NERVOUS SYSTEM OF FISHES. 203 



57 _ f { „ ih by the pigmental layer or not, is called the fahd- 

 k -^^^^^^^^ form process ' (c) ; it carries before it a fold of 

 M^'^S^r^i^) the proper tunic of the vitreous humour (' mem- 

 '-iiA '" Jl^'^ brana hyaloidea '), and usually extends to the 

 T-^^fe" 'ij' "'^'^^r^ ^ capsule of the lens {d\ to which it is attached 

 «^§4^ ^C^^ by means of a clear but firm substance, called 

 'i\\'k\. *^*^ ' campanula Halleri.^ 

 JW^ The posterior or outer layer of the retina con- 

 Eye of swoni-fish ; sigts of the cellular basis, supporting the stratum 



one-tniru natural size. . 



of cylindricules, standing vertically upon its con- 

 cave surface, with the interblended twin-fusiform corpuscles, both of 

 which microscopic structures are more easily demonstrated in the 

 present than in the higher classes of Vertebrata. Each twin-corpuscle 

 is surrounded by a circle of cylindricules. The primitive nerve-fibres 

 radiate over the cylindricules, without anastomosing, and terminate 

 in free ends, not by loops, at the basis of the ciliary zone. A delicate 

 but well-defined raised rim or 'bead' runs along both the anterior 

 margins of the retina, and along those which form the falciform slit. 



The crystalline lens (d) is spherical, large, firm, with a dense 

 nucleus : it is almost buried in the vitreous humour, where it is 

 steadied by the attachment of the falciform ligament to its thin cap- 

 sule : the fore part projects through the pupil against the flat cornea, 

 and so nearly fills the anterior chamber, that but a very small space 

 is left for ' aqueous humour.' 



The radiating fibres and elongated cells of the hyaloid tissue *, 

 with the interstitial ' vitreous humour,' present a firmer consistency 

 than in the human eye, and show their intimate structure and 

 arrangement more clearly under the microscope than in Mammalia. 



The membranes situated between the retina and sclerotica, called 

 collectively ' choroid tunic,' are three in number : the external layer 

 in Osseous Fishes, called ' memhrana argentea' (e), is composed cliiefly 

 of microscopical acicular crystals reflecting a silvery, or sometimes a 

 golden lustre, with a delicate cellular basis, which assumes more 

 flrmness where it is continued upon the ' iris.' The second or middle 

 layer is the ^memhrana vasculosa,' seu ^ Halleri,' (/'), and, as its 

 name implies, is the chief seat of the ramifications of the choroid 

 vessels : it also supports the ciliary nerves. The innermost layer is 

 the ' membraiia picta' seu ' Riii/schiana,^ (g), also called ' uvea,' 

 which is composed of hexagonal pigment-cells, usually of a deep 

 brown or black colour. In the Grey Shark {Galeus) the silvery layer 



