174 LECTURE VIIT. 



As the niyelon approaches the enceplialon it expands, and the 

 following changes may be here observed, in the Cod and Sliark : in 

 the ventral columns a short longitudinal groove di- 

 vides a narrower median ' pre-pyramidal ' tract {^fig. 

 48. a\ from a broader lateral 'olivary' tract {ih. b) : 

 in the dorsal columns a median ' funicular ' tract {ib. e), 

 is similarly marked off from a lateral ' post-pyramidal ' 

 tract {(I) ; this is now, also, distinguished by a deeper 

 fissui-e from the true lateral or 'restiform' tract (c), 

 "^uir^iaT"^' ^"'' ^^ *^^® inferior part of which a distinct slender por- 

 tion is also sometimes defined. Tlie post-pyramidal 

 tracts diverge, expand and blend anteriorly with the similarly bul- 

 ging restiform tracts, forming the side-walls of a triangular or 

 rhomboidal cavity, called the ' fourth ventricle ' : the pre-pyramidal 

 and olivary tracts forming the. floor of the ventricle, are covered below 

 by a thin superficial layer of transverse ' arciform fibres ' * {ib. m ) 

 concealing their boundary fissures. At the bottom of the ventricle 

 the myelonal canal is exposed, and its sides swell and rise as rounded 

 or ' teretial ' tracts {ib. f) "j" from the floor of the ventricle, diverging 

 slightly as they advance, and exposing an intermediate ' nodular ' 

 tract ; this structure is well seen in the Stui-geon and Selache : two 

 lateral prominent ' vagal ' columns, also, project inwards into the 

 ventricle, from tlie conjoined restiform and post-pyramidal tracts ; 

 these vagal columns present a series of nodules, corresponding with 

 tlie fasciculi of the root? of tlie great vagal nerve in Selache : (Prep. 

 1311 A). 



In the Cyprinoid fishes the median inferior tract rises into the 

 ventricle, and is developed into a smooth hemispheric mass, the 

 ' nodulus ' {Jig. 51. 1i) : the conjoined post-pyramidal and restiform 

 walls swell outwards, and form large lateral ' vagal ' lobes ( Jig. 

 51. /<) : these are remarkably developed, and are nodulated in the 

 Carp, wliich is so tenacious of life. The vagal lobes are enormously 

 developed in the Torpedo ; they join the trigeminal lobes, and present 

 a yellowish colour in the recent fish : many non-nucleated cells are 

 present in their substance ; they give origin to the nerves of the 

 electric organs, and have been called 'lobi electrici ' ; but the vagal 

 lobes are scarcely less remarkable for their size in the Gymnotus, 

 where they have no direct connection with any of the nerves of the 

 electric organs. In the Cod the vagal ganglions are obsolete, and 



* Homologous with the " (ilainenti aiciforml " of Rolando, lviji. p. 170. t. i. 

 fig. 2. 



•j- Three are failed " vordero pyraniideii " by Dr. Stanniii»i, i.vi. p. 43. 



