THK BKACHIAL PLEXUS. 



293 



The posterior thoracic nerve (n. thoracalis longus] usually arises by three 

 roots from the fifth, sixth, and seventh cervical nerves. The upper and middle roots 

 perforate the scalenus rnedius and join either in the substance or on the surface of 

 that muscle : the lower root passes in front of the scalenus medius and joins the 

 trunk opposite or below the first rib. The nerve descends behind the brachial 

 plexus and the first part of the axillary artery, on the outer surface of the serratus 

 inagnus, nearly to the lower border of that muscle, supplying it with numerous 

 branches. The fibres derived from the several roots are distributed to the slips of 

 the muscle in order from above downwards. 



Varieties. The root from the seventh nerve is sometimes wanting 1 . In three instances 

 the nerve was found by Lucas receiving a fourth root from the eighth cervical nerve. The 

 root from the fifth nerve sometimes remains separate, being- distributed only to the upper 

 division of the muscle. 



The suprascapular nerve arises from the trunk formed by the union of the 

 fifth and sixth cervical nerves, but its fibres are derived mainly from the fifth nerve. 

 It passes outwards and backwards beneath the trapezius and omo-hyoid muscles to 



Fig. 194. DISTRIBUTION OP THE SUPRA- 

 SCAPULAR AND CIRCUMFLEX NERVES. 



(Hirschfeld and Leveille. ) ^ 



a, scalenus medixis and posticus muscles ; 

 b, levator anguli scapulae ; c, acroraion ; d, 

 deltoid muscle, of which the back part has 

 been removed ; e, rhomboid muscles ; /, 

 teres major ; g, latissimus dorsi ; 1, the 

 bra hial plexus, seen from behind ; ]', nerve 

 to the rhomboid muscles ; 2, placed on the 

 clavicle, the suprascapular nerve ; 3, its 

 branches to the supraspinatus muscle ; 4, 

 branch to the infraspinatus ; 5, the circum- 

 flex nerve, passing out of the quadrangular 

 interval ; 6, its branch to the teres minor 

 muscle; 7, branches to the deltoid; 8, 

 cutaneous branch. 



the uppf r border of the scapula, where it enters the supraspinous fossa through the 

 suprascapular notch, below the ligament of the same name. In the supraspinous 

 fossa, the nerve supplies branches to the supraspinatus muscle, and a slender 

 articular filament to the shoulder-joint ; and it then descends through the great 

 scapular notch to the lower fossa, where it ends in the infraspinatus muscle, 

 furnishing sometimes a second twig to the articulation of the shoulder. 



Varieties. The suprascapular nerve is sometimes derived eolely from the fifth nerve ; or 

 it may receive a few fibres from the fourth nerve. It has been seen dividing 1 into two parts, 

 the one of which passes through the suprascapular notch, while the other pierces the bone 

 just below the notch. Very rarely a branch is given to the upper part of the subscapularis 

 muscle, or to the teres miuor. Filaments to the acromio-clavicular articulation have been 

 noticed by Riidinger, and to the periosteum and substance of the scapula by Swan and Ellis. 



The nerve of the subclavius muscle, a slender branch given off from the 

 front of the upper trunk of the plexus, is also derived mainly or wholly from the 

 fifth cervical nerve. It descends over the third part of the subclavian artery and 

 behind the clavicle to the posterior surface of its muscle. 



Varieties. The nerve to the subclavius often sends a branch inwards to join the phrenic 

 nerve either at the root of the neck (fig. 193) or, less frequently, in the thorax. A 

 communicating twig to the external anterior thoracic nerve, and a branch to the clavicular 

 head of the sterno-mastoid have also been observed (Turner). 



