THE BRACHIAL PLEX 837 



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 

 fifth cervical, which joins that nerve on the Anterior scalenus muscle : the cervi- 

 cal and first dorsal nerves are also joined by filaments from the middle and 

 inferior cervical ganglia of the sympathetic, close to their exit from the interver- 

 tebral foramina. 



Relations. In the neck the brachial plexus lies at the first between the Anterior 

 and Middle scaleni muscles, and then above and to the outer side of the subclavian 

 artery : it then passes behind the clavicle and Subclavius muscle, lying upon the 

 first serration of the Serratus magnus, and the Subscapularis muscles. 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. The branches of the brachial plexus are arranged in two groups 

 viz. those given off above the clavicle, and those below that bone. 



Branches above the Clavicle. 



Communicating. Posterior thoracic. 



Muscular. Suprascapular. 



The communicating branch 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 muscular branches supply the Longus colli, Scaleni, Rhomboidei, and 

 Subclavius muscles. Those for the Longus colli and Scaleni arise from the lower 

 cervical nerves at their exit from the intervertebral foramina. The Rhomboid 

 branch arises from the fifth cervical, pierces the Scalenus medius, and passes 

 beneath the Levator anguli scapulas, which it occasionally supplies, to the Rhomboid 

 muscles. The nerve to the Subclavius is a small filament which arises from the 

 fifth cervical at its point of junction with the sixth nerve: it descends in front 

 of the subclavian artery to the Subclavius muscle, and is usually connected by a 

 filament with the phrenic nerve. 



The posterior thoracic nerve (long thoracic, external respiratory of Bell) 

 (Fig. 500) supplies the Serratus magnus, and is remarkable for the length of its 

 course. It sometimes arises by two roots from the fifth and sixth cervical nerves 

 immediately after their exit from the intervertebral foramina, but generally by 

 three roots from the fifth, sixth, and seventh nerves. These unite in the substance 

 of the Middle scalenus muscle, and, after emerging from it, the nerve passes down 

 behind the brachial plexus and the axillary vessels, resting on the outer surface of 

 the Serratus magnus. It extends along the side of the chest to the lower border 

 of that muscle, supplying filaments to each of its digitations. 



The suprascapular nerve (Fig. 501) arises from the cord formed by the fifth and 

 sixth cervical nerves : passing obliquely outward beneath the Trapezius, it enters 

 the supraspinous fossa, through the notch in the upper border of the scapula, and, 

 passing beneath the Supraspinatus muscle, curves in front of the spine of the 

 scapula to the infraspinous fossa. In the supraspinous fossa it gives off two 

 branches to the Supraspinatus muscle, and an articular filament to the shoulder- 

 joint : and in the infraspinous fossa it gives off two branches to the Infraspinatus 

 muscle, besides some filaments to the shoulder-joint and scapula. 



Branches below the Clavicle. 



The branches below the clavicle are derived from the three cords, as follows : 

 Fi-fttn the oittfr eorJ arise the external of the two anterior thoracic nerves, the 



musculo-cutaneous nerve, the nerve to the Coraco-brachialis muscle, and the outer 



head of the median. 



