. /'/,/: ATX 



of it. In tlu> axillary fossa the cords arc arranged annin<l the axillary artery, the 

 oilier CM i rd lyinu; to I lie outer side, the inner cord to the inner side, and I lie po.-icrior 

 curd behind the artery. In this region the [xtsterior relations of t he plexus are the fat 

 in the upper part of the fossa anil the suhscapularis muscle, and it is covered in from by 

 the pectoral muscles and the eoraco-clavicular fascia. The lower liorder of the 

 plexus is in relation in the posterior triangle and the root of the neck \\ ith the pleura 

 and the first rib, and it is overlapped in front by the third part of the subclavian 

 artery. In the axillary fossa the lower cord which forms the lower border of the 

 plexus is overlapped anteriorly by the axillary vein. The upper and outer border 

 of the plexus has no very important relations. 



The branches of the plexus are given ofT either from the roots above the clavicle, 

 or from the cords behind ami below t lie clavicle. The branches from the cords are 

 laieral branches and terminal branches. 



The branches from the supra-clavicular portion. After the roots of the 

 plexus have received communications from the sympathetic, which have already been 

 referred to. t hey give off a series of muscular branches, \-i/. the posterior thoracic 

 nerves (the dorsal scapular and the long thoracic nerves), the supraseapular nerve, a 

 communicating twig to the phrenic, the nerve to the subclavius, and small twigs to 

 the scalene muscles and the longus colli muscle. 



The posterior thoracic nerves are two in number: (a) the dorsal scapular 

 (nerve to the rhomboids) arises principally from the fifth cervical nerve, but it fre- 

 quently receives a twig from the fourth nerve (fig. 675). It passes downwards and 

 dorsalwards, across the middle scalene, parallel with and below the spinal accessory 

 nerve to the anterior border of the levator scapula 1 , under which it disappears. It 

 continues its descent under cover of the levator scapulae and the rhomboids almost 

 to the lower angle of the scapula, lying a little internal to the posterior border of the 

 bone, and it supplies the lower fibres of the levator and the smaller and larger rhom- 

 boid muscles. 



(6) The Long Thoracic Nerve (external respiratory of Bell) usually arises, by 

 three roots, from the fifth, sixth, and seventh cervical nerves. The last is sometimes 

 'absent (figs. 675 and 678). The upper two roots traverse the substance of the 

 scalenus medius; the root from the seventh passes in front of that muscle. Twigs 

 are furnished to the superior portion of the serrat us anterior by the upper two roots; 

 lower down they unite and are subsequently joined by the root from the seventh 

 when present. The trunk of the nerve passes downwards Irehind the brachial plexus 

 and the first stage of the axillary artery, and runs along the axillary surface of the 

 serrat us anterior (magnus), supplying twigs to each of the digitations of that muscle 

 dig. 679). 



The Supraseapular Nerve (fig. 675) receives fibres from the fifth and sixth cervi- 

 cal nerves, and occasionally also a twig from the fourth nerve. It is a nerve of con- 

 siderable size, and it passes downwards and dorsalwards parallel with the dorsal scapu- 

 lar nerve, at first along the upper border of the posterior belly of the omo-hyoid 

 muscle, then internal to the latter muscle and under cover of the anterior border of 

 the trapezius to the suprascapular notch (tig. 670), where it comes into relation with 

 the transverse, scapular (suprascapular) artery. 1 1 is separated from the artery at the 

 notch by the superior transverse ligament, the nerve passing through the notch and 

 the artery above the ligament. After entering the supraspinous fossa the nerve 

 supplies branches to the supraspinat us and a branch to the shoulder-joint; then it 

 descends through the great scapular notch bet ween the bone and the inferior trans- 

 verse ligament to the infraspinous fossa, where it terminates in the infraspinatus 

 muscle. 



The communicating twig to the phrenic (fig. 675) arises from the fifth nerve 

 dose to the point where the latter nerve receives its communicating twig from the 

 cervical plexus. 



The nerve to the subclavius (fig. 679) is a small twig which arises from the fifth 

 nerve or from the upper trunk of the plexus, but occasionally it receives additional 

 fibres from the fourth and sixth nerves. It runs downwards in front of the lower part 

 of the plexus and the third stage of the subclavian artery and. after giving off some- 

 times a communicating branch to the phrenic, pierces the posterior layer of the eoraco- 

 clavicular fascia, and enters the subclavius at its lower border. 



Variety. In rare cases the entire phrenic nerve may pass rtd the nerve to the subclavius in 



front of the third stage of the subclavian artery. 



