Johnston, Functional Divisions of Nervons System. 95 



and cerebellum. In all vertebrates a part of the fibers of this 

 system, the spinal VIII tract, runs caudally parallel with the 

 spinal V tract and ends in a special nucleus at the caudal end of 

 the medulla. In fishes this nucleus is closely associated with 

 (Acipenser) or lorms an integral part of (Petromyzon) the com- 

 mon nucleus of the spinal V and dorsal tracts. In contrast to 

 this, which indicates the close connection between the two sub- 

 divisions, stands the high degree of histological differentiation 

 of the acusticum and cerebellum, which form the chief centers 

 for the acustico-lateral components. The considerations regard- 

 ing the origin and development of the cerebellum and its rela- 

 tion to the acusticum which have been urged by the writer (12, 

 13), have been quite fully confirmed by the results of Houser's 

 work on Mustelus. The cerebellum contains the same types 

 of cells as the acusticum, and the characteristic elements of the 

 cerebellum, the Purkinje cells, are present in the acusticum also 

 and are developed from the ordinary large cells of the acusti- 

 cum. The cerebellum is, therefore, a derivative of the rostral 

 end of the acusticum. It may be said, then, that the somatic 

 sensory division of the nervous system has a large center in the 

 hind brain which is essentially a unit with the dorsal horn, but 

 presents a remarkable degree of differentiation as compared 

 with the dorsal horn. Almost all the stages of the develop- 

 ment of the complex acusticum and cerebellum from the simple 

 structure of the dorsal horn can be traced in the brain of exist- 

 ing fishes. 



The splanchnic sensory division has its center in the cord 

 in the region of Clarke's column probably in all vertebrates. 

 This conclusion rests on Gaskell's (4) experimental and the- 

 oretical considerations, on Cajal's (i) study of the upper part 

 of the cervical cord in young mammals, on Onuf and Collins' 

 (15) experimental researches on the sympathetic in mammals, 

 and upon the independent investigation of the transitional 

 region between medulla and cord in lower vertebrates by Her- 

 RiCK ((7) Menidia, Mugil) and the writer ((12) Amia, Acipen- 

 ser, Coregonus, Catostomus, Rana; (13) Petromyzon). The 

 fibers ending in this center come from the sympathetic system 



