THE BRACHJAL PLEXUS 1U31 



by the junction of the fifth and .sixth cervical nerves; a middle one, consisting of 

 the seventh cervical nerve; and a lower one, formed by the junction of the eighth 

 cervical and first thoracic nerves. As they pass beneath the clavicle, to compose 

 the infraclavicular part of the plexus (par's ii/fradavicularis), each of these three 

 trunks divides into two branches, an anterior and a posterior. The anterior divi- 

 sions of the upper and middle trunks then unite to form a common cord, which 

 i s situated on the o u ter side of the middle part of the axillary artery, and is called 

 the outer cord of the brachial plexus (fasciculus laleralis). The anterior division 

 of the lower trunk passes distally on the inner side of the axillary artery in the 

 middle of the axilla, and forms the inner ccrd of the brachial plexus (fasciculus 

 medial in). The posterior divisions of all three trunks unite to form the posterior 

 cord of the brachial plexus (fasciculus posterior}, which is situated behind the second 

 portion of the axillary artery. From this posterior cord are given oft' the two 

 lower subscapular nerves, the upper sub-scapular nerve being given off from 

 the posterior division of the upper trunk prior to its junction with the posterior 

 division of the lower and middle trunks. The posterior cord divides into the 

 circumflex and musculospiral nerves. 



Branches of Communication. The brachial plexus communicates with the cer- 

 vical plexus by a branch from the fourth to the fifth cervical nerve, and with the 

 phrenic nerve by a branch from the fifth cervical, which joins that nerve on the 

 Anterior scalenus muscle; the fifth and sixth cervical nerves are joined by filaments 

 to the middle cervical ganglion of the sympathetic, the seventh and eighth cervical 

 to its inferior ganglion, and the first thoracic nerve to its first thoracic ganglion. 

 (lose to their exit from the interver tebrai foramina the nerves give off the filaments 

 to the ganglia. 



Relations. In the neck, the brachial plexus lies in the posterior triangle, being covered 

 by the skin, Platysma, and deep fascia; it is crossed by the posterior belly of the Omohyoid 

 muscle and by the transversalis colli artery. When the dorsalis scapulae artery arises from the 

 third part of the subclavian it usually passes between the roots of the plexus. The plexus lies 

 at first between the Scalenus anticus and medius, and then above and to the outer side of the 

 subclavian artery; it next passes behind the clavicle and Subclavius muscle, lying upon the first 

 serration of the Serratus magnus, and the Subscapularis muscles. It is in close relation with 

 the apex of the lung (Luschka). In the axilla it is placed on the outer side of the first portion of 

 the axillary artery; it surrounds the artery in the second part of its course, one cord lying upon 

 the outer side of that vessel, one on the inner side, and one behind it, and at the lower part of the 

 axillary space gives off its terminal branches to the upper extremity. 



Branches of Distribution. The branches of the brachial plexus are arranged 

 in two groups viz., those given off above the clavicle, and those below the clavicle. 



SUPHACLAVICULAR BRANCHES. 



Communicating .< 'ith phrenic . . . ....... 5, C. 



| with sympathetic ..... ..... 5, 6, 7, 8, C. 1 , T. 



Muscular 



Rhomboids (posterior scapular) ...... 5, C. 



Supraspinatus ) ~ 



Infraspinatus f Supraacapular ...... 5, 6, C, 



Subclavius . ........... 5 , 6 , C. 



Serratus magnus (posterior thoracic) ..... 5, 6, 7,'c. 



Longus colli ............ 5, 6, 7, 8, C. 



I Scaleni ............. 5, 6, 7, 8, C. 



The Communicating Branch (Figs. 756 and 759) with the phrenic is derived from 

 the fifth cervical nerve or from the loop between the fifth and sixth; it joins the 

 phrenic on the Anterior scalenus muscle. The communications with the sympa- 

 thetic have already been referred to. 



The Muscular Branches (rami in iisculares) supply the Longus colli, Scaleni, 

 Hhomboidei, and Subclavius muscles. Those for the Longus colli and Scaleni 



