314 A MANUAL OF ANATOMY. 



dorsal nerve, with branches from the fourth cervical and the 

 second dorsal nerves. 



The cervical nerves emerge along the side of the neck 

 between the anterior and middle scaleni muscles. The 

 first dorsal passes over the upper border of the first rib to 

 join the last cervical nerve. 



The plexus is conveniently divided into stages for de- 

 scription : 



(i) The nerve roots. (2) The junction of the nerves to 

 form trunks, the fifth and sixth nerves forming the upper, 

 the seventh the middle, and the eighth and first dorsal the 

 lower trunks. (3) The division of these trunks into 

 anterior and posterior branches. The division is about 

 equal in the upper and middle trunk, but in the lower the 

 anterior division is much larger than the posterior. (4) 

 The union of these divisions to form cords. The anterior 

 divisions of the upper and middle trunks forming the outer 

 cord. The anterior division of the lower trunk continuing 

 as the inner cord, and the junction of the posterior 

 divisions of all three trunks forming the posterior cord. 

 The first and second portions of the plexus lie in the neck 

 above and to the outside of the subclavian artery. The 

 third portion is behind the clavicle and the subclavius 

 muscle and at the outside of the axillary artery. The 

 fourth portion surrounds the axillary artery, the outer 

 cord being to the outside, the inner cord to the inside, and 

 the posterior cord behind, the axillary artery. 



Branches of the Plexus. 



(i) From the first portion of the plexus. 



(a) From the fifth nerve, the rhomboid branch. (U) The 

 communicating to the phrenic, (c) The first part of the 

 posterior or long thoracic. (d) From the sixth and the 

 seventh, branches to the scaleni, longus colli muscles, and 



