THE BRACHIAL PLEXUS. 



765 



Hon is subject to considerable variation, so that no one plan can be given 

 as applying to every case. The following appears, however, to be the most con- 

 stant arrangement: the fifth and sixth cervical unite together soon after their 

 exit from the intervertebral foramina to form a common trunk. The eighth cervi- 

 cal and first dorsal also unite to form one trunk. So that the nerves forming the 

 plexus, as they lie on the Scalenus medius external to the outer border of the 



From, ith 



VCervtcal 



VIC, 



Rhomboid 



Subclavian 



C. with Phrenic 

 VII 



Branches to Longus 

 Colli and Scaleni 



VIII C< 



Anterior division 

 of Middle Trunk 



I Dorsal 



Lower Trunk 



Posterior division of Middle Trunk 



Suprascapular 

 Upper Trunk 



Middle Trunk 



Anterior division of Upper Trunk 



External Anterior Thoracic 

 Posterior division of Upper Trunk 

 Upper Sub-scapular 

 Middle and Lower 

 Sub-scapular 



Circumflex 



Posterior division of Lower Trunk 



Internal Anterior Thoracic 



Musculo-cutaneous 





Lesser Internal Cutaneous 



Internal Cutaneous Lj 



Ulnar 



Musculo-spiral 

 FIG. 410. Plan of the brachial plexus. 



Scalenus anticus, are blended into three trunks an upper one, formed 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 dorsal nerves. As they pass beneath the clavicle, each of these 

 three trunks divides into two branches, an anterior and & posterior. 1 The anterior 

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

 which is situated on the outer side of the middle part of the axillary artery, and 

 is called the outer cord of the brachial plexus. The anterior, division of the 

 lower trunk passes down on the inner side of the axillary artery in the middle 

 of the axilla, and forms the inner cord of the brachial plexus. The posterior 

 divisions of all three trunks unite to form the posterior cord of the brachial 

 plexus, which is situated behind the second portion of the axillary artery. From 

 this posterior cord are given off 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 musculo-spiral nerves. 



The brachial plexus communicates with the cervical plexus by a branch from 

 the fourth to the fifth nerve, and with the phrenic nerve by a branch from the 



1 The posterior division of the lower trunk is very much smaller than the others, and is frequently 

 derived entirely from the eighth cervical nerve. 



