THE NERVOUS SYSTEM 437 



towards an intervertebral exit for both roots, but even in 

 birds and mammals there are cases of an exit through a verte- 

 bra. Thus in the pre-sacral vertebrae of birds there are two 

 foramina on each side in the bodies of vertebrae for the separate 

 exit of the two roots; there are also many instances among 

 mammals of the piercing of a vertebra for nerve exits; for 

 example, the majority of the cervical and dorsal vertebrae in 

 pigs, or the dorsal and lumbar vertebrae among ruminants. 



Regarding the union of dorsal and ventral roots ; in the cy- 

 clostome Petromyzon the two remain separate, although in 

 other cyclostomes, (e.g., Myxine) they unite. In true fishes 

 the union of the two roots takes place outside of the vertebral 

 canal ; in the higher forms the union is within it and the united 

 nerves pass through the inter- (or intra-) vertebral foramen. 

 The spinal ganglia, which, in the higher forms, are exclusively 

 associated with the sensory roots, are often in the lower con- 

 nected with both. They may possibly be homologous with the 

 subcutaneous ganglia found on the sensory nerves of Amphi- 

 v.vus, but their development from a ridge along the spinal cord 

 does not seem to support this idea. 



In studying the distribution of the peripheral nervous system 

 there are two fundamental principles to be first considered, ( I ) 

 that of the exact relation between the size of a nerve and the 

 amount of development of the part to which it is distributed ', 

 and (2) that of the permanence of nerve distribution. The 

 first follows from the fact that every cell or related group of 

 cells in a given organ has each its own nerve fiber, and there 

 are thus as many fibers in the nerve bundle supplying the organ 

 as there are such units in the organ itself. If, then, a part re- 

 duces or increases its total number of cells, the change is di- 

 rectly indicated in a corresponding reduction or addition of 

 nerve fibers; and, furthermore, as the separate nerve fibers 

 must each reach a central cell in the brain or cord, there are 

 changes there also. These latter are sometimes sufficient to 

 become easily noticeable, as in the case of the intumescentias of 

 the spinal cord, which are correlated with the development of 

 the limbs. 



