644 



THE BRACHIAL PLEXUS. 



The nerve of the subdavius muscle, of small size, arises from the front of the cord 

 which results from the union of the fifth and sixth cervical nerves. It is directed 

 over the outer part of the subclavian artery to the deep surface of the subclavius 

 muscle. This small nerve is commonly connected with the phrenic nerve in the neck 

 or in the che-st, by means of a slender filament. 



Branch to join the phrenic nerve. This small and short branch is an offset from 

 the fifth cervical nerve ; it joins the phrenic nerve on the anterior scalenus muscle. 



POSTERIOR THORACIC NERVE. 



The posterior thoracic nerve (nerve of the serratus magnus, external 

 respiratory of Bell) is formed in the substance of the middle scalenus muscle 

 by two roots, one from, the fifth and another from the sixth nerve, and reaches 

 the surface of the scalenus lower than the nerve of the rhomboid muscles, 

 with which it is often connected. It descends behind the brachial plexus 

 on the outer surface of the serratus magnus, nearly to the lower border of 

 that muscle, supplying it with numerous branches. 



SUPRASCAPULAR NERVE. 



The suprascapular nerve arises from the back of the cord formed by the 

 union of the fifth and sixth nerves, and bends beneath the trapezius to the 

 upper border of the scapula, where it passes between the muscles and the 

 bone. Entering the supraspinous fossa of the scapula, through, the supra- 

 scapular notch (beneath the ligament which crosses the notch), the supra- 

 scapular nerve supplies two branches to the supraspinatus, one near the 

 upper, the other near the lower part of the muscle ; and it then descends 

 through the great scapular notch into the lower fossa, where it ends in the 

 infraspinatus muscle. 



In the upper fossa of the scapula, a slender articular filament is given to the 

 shoulder-joint, and in the lower fossa other twigs of the nerve enter the same joint 

 and the substance of the scapula. 



BRANCHES BELOW THE CLAVICLE. 



Origin of nerves from the plexus. The several nerves now to be described 

 are derived from the three great cords of the plexus in the following order. 



From the upper or outer cord, the external of the two anterior thoracic 

 nerves, the musculo-cutaneous, and the outer root of the median. 



From the lower or inner cord, the inner of the two anterior thoracic, the 

 nerve of Wrisberg, the internal cutaneous, the ulnar, and the inner root of 

 the median. 



From the posterior cord, the subscapular nerves, the circumflex, and 

 the musculo-spiral. 



The nerves traced to the spinal nerves. If the fasciculi of which the principal 

 nerves are composed be followed through the plexus, they may be traced to those of 

 the spinal nerves which in the subjoined table are named along with each trunk. 

 The higher numbers refer to the cervical nerves, the unit to the dorsal nerve : 



5.6.7.8. 



Subscapular from 



Circumflex . 



Musculo-spiral . 



External cutaneous . 



Median 5.6.7.8.1. 



1. or 7.8.1. 



Ulnar 



Internal cutaneous . ) 



Small internal cutaneous ) 



outer 5.6.7 

 inner 8.1. 



Anterior thoracic 



The outline in Fig. 423, taken from a dissection, represents one of the most 

 common arrangements. 



