3 86 NERVOUS SYSTEM. 



(4) and fifth are distributed to the muscles and integument of 

 the sides of the neck. A branch of the fourth supplies the in- 

 tegument in the hollow of the shoulder, and one from the fifth 

 follows the vena cephalica and supplies the integument over 

 the shoulder. The fifth by sending a branch to aid in forming 

 the phrenic nerve (Fig. 157, /) may be considered to enter 

 partly into the brachial plexus. 



Owing to the intercommunicating branches between the 

 ventral roots of the first five cervical nerves, these are some- 

 times considered as forming a loose plexus which receives the 

 name cervical plexus. 



The sixth, seventh, and eighth cervical nerves (with a part 

 of the fifth) become interconnected with each other and with 

 the first thoracic to form the brachial plexus. 



The Brachial Plexus (Fig. 159). The brachial plexus is 

 formed by the ventral rami of the fifth, sixth, seventh, and 

 eighth cervical nerves and the first thoracic. Of the fifth cer- 

 vical only a small part enters into the plexus, forming part of 

 the phrenic nerve. The formation of the plexus is due to the 

 union of the different nerves by means of strong connecting 

 branches or ansae. The plexus lies in the axilla, along with 

 the axillary artery and vein ; all its component nerves pass 

 laterad in front of the first rib. Its branches supply the arm 

 and shoulder. 



The precise arrangement of the different strands is 

 somewhat variable. The plexus is commonly made up in 

 approximately the following manner (Fig. 159). From the 

 fifth cervical nerve ( F) a small branch joins a similar one 

 from the sixth to form the phrenic nerve (a); the remainder of 

 the fifth does not enter into the plexus. From the sixth cer- 

 vical (VI] arise parts of the phrenic nerve (a), the suprascapular 

 (ft], the cranial one of the three subscapular nerves (c\ the 

 axillary (d}, and the musculocutaneus (/"). The sixth also 

 gives off close to its origin a nerve (b'} which passes to the 

 inner surface of the levator scapulae and ramifies over the sur- 

 face, supplying this muscle and extending to the rhomboideus, 

 which it also innervates. The seventh cervical ( VII) is the 

 largest nerve entering into the plexus ; it furnishes parts of one 



