582 THE BRACHIAL PLEXUS. 



BRACHIAL PLEXUS. 



This large plexus, from which the nerves of the upper limbs are 

 supplied, is formed by the union of the anterior trunks of the four 

 lower cervical and first dorsal nerves ; and it further receives a fas- 

 ciculus from the lowest of the nerves (fourth), which goes to form the 

 cervical plexus. The plexus extends from the lower part of the neck to 

 the axillary space, and terminates opposite the coracoid process of the 

 scapula in large nerves for the supply of the limb. 



The manner in which the nerves are disposed in the plexus is liable 

 to some variation, but the following may be regarded as the arrange- 

 ment most frequently met with. The fifth and sixth cervical join 

 together at the outer border of the scalenus medius to form an upper 

 trunk ; similarly the eighth cervical and first dorsal unite together 

 between the scaleni muscles to form a lower trunk, while the seventh 

 cervical remains single, forming a middle trunk. 



Soon after passing the outer border of the scaleni muscles each pri- 

 mary trunk divides so as to form an anterior and a posterior branch 

 The' anterior branches of the upper and middle trunks unite together 

 to form what is called the upper or outer cord of the plexus ; the ante- 

 rior branch of the lower trunk forms by itself the lower or inner cord of 

 the plexus, and the posterior branches of all three trunks unite together 

 to form the middle or posterior cord. The cords thus formed lie side by 

 side in the fore part of the plexus, and external to the first part of the 

 axillary vessels, but lower down they are placed, one on the outer side 

 of the axillary artery, one on the inner side, and one behind that vessel, 

 and are continued into the principal nerves for the arm. 



Varieties. — Instead of three primary trunks, only two are found in a great 

 number of instances. This arises from the frequent union of the seventh nerve 

 with the trunk formed by the fifth and sixth nerves, which takes place at the 

 outer border of the scaleni muscles. The fasciculi which unite to form the 

 posterior cord are generally separated at a higher level than the formation of the 

 two other cords, but they are also frequently given off as low as the clavicle, or 

 even farther doivn. As regards position, the fifth and sixth nerves or the fifth 

 alone not unfrequently pierce the upper fibres of the anterior scalenus muscie ; 

 the fifth nerve has even been found to pass altogether in front of that muscle. 

 (Turner, Nat. Hist. Review, 1S(U. and Joiu-n. of Anat,, 1872 ; Henle. Systematic 

 Anatomy, vol. iii. ; Lucas, Guy"s Hosp. Reports, 1875, &c.) 



Branches. — The branches proceeding from the plexus are numerous, 

 and maybe conveniently divided into two classes^viz., those that arise 

 above the clavicle, and those that take origin below the bone. 



BRANCHES ABOVE THE CLAVICLE. 



Above the clavicle there arise from the trunks of the orachial plexus, 

 the posterior thoracic and suprascapular nerves, a nerve for the rhom- 

 boid muscles, auotlier for the subclavius, irregular branches for the 

 scaleni and longus colli, and a branch to join the phrenic nerve. 



Small muscular branches. — The branches for the scaleni eind longus colli 

 omisclcs spring in an irregular manner from the lower cervical nerves close to 

 their place of emergence from the vertebral foramina. 



The branch for thu rhomhoid muscles arises from the fifth nerve, and is directed 

 ..backwards to the base of the scapula through the fibres of the middle calenus 



