THE SPINAL NERVES. 6 1 



part of the tail, and note the small caudal spinal nerves 

 as they appear through their foramina of exit. 



156. Running out across the carapace, note the main 

 branches of the dorsal spinal nerves, from the second 

 (which appears between the second and third dorsal 

 vertebrae) to the seventh, inclusive. Each consists of 

 two main divisions which lie side by side. 



157. The first dorsal nerve gives off, close to the ver- 

 tebral column, a branch which enters the brachial 

 plexus (159), and then runs out dorsal to the shoulder- 

 girdle. Parallel to the communicating branch to the 

 brachial plexus, and close to it, is a branch of the main 

 sympathetic chain, which may be noted as a guide in 

 the subsequent dissection of the sympathetic system. 



158. There are nine cervical spinal nerves, which, ex- 

 cept the first, may be found on clearing away the mus- 

 cles on the sides of the neck. The first (sub-occipital 

 nerve) is very small and has no posterior root. To find 

 it flex the head on the spine, open the membrane be- 

 tween occiput and atlas, and with bone forceps cut 

 away the dorsal half of the latter bone. On then gently 

 raising the exposed part of the spinal cord and pushing 

 it to one side, the first cervical nerve will be seen run- 

 ning out between the occipital bone and the atlas. Do 

 not confound it with the spinal accessory (i8o, //), or 

 hypoglossal (180, /), which can also be seen ; the former 

 runs on into the cranial cavity ; the latter passes out 

 through a foramen in the exoccipital bone. 



159. The seventh, eighth, ninth, 'and tenth cervical 

 spinal nerves form the brachial plexus, which also gets 



