THE NERVOUS SYSTEM 235 



and in the region of the sacrum, the sacral plexus (Figs. 114 and 



IIS). 

 There are eight cervical nerves, the first of which does not 



make its exit through the intervertebral foramen, as do all the 

 other spinal nerves, but traverses a foramen in the atlas. The 

 first five cervical nerves, the three posterior cranial nerves, and 

 branches from the sympathetic trunk, form the cervical plexus. 

 These five cervical nerves supply mainly the structures of the 

 neck. 



There are thirteen pairs of thoracic nerves. The ventral 

 branches of the sixth, seventh, and eighth cervical nerves and 

 the first thoracic nerve form the brachial plexus. This may be 

 displayed by remo\ing the cephalo-humeral muscle and cutting 

 through the pectoral muscles about two centimeters from their 

 origin (Fig. 48) . The manner in which the nerves anastomose 

 varies somewhat, but the following will be found approximately 

 correct : 



The sixth cervical nerve gives off a small branch to the rhom- 

 boideus and levator scapulae muscles of the shoulder, and then 

 divides into two nearly equal branches, one of which supplies 

 the muscles on the lateral aspect of the scapula, and the other 

 joins with the seventh cervical nerve. Small branches from 

 the fifth and sixth cervical nerves unite to form the phrenic 

 nerve supplying the diaphragm. 



The seventh cervical nerve gives off three small branches at 

 about the same point, one of which is the posterior thoracic 

 supplying the serratus magnus muscle, a second helps to form 

 the musculocutaneous, and the third forms part of the median 

 and anterior thoracic. The main portion of the seventh cer- 

 vical unites with the eighth and first thoracic, to form the mus- 

 culospiral (radial) nerve. The circumflex (axillaris) and sub- 

 scapular branches supplying the deltoid and subscapular muscles 

 are also derived from the seventh. 



The eighth cervical nerve, after giving off a small branch to the 

 pectoral muscle, a small twig to the median nerve, and a large 



