834 



THE NERVOUS SYSTEM. 



The Brachial Plexus (Fig. 497). 



The Brachial Plexus is formed by the union of the anterior branches of the 

 four lower cervical and the greater part of the first dorsal nerves, receiving also a 

 fasciculus from the fourth cervical nerve. It extends from the lower part of the 

 side of the neck to the axilla. It is very broad, and presents little of a plexiform 

 arrangement at its commencement. It is narrow opposite the clavicle, becomes 

 broad and forms a more dense interlacement in the axilla, and divides opposite 

 the coracoid process into numerous branches for the supply of the upper limb. 

 The nerves which form the plexus are all similar in size, and their mode of com- 

 munication is subject to considerable variation, so that no one plan can be given 

 as applying to every case. The following appears, however, to be the most con- 

 stant arrangement : the fifth and sixth cervical unite together soon after their 

 exit from the intervertebral foramina to form a common trunk. The eighth cervi- 

 cal and first dorsal also unite to form one trunk. So that the nerves forming the 

 plexus, as they lie on the Scalenus medius external to the outer border of the 



TROM 4. ! 



A-SCAPULAR 



BRS TO LOW 

 US COULI 

 SCALEN 



XT: ANT: THORACIC 



UPPER SUB-SCAPULAH. 



SUB-SCAPULAR 

 CIRCUMFLEX 



FIG. 497. Plan of the brachial plexus. 



Scalenus anticus, are blended into three trunks an upper one, formed by the 

 junction of the fifth and sixth cervical nerves ; a middle one, consisting of the 

 seventh cervical nerve; and a lower one, formed by the junction of the eighth 

 cervical and first dorsal nerves. As they pass beneath the clavicle, each of these 

 three trunks divides into two branches, an anterior and a, posterior. The anterior 

 divisions of the upper and middle trunks then unite to form a common cord, 

 which is situated on the outer side of the middle part of the axillary artery, and 

 is called the outer cord of the brachial plexus. The anterior division of the 



