BRA.WHES OF BRACHIAL P LEXISES 787 



tlif cumjjoiicnt nerves are sejKinite from <'aeh other. In the secund stage the nerves 

 unite witli one another to form three trunks. In the third stage the trunks divide 

 into anterior and posterior divisions. In the fourth stage tlie six divisions 

 l)eeonie collected into three rounded bundles, which are termed cords. 



The tifth and sixth, and usually a communicating twig from the fourth, cervical 

 nerves unite to form the upper trunk; the seventh remains indeiiendent and forms 

 the middle trunk; the eiglith cervical and first thoracic, with occasionally a c(tm- 

 municatiiiLT twig from the second thoracic, imite to form the lower trunk. Eacli of 

 these trunks divides into an anterior and a jtosterior division. The anterior and 

 posterior divisions of the upper and middle trunks are al>out equal in size. The 

 anterior division of the lower trunk is very much larger than the j)osterior division. 

 The three posterior divisions unite to form the posterior cord. The anterior 

 divisions of the upper and middle trunks join to form the outer cord. The anterior 

 division of the lower trunk is c(Mitinued into the inner cord. 



The first stage of the plexus is situated upon the middle scalene in the 

 posterior triangle of the neck. The second stage is placed in th<' posterior inferior 

 triangle of tlie neck, the lower trunk of the ]ilcxus l)eing Itehind the third stage of 

 the subclavian artery and the upper and middle trunks aljove and on a plane 

 posterior to that vessel. When a posterior scapular artery arises from the third 

 stage of the subclavian, it passes backwards Ix-tween the trunks of the plexus. In 

 its third stage the plexus lies under cover of the clavicle and subclavius muscle, 

 and is placed al)Ove, external, and on a plane posterior to the axillary artery. In 

 its fourth stage the plexus is situated under cover of the pectoral muscles, and the 

 cords surround the second stage of the axillar}' artery, occupying the positions 

 indicated by their names, the external cord being in contact with the outer side of 

 the artery, the internal cord on the inner side, and the posterior cord behind the 

 vessel. A little external to the outer border of the pectoralis minor, the plexus 

 terminates by dividing into large nerves, wliich surround the third stage of the 

 axillary artery. 



The branches of the brachial plexus are classified into branches which arise 

 above the clavicle, and branches which are given off beloAv that bone. The branches 

 which arise below the clavicle may be conveniently subdiA-ided into short branches 

 which end in the axilla, and long branches which arise from the terminations of the 

 three cords and are destined to supply the shoulder, arm, forearm, and hand. 



Braxciies given off Ai«nE THE Clavicle 



These are all nuiscular l>ranches, and com])rise the following nerves: — (\) Supra- 

 scapular; (2) rhomboid; To) posterior thoracic; (4) communicating to the phrenic; 

 nerves to (5) the sul)clavius, ((>; scaleni, and (7) longus colli. 



(1) The suprascapular nerve arises from the upper trunk of the plexus, and 

 therefore contains fibres derived from the fifth and sixth, and in some cases from 

 the fourth, cervical nerves. It runs downwards and outwards, crosses internal to 

 the posterior belly of the omo-hyoid muscle, and passes imder cover of the trapezius 

 to reach the suprascapular notch; here it crosses the suprascapular artery, and takes 

 a lower position than that vessel. It then passes through the suprascapular notch 

 beneath the transverse ligament, and, after furnishing a twig to tlie .«houlder joint, 

 divides into a supraspinous and an infras]iin(»us l)ranch. The sujn-aspinous branch 

 inclines inwards and ends in the supraspinatus muscle. The infraspinous branch 

 passes through the great scapular notch under cover of the spino-glenoid ligament, 

 and terminates in the infrasjnnatus muscle. 



(2) The nerve to the rhomboids arises from the fifth cerA'ical nerve shortly 

 after its exit from the intervertebral foramen. It arises in connnon with the u]»per 

 root of the posterior thoracic nerve, and passes V)ackwards and outwards through 

 the fibres of the scalenus medius. It then usually divides in a jdexiform manner 

 into several ])ranches, the strongest of which pa.^ses below the levator anguli 

 scapula^ while the others traverse the lowest division of that nuiscle and supply it 

 with one or more twigs. In the c(^llular interval l)etween the levator and therhom- 

 boideus minor, the twigs reunite to form one nerv<\ which jia.^ses to the deeji surface 

 of the two rhomboid muscles and supplies them. 



