NEHVOUS SYSTEM Ol' FISHES. 



r 



Brain and Mye 



Ion, DiudiiH. 



nat. size. 



pair af spinal nerves. The absence of this ganglion in tlic Sliark 

 shows that it relates not to the strength of the tail but to its form, as 

 depending on the concentration and coalescence of the 

 terminal vertebras ; except, indeed, where such meta- 

 morphosis is extreme, as, e. g. in Orfhayurisciis mola, 

 and where it affects the entire condition of the myelon, 

 which has shrunk into a short, conical, and, according 

 to Arsaki (liii. tab. iii. fiy. 10.), gangliated appendage 

 to the encephalon. A like singular modilication, but 

 without the ganglionic structure, obtains in Tetrodon 

 and Diodon, in a species of which latter genus I found 

 the uiyelon {Jig. 47. m.) only four lines long in a Hsh 

 of seven inches in length, and measuring three inches 

 across the head. The neural canal in these Plec- 

 tognathic fishes is chiefly occupied by a long ' cauda 

 equina' {^ib. c. c). But, insignificant as the myelon 

 here seems, it is something more than merely unre- 

 solved nerve fibres : transverse white striae are dis- 

 cernible in it, with grey matter, showing it to be a 

 centre of nervous force, not a mere conductor. 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 Avith 

 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 ventral surface, and 

 a deeper one the dorsal surface, of the myelon, into equal moieties : 

 a feeble longitudinal 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 enlai'gement 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 Cuvicr* might lead liini to expect. 



* xxui. i. p. 323. ; xni. iii. p 



