94 Journal of Comparative Neurology. 



highly speciaUzed acustico-lateral centers from the more simple 

 general cutaneous centers can be traced in all its stages in the 

 fishes. The lowest stage of histological development is found 

 in the lowest form (Petromyzon). It will appear from what 

 follows that the arrangement of the centers gives further evi- 

 dence that they are sub-divisions and not separate divisions. 



The general cutaneous components reach their distribution 

 in various rami, innervating all the skin rostral to the innervation 

 territory of the first free spinal nerve, except in those cases in 

 which one or more dorsal roots of the spino-occipital nerves 

 persist. Corresponding to this wide innervation territory, the 

 general cutaneous centers have a great extent in the brain. 

 They include the nucleus common to the dorsal tracts of the 

 cord and the spinal V tract (all vertebrates) ; special collections 

 of cells accompanying the latter tract (Petromyzon), or the 

 tuberculum acusticum in the region of the X to V nerves (Aci- 

 penser), or both (Mustelus, lo) ; and the granular layer of the 

 cerebellum (Acipenser, Petromyzon, Scyllium (2) ). This wide 

 central distribution of the general cutaneous fibers shows clearly 

 that the whole extent of the somatic sensory centers in the 

 brain originally belonged, and still belongs in part, to the gen- 

 eral cutaneous system. In other words, the acustico-lateral 

 system is an offspring of the general cutaneous system and the 

 latter still claims the tuberculum acusticum and cerebellum as 

 parts of its center. There is no particular "general cutaneous 

 nucleus. " The end-arborizations of the general cutaneous fibers 

 lie mingled with those of the acustico-lateral components in 

 each of the chief portions of the somatic sensory column of the 

 brain. This does not mean that the two components have pre- 

 cisely the same central relations, since different connections may 

 be set up by the neurites of the cells in the sensory nuclei. It 

 is true, however, that the central relations of the general cuta- 

 neous and acustico-lateral components are so closely similar as 

 to indicate that their centers are genetically related. 



The acustico-lateral sub-division, although not separated 

 centrally from the general cutaneous, is responsible for the great 

 development and histological differentiation of the acusticum 



