BRANCHES OF THE BRACHIAL PLEXUS. 



643 



between the scaleni muscles. The two cords thus formed lie side by side 

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

 vessels.. At the same place, or lower down, a third intermediate or posterior 

 cord is produced by the union of fasciculi from each of the other two cords, 

 or separately from the nerves forming them. The three cords of which the 

 plexus now consists, 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. 



The two fasciculi which unite to form the intermediate of the three 

 trunks are generally separated at a higher level than the formation of the 

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

 or even farther down ; this gives rise to some varieties, more apparent than 

 real. The seventh nerve also may give a branch to the cord below it. 



The branches proceeding from the plexus are numerous, and may be 

 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 brachial plexus, the 

 posterior thoracic and supra scapular nerves, a nerve for the rhomboid 

 muscles, another for the subclavius, irregular branches for the scaleni and 

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



The branches for the scaleni and longus colli muscles spring in an irregular manner 

 from the lower cervical nerves close to their place of emergence from the vertebral 

 foramina. 



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

 backwards to the base of the scapula through the fibres of the middle scalenus and 

 beneath the levator anguli scapulae. It is distributed to the deep surface of the 

 rhomboid muscles, and gives sometimes a branch to the levator scapulae. 



Fig. 426. DISTRIBUTION OP 



TOE SUPRASCAPULAR AND 



CIRCUMFLEX NERVES (from 

 HirschfeldandLeveille). | 



Fig. 426. 



1' 



a, the scalenus medius and 

 posticus muscles ; b, levator 

 anguli scapulae ; c, acromion ; 

 d. deltoid muscle, of which 

 the back part has been de- 

 tached from the scapula and 

 in part removed ; e, rhomboid 

 muscle ; /, teres major ; g, la- 

 tissimus dorsi ; 1 , the brachial 

 plexus of nerves as seen from 

 behind ; 1', the nerve of the 

 levator scapulae and rhomboid 

 muscles ; 2, placed on the 

 clavicle, marks the supra- 

 scapular nerve ; 3, its branch 

 to the supraspinatus muscle ; 



4, branch to the infraspinatus ; 



5, placed on the back of the 

 humerus below the insertion 



of the teres minor, marks the circumflex nerve passing out of the quadrangular interval ; 



6, its branch to the teres minor muscle ; 7, branches to the deep surface of the deltoid ; 

 8, cutaneous branch to the back of the shoulder. 



