THE CBKEBRO-SPINAL SYSTEM OP NERVES. 591 



the brain they pass a ganglion corresponding to the stationary 

 posterior ganglion of the posterior root of a spinal nerve. 

 These being, however, neither in roots nor ganglion functional, 

 are to be regarded as the phylogenetically (ancestrally) degen- 

 erated remnants of what were once functional ganglia and 

 nerve-flbers ; in other words, the afferent roots of these nerves 

 and their ganglia have degenerated. 



The hindmost group of cranial nerves also answers to the 

 spinal nerves. They arise from nuclei of origin in the medulla 

 and in the cervical region of the spinal cord, directly continu- 

 ous with corresponding groups of nerve-cells in other parts of 

 the spinal cord ; but in these nerves there is a scattering of the 

 components of the corresponding spinal nerves. Certain pecul- 

 iarities of these cranial nerves seem to become clearer if it be 

 assumed that, in the development of the vertebrate, degenera- 

 tion of some region once functional has occurred, in conse- 

 quence of which certain portions of nerves, etc., have disap- 

 peared or become functionless. 



It is also to be remembered that a double segmentation ex- 

 ists in the body, viz., a somatic, represented by vertebrae and 

 their related muscles, and a splanchnic represented by visceral 

 and branchial clefts, and that these two have not followed the 

 same lines of development; so that in comparing spinal nerves 

 arranged in regard to somatic segments with cranial nerves, 

 the relations of the latter to the somatic muscles of the head 

 must be considered; in other words, like must be compared 

 with like. 



