FISHES. 339 



More internally is a very thin membrane, almost without con- 

 sistence, which at first sight resembles only a silver or gold colour, 

 and which encloses all the n ore internal parts. It is this same layer 

 which is continued outside the iris, and which gives it that beautiful 

 metallic lustre which is generally so brilliant in the fishes. 



The pupil of fishes has not generally the faculty of changing its 

 diameter ; but we must remark the singular truncated prolongation in 

 form of a palm which its superior border forms in the rays and 

 pleuixmectes, anrl which close the opening of the pupil, in the same 

 way as a window-blind. 



The posterior surface of the iris or the uvea, is formed by another 

 membrane, which lines the whole interior of the eye, and of which 

 the internal surface is generally furnished with a coat or sort of 

 varnish more or less black. This membrane, which is very finely 

 vascular, may be divided into two lamina : the internal one, very 

 thin and simple, is a true ruyschian ; the external one, which is, pro 

 perly speaking, the vascular lamina, is rather thick ; this is the 

 choroid. 



In large eyes the ruyschian sensibly forms, at the internal surface 

 of the uvea, at the part where are situated the ciliary processes 

 of mammifera, a circle of radiated and very fine folds ; but these 

 folds do not project, neither do they extend so far as the capsule 

 of the cristalline, so that they cannot be called true ciliary processes. 

 These folds, as well as the rest of the urea, immediately touch the 

 vitreous body, and strongly adhere to it; and the anterior convexity 

 of the cristalline frequently projects through the pupil in such a 

 manner, that the aqueous humour has no posterior chamber. 



Between the choroid and the membrane of the metallic colour 

 which envelops it, is an apparatus proper only to fishes, and even to 

 osseous fishes, for it is not present in the chondropterygians. It is 

 a band or pad variously curved (h h, fig. v.), and forming an irregular 

 and incomplete ring, which surrounds for some distance the entrance 

 of the optic nerve. This pad is sometimes divided into two parts ; 

 at others it presents a large crossing, but it has always a solution of 

 continuity with its inferior part. It is always very red ; its tissue 

 presents blood vessels which are either transverse, densely compact, 

 or parallel with one another. It gives out other vessels, frequently 

 very tortuous always very much ramified, and which form 4 in the 

 thickness of the choroid, a compact net- work, Avhich Haller con- 

 sidered as a particular membrane. 



The nature of this pad is not easy to determine. Some have 

 thought it muscular ; but the red stria?, which are seen in it, are vas- 

 cular, and not fibrous ; others have regarded it as glandular ; but it 

 appears to me to arise only from blood vessels. Perhaps it is an 

 erectible tissue, analogous to that of the carniverous bodies, and 

 which has some influence in accommodating the form of the eye to 

 the distances and density of the media. 



The optic nerve (i i, fig. iii. to vii.), as we have said above, is com- 

 posed, in many fishes (at least amongst the acanthopterygians), of a 

 plaited membrane, enveloped in a tunic more or less strong, which 

 terminates at the sclerotic : it extends in a point of the eve sufficiently 



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