THE SUPERIOR EXTREMITY 



the first thoracic nerve. The manner in which the nerves 

 join to form the plexus is very constant. The fifth and sixth 

 cervical nerves unite to form an upper trunk ; the seventh remains 

 single and proceeds distally as a middle trunk ; whilst the 



Upper trunk 



Scalenus anterior and longus colli 

 Scalenus medius and posterior 



Phrenic nerve 



Seal, anterior and longus colli 

 Seal, medius and posterior 

 Middle trunk 



Seal, anterior and longus colli 

 Seal, medius and posterior 



Scalenus anterior and longus colli 

 Scalenus medius and posterior 



Lower trunk 



T2 



ANTERIOR 



N.INTE.RCOSTAUS 



FIG. 20. Diagram of the Brachial Plexus. (After Paterson.) 



eighth and first thoracic nerves join, to constitute a third or 

 lower trunk. A short distance above the clavicle each of the 

 three trunks splits into an anterior and a posterior division. 

 When the three anterior divisions are raised on the handle 

 of a knife, the three posterior divisions will be seen uniting to 

 form the posterior cord of the plexus. It should be noted that 

 the lowest or most medial of the posterior divisions is much 



