ENCEPHALON OF EISHES. 273 



in the ventral columns a short longitudinal groove divides a 

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

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

 ' funicular ' tract, ib. e, 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 ' restiform ' tract, c, at the 

 inferior part of which a distinct slender portion is 

 also sometimes defined. The post-pyramidal tracts ^ 2 



diverge, expand and blend anteriorly with the 

 similarly bulging restiform tracts, forming the side- 

 walls of a triangular or rhomboidal cavity, called the 

 1 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 ' ' ib. m, concealing their Sectimi of medulla 

 boundary fissures. At the bottom of the ventricle ^ ngata ' Garcha - 

 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, e, corresponding with the fasciculi of 

 the roots of the Great va^al nerve in Selache. 3 



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 

 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 ' filamcnti arciformi ' of Rolando, lviii. p 170, t. i. fig. 2. 



2 These are called ' vorderc pyramiden ' by Dr. Stannius. lvi. p. 43. 



3 xx. vol. in. p. 22; Prep. no. 1311a. 

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