ORGAN OF SIGHT IN FISHES. 



333 



217 



P 



c: 

 c 



L. 





:i.^nilled, showing inter- 

 Lied margiDS. ccxiii 



meridians of a globe, to two opposite points or poles of tlie sphe- 

 roid : in the Sahnonidce and Shark, they converge to a linear tract 

 or septum at each pole, as in fig 

 218. In the fibres of the lens 

 of a cod Brewster discovered 

 the marginal teeth, like those of 

 rack-work, by which the fibres 

 are interlocked together, as in 

 fig. 217. 



This acute observer computes 

 five millions of fibres and sixty- 

 two thousand five hruidred mil- 

 lions of teeth in the lens of a cod : 

 yet in the living and fresh state 

 this organ is transparent. 



The radiatino; fibres and elona:- 

 ated cells of the hyaloid tissue,' 

 with the interstitial ' vitreous 

 hiunour,' present a firmer con- 

 sistency 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 num- 

 ber : the external layer in 

 Osseous Fishes, called ' mem- 

 hrana argentea^ fig. 216, e, is 

 composed chiefly of micro- 

 scopical acicular crystals 

 reflecting a silvery, or some - 

 times a golden lustre, with a 

 delicate cellular basis, which 

 assumes more firmness where 

 it is continued upon the iris. 

 The second or middle layer 

 is the ' membrana vasculosa,'' 

 seu ' Halleri,^ ib. f, and, as 

 its name implies, is the chief 

 seat of the ramifications of 

 the choroid vessels : it also supports the ciliary nerves. The 



218 



Arrangem 1 1 f 



