ENCEPHALON OF FISHES. 273 



in the ventral columns a short longitudinal groove divides a 

 narrower median f pre-pyramidal' tract, fig. 172, a, from a broader 

 lateral ' olivary ' tract, ib. b : in the dorsal columns a median 

 ' funicular ' tract, ib. <?, is similarly marked off from a lateral 

 ' post-pyramidal ' tract, d : this is now, also, distinguished by a 

 deeper fissure from the true lateral or c restiform ' tract, c, at the 

 inferior part of which a distinct slender portion is 

 also sometimes defined. The post-pyramidal tracts 172 



diverge, expand and blend anteriorly with the 

 similarly bulging restiform tracts, forming the side- 

 walls of a triangular or rhomboidal cavity, called the 

 ' fourth ventricle,' fig. 173, y : the pre-pyramidal and 

 olivary tracts, forming the floor of the ventricle, are 

 covered below by a thin superficial layer of trans- 

 verse 'arciform fibres' 1 ib. m, concealing their Section of medulla 

 boundary fissures. At the bottom of the ventricle <>^^,ca,-cha- 

 the myelonal canal is exposed, and its sides swell 

 and rise as rounded or ' teretial ' tracts, 2 ib. f, from the floor of 

 the ventricle, diverging slightly as they advance, and exposing an 

 intermediate ' nodular ' tract ; this structure is well seen in the 

 basking shark ( Selache) : two lateral prominent ( vagal ' columns 

 also project inwards into the ventricle, from the conjoined resti- 

 form and post- pyramidal tracts; these vagal columns present a 

 series of nodules, fig. 173, f, corresponding with the fasciculi of 

 the roots of the great vagal nerve in Selacke. 3 



o o 



In the Cyprinoid Fishes the median inferior tract rises into the 

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

 ( nodulus,' fig. 178, k: the conjoined post-pyramidal and restiform 

 walls swell outwards, and form lateral 'vagal' lobes, large and 



' o o 



nodulated in the Carp, fig. 178, h, which 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 con- 

 nection with any of the nerves of the electric organs. In the 

 Cod the vagal ganglions are obsolete, and the nodulus slightly 

 swells above, and obliterates the ( calamus scriptorius.' In the 

 Lucioperca the vagal lobes are not very distinct, but they mark 



1 Homologous with the ' filament! arciformi ' of Rolando, LVIII. p 170, t. i. fig. 2. 



2 These are called ' vordere pyramiden ' by Dr. Stanuius. LVI. p. 43. 



3 xx. vol.. in. p. 22; Prep. DO. 131 lA. 



VOL. I. T 



