272 



ANATOMY OF VERTEBRATES. 



171 



to Arsaki (LIII. tab. iii. fig. 10), gangliated appendage to the 

 encephalon. A like singular modification, but without the 

 ganglionic structure, obtains in Tetrodon and Diodon, in a species 

 of which latter genus I found the myelon, fig. 171, M, only four 

 lines long, in a fish of seven inches in length and mea- 

 surinff three inches across the head. The neural canal 



o 



in these Plectognathic fishes is chiefly occupied by 

 a long l cauda equina,' ib. c e. But, insignificant as 

 the myelon here seems, it is something more than 

 merely unresolved nerve-fibres : transverse white 

 stria3 are discernible in it, with grey matter, showing 

 it to be a centre of nervous force, not a mere con- 

 ductor. In the Lophius a long cauda equina partly 

 conceals a short myelon, which terminates in a point 

 at about the twelfth vertebra. In other fishes the 

 myelon is very nearly or quite co-extensive with 

 the neural canal, and there is no cauda equina, or 

 bundle of nerve-roots, in the canal : a tendinous 

 thread sometimes ties the terminal o;anoiioii to the 



O O 



end of the canal. 



A shallow longitudinal fissure divides the ventral 



~ 



surface, and a deeper one the dorsal surface, of 

 the myelon, into equal moieties : a feeble longi- 

 tudinal lateral impression (Sturgeon) subdivides 

 these into dorsal and ventral columns ; in other fishes 

 (Cod, Herring) these are separated by a lateral tract, and six 

 columns or chords may be distinguished in the myelon - - two 

 dorsal or sensory, two ventral or motory, and two lateral or 

 restiform tracts. A minute cylindrical canal extends from the 

 fourth ventricle, beneath (ventrad of) the bottom of the dorsal 

 fissure, along the entire myelon ; this canal is not exposed in the 

 recent fish by merely divaricating the dorsal columns. Both 

 lateral halves of the myelon have grey matter in their interior, 

 and white transverse stria?. Although many fishes (Bream, 

 Dorsk) show a slight enlargement at each junction of the nerve- 

 roots with the myelon, the anatomical student will look in vain in 

 the recent Eel, or Lump-fish, for that ganglionic structure of the 

 myelon which the descriptions of Cuvier 1 might lead him to 

 expect. 



As the myelon approaches the encephalon, it expands ; and the 

 following changes may be here observed in the Cod and Shark :- 



Brain and mye- 

 lon, Dtodcm, 

 natural size 



1 xxiii , i p. 323 ; xm. iii. p. 



176. 



