272 



ANATOMY OF VERTEBRATES. 



to Arsaki (liii. tab. iii. fig. 10), gangliated appendage tc th" 

 encephalon. A like singular modification, but without tlie 

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

 of wliich latter genus 1 found the myelon, fig. 171, M, only four 

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

 '^'^^ surino- three inches across the head. The neural canal 



in these Plectognathic fishes is chiefly occupied by 

 a long "^ Cauda equina,' ib. c e. But, insignificant as 

 the myelon here seems, it is something more than 

 merely unresolved nerve-fibres : transverse white 

 strice are discernible in it, with grey matter, showing 

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

 ductor. In the Lojohius a long cauda equina jiartly 

 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 ganglion to the 

 end of the canal. 



A shallow longitudinal fissure divides the -N'entral 

 surface, and a deeper one the dorsal siu'face, 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) tlic bottom of the dorsal 

 fissure, along the entire myeton ; 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 strite. Although many fishes (Bream, 

 Dorsk) show a slight enlargement at each junction of the ner\-c- 

 roots with the myelon, the anatomical student will look in ^-ain in 

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

 myelon which the descriptions of Cuvicr ' might lead him to 

 expect. 



As the myelon a])in-(iachcs the encephalon, It expands; and the 

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



Bniiii juid my 

 loll, Dio'ioii, 

 liatui-al size 



xxiii , i ji. 323; XJII. iii. ji. 170. 



