THE NERVOUS SYSTEM 22$ 



roots an anterior or motor root, and a posterior or sensory 

 root. We will now consider the several spinal nerves 

 individually. 



The brachial plexus, sometimes called the axillary, is 

 made up of the anterior divisions of the four lower cervical 

 and first dorsal nerves. 



The nerves forming the plexus communicate as follows : 

 The fifth and sixth nerves unite near their exit from the 

 spine to form a common trunk; the seventh nerve joins 

 this trunk near the outer border of the middle scalenus 

 muscle, and the three nerves thus form one large single 

 cord. The eighth cervical and the first dorsal nerves form 

 a common trunk. Thus two large trunks are formed, the 

 upper one by the union of the fifth, sixth, and seventh 

 cervicals; and the lower by the eighth cervical and the 

 first dorsal. These two trunks accompany the subclavian 

 artery to the axilla, lying upon the outer side, the trunk 

 formed by the union of the last cervical and the first dorsal 

 being nearest the vessel. Opposite the clavicle, and some- 

 times in the axilla, each of these cords gives off a fascicu- 

 lus, which, uniting, form a third trunk, so that in the center 

 of the axilla three cords are formed. The brachial plexus 

 communicates with the cervical plexus by a branch from 

 the fourth to the fifth nerve, and with the phrenic by a 

 branch from the fifth cervical, which joins that nerve on 

 the scalenus muscle. It is through this latter addition 

 that in cardiac pains the sensation is felt in the elbow. 

 The three cords of the brachial plexus are placed one on the 

 inner side of the axillary artery, one behind, and the third 

 on the outer side. 



The plexus is broad, becoming narrower as it passes 

 down the arm, but at the axilla, where it divides into 

 terminal branches, it is larger. This plexus communi- 

 cates with the cervical plexus by a branch from the fourth 

 nerve, and also receives branches from the two inferior 

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