NERVOUS SYSTEM 



855 



nection also, but it is obscured by the greatly enlarged correlation cen- 

 ters of which the cerebral cortex is the most important. The distinct 

 course in neurology given in the schools deals largely and primarily with 

 the histological structure and function of this cortex. 



Because the cerebral cortex is found only in the higher forms of ver- 

 tebrates and, therefore, is supposed to have developed later in the evolu- 

 tionary scale than the simpler type such as the fish displays, it has been 

 called the neencephalon in contradistinction to the "fish type of brain 

 which is then known as the old-brain or palaeencephalon. 



Another point to note is that the "ear brain," the "skin brain," and 

 the "visceral brain"" are all contained in the rhombsncephalon. In fact, 

 the "stem" of the rhombencephalon (also called the segmental portion) 

 is made up of these sensory "brains" and their corresponding motor 

 centers. 







B 



an. 2, 



oph sup. 



A and B, Schematic diagrams of sections of the skin. The sections pass 

 through the lateral line organs. A, of a Teleost, B, of a dogfish. N, lateral 

 line nerve ; S, sensory nerve ending ; the asterisk shows the cutaneous ori- 

 fice ; the arrows indicate the direction of the stimulus. 



C, Lateral line nerve of a fish, anl, anastomosis between the anterior 

 and posterior portions of the lateral nerve ; an2, transverse anastomosis be- 

 tween the right and left lateral nerve ; buc., buccal branch of lateral nerve ; 

 g.l., lateral nerve ganglion ; mand.ex., mandibular branch of lateral nerve ; 

 m., spinal cord ; oph.sup., superficial ophthalmic branch of lateral nerve ; r.U., 

 branch which follows the lateral line X, X cranial nerve (dotted) to show 

 partial fusion with lateral nerve. (From Vialleton, A and B after Dean.) 



