788 THE NERVOUS SYSTEM 



(3) The posterior thoracic nerve (external respiratory of Bell) arises, by three 

 roots, from the fifth, sixth, and seventh eervical nerves. The upper two roots 

 traverse the substance of the scalenus niedius; the root from the seventh passes in 

 front of that muscle. Twigs are furnished to the superior portion of the serratus 

 magnus 1)V the upper two roots; lower down they unite and are su))sequently joined 

 by the root from the se\'enth. The nerve now passes downwards behind the brachial 

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

 of the serratus magnus, supplying twigs to each of the digitations of that muscle. 



(4) The communicating twig to the phrenic arises from the tifth nerve close 

 to the point where the latter nerve receives the connnunicating twig from the cervical 

 plexus. 



(5) The nerve to the subclavius is a small twig which arises from the fifth 

 nerve or from the upper trunk of tlie plexus. Its tiln-es are derived from the fifth 

 or in some cases from the fourth cervical nerve through the connnunicating twig 

 from the cervical plexus. It run downwards in front of the tliird stage of the sub- 

 clavian artery and, after giving off a communicating l:>ranch to the phrenic, pierces 

 the posterior layer of the costo-coracoid membrane, and enters the subclavius at 

 the lower border of that muscle. 



Variety, — In rare cases the entire phrenic nerve may pass via the nerve to the suhclavius in 

 front of tlie third stage of the subclavian artery. 



(6) The scnleni and (7) longus colli are supplied by twigs Avliich arise from 

 the lower two or three cervical nerves immediately after their exit from tlie 

 intervertebral foramina. 



Branches given off below the Clavicle 



The AXILLARY or SHORT BRANCHES are (1) the external and (2) internal 

 anterior thoracic, and (3) the three subscapular nerves. 



(1) The external anterior thoracic nerve arises from the outer cord of the 

 plexus; it contains fibres from the fifth, sixth and seventh nerves. After communi- 

 cating with the internal anterior thoracic, it pierces the costo-coracoid membrane 

 and ends in branches which supply the pectoralis major. 



(2) The internal anterior thoracic nerve arises from the inner cord and passes 

 forwards between the first stage of the axillary artery and the axillary vein. It 

 contains fibres of the eighth cervical and first dorsal nerves, and it gives branches to 

 the pectoralis minor, some of which pass through the latter muscle and end in the 

 great pectoral. The nerve then unites with a branch from the external anterior 

 tlioracic, and forms a loop which is placed in front of the first stage of the axillary 

 artery. From this loop additional branches are furnished to the greater pectoral 

 muscle. 



(3) The subscapular nerves are branches of the posterior cord. They are 

 three in number, are distinguished as upper, middle, and lower, and are distributed 

 to the subscapularis, latissimus dorsi, and teres major muscles. 



(a) The upper or short subscapular nerve is derived from the fifth and sixth 

 cervical nerves. It is distributed exclusively to the subscapularis muscle. It is 

 occasionally double. 



(h) The middle or long subscapular nerve contains fibres of the seventli 

 cervical nerve; it accompanies the subscapular artery along the axillary margin of 

 the subscapularis muscle and ends in the latissimus clorsi. 



(c) The lower subscapular nerve, carrying fibres of the fifth and sixth cervi- 

 cal nerves, ]iasses behind the subscapular artery, beloAV the dorsalis scapulae branch, 

 and is distril)uted to the teres major; furnishing one or two twigs to the subscapu- 

 laris, Avliich enter the subscapularis near the axillary margin of that muscle. 



TERMINAL OR LONG BRANCHES. — These are given off as follows: from the 

 outer cord, the musculo-cutaneous and the outer head of the median; from the 

 inner cord, the inner head of the median, the ulnar, the internal cutaneous, and 

 the lesser internal cutaneous; from the posterior cord, the muscido-sj)iral and the 

 circumflex nerves. The circumflex, lesser internal cutaneous, internal cutaneous, 



