the pectoralis minor and subscapularis muscles. 



Constitution and Plan. — In the various weavings of the component elements 

 of the plexus five stages can be recognized : (a) anterior primary divisions of 

 the spinal nerves, {b) trunks, (r) divisions, {a) cords and (e) terminal branches 

 (Fig. 1091). 



Fig. 1091. 



C»T_»NT_T^, 



Diagram illustrating plan of brachial plexus. 



Emerging from the interval between the anterior and middle scalene muscles, 

 the fifth and sixth cervical nerves unite to form the outer or upper trunk, the 

 seventh alone is continued into the middle trunk, whilst the eighth cervical and first 

 thoracic fuse to form the inner or lower trunk. These trunks continue undivided 

 until slightly beyond the lateral margin of the scalenus anticus, each one then sepa- 

 rating into an anterior and a posterior division. These are of about equal size, 

 with the exception of the posterior division of the inner trunk, which is much smaller 

 than the others because the first thoracic nerve sends few if any fibres to the posterior 

 division. The six divisions, three anterior and three posterior, unite differendy to 

 form three cords. The outer cord (fasciculus lateralis) is the bundle formed by the 

 union of the anterior divisions of the outer and middle trunks. The posterior cord 

 (fasciculus posterior) is the result of the fusion of the posterior divisions of all of the 

 trunks and the inner cord (fasciculus medialis) is the continuation of only the 

 anterior division of the inner trunk. The trunks are named in correspondence with 



