Till-: 11RACIIIAL I'LHXI'S 



917 



THK BRACHIAI. I'l.KXTS 



The brachial plexus (figs. C)7."), (j7s. <>7'J) is fonnod by the anterior ])riin;iry 

 divisions of I lie four lower cervical nerves and the greater part of t lie lirst thoracic 

 nerve. It is usually joined by small communicating twigs from the fourth cervical 

 ami second thoracic nerves. 



The anterior primary divisions of the lower four cervical nerves, after crossing 

 behind the, vertebral artery and between the anterior and posterior parts of the inter- 

 transver-e muscles, pa>s into the posterior triangle in the interval between the adjacent 

 borders of the anterior and middle scalene muscles, where the fifth and sixth nerves 

 receive a irrey ra mus comniunicans each from the middle cervical sympathetic ganglion, 

 and the seventh and eighth nerves each receive a grey ramus from the inferior cervical 

 sympathetic ganglion. The first thoracic is connected by two rami communicantes 

 wit h the first t horacic sympat het ic ganglion, and it divides into a smaller and a larger 

 branch. The smaller branch passes along the intercostal space a.- the intercostal 

 nerve, and t ne larger branch, after being joined by a communication from the second 



FIG. 678. DIAGRAM OF A COMMON FORM OF BRACHIAL PLEXUS. 



The posterior cord of the plexus is darkly shaded. 



FROM FOURTH CERVICAL 



vr.l/V/,.4* 

 TO I'HKKXIC 



NERVE TO SfBCLAVIUS 



tH'PJlASCAPl'LAK 



f.o\i; ril'ii: [CIO 

 rt/;*r THORACIC 



FIRST 



INTERCOSTAL 

 SECOND 

 THORACIC 

 SECOND 

 INTERCOSTAL 



TIlIRlt 

 THORACIC 



> . I .V TKK10K Till in A CIC NER I '/> 



Ol'TER CORD OF PIJ..VIS 



AI1L1.ARY \ClKri-Ufl.KX, 



MUSCULO-CUTA NEO US 



RADIAL Oll'SCVLO-SPIK.II.) 



IXXER CORD OF PLEXUH 

 Tltoll.K'O-DORSAL 

 MEDIAN 



ULNAR 



MI-:iHM. .I.V77- 

 /,/.' I' ///.(/. 

 CVTANEOl > 



Mi: it i 11 l;i: \< KlAl, 

 CUTANEOUS 

 ( M-:I: i 

 WRISBERO) 

 LA TERA L Cl TA KEO US 



thoracic nerve, passes upwards and outwards, in front of the neck of the first rib and 

 behind the apex of the pleural sac, into the lower part of the posterior triangle of the 

 neck, where it takes part in the formation of the plexus. 



The anterior primary divisions of those cervical nerves that form the brachial 

 plexus may be considered as typically giving off anterior and posterior branches, ex- 

 cept that the fifth and sixth nerves often unite before branching and give off their 

 posterior branches as a common trunk, and the eighth nerve often receives its com- 

 munication from the first thoracic nerve before giving off its posterior branch. 



It is on account of this variation in the point of union of the fifth and sixth cervical 

 nerves and of the eighth cervical and first thoracic nerves that so manV different 

 forms of the plexus have been pictured and described. But if the differences in pri- 

 mary branching be borne in mind, the formation of the plexus is always uniform and 

 simple, notwithstanding its different appearances. 



The three cords are formed from these branches in the following manner: (1) 

 The outer cord is formed by the anterior branches of the fifth, sixth, and seventh 

 nerves; (2) the inner cord, by the anterior branches of the eighth cervical and first 



