034 ANATOMY OF VERTEBRATES. 



innermost layer is the ' memhrana -picta^ seu ' TLiajscliiana^ g, 

 also called ' uvea,' wliicli is composed of hexagonal pigment-cells, 

 usually of a deep Ijrown or black colour. In the Grey Shark 

 (Galeiis), the silvery layer is laid upon the central surface, not 

 the i^eriphery of the choroid.' 



The formation of the iris, h, by the production of all these mem- 

 branes is well shown in the eye of the Sword -tish Xiphios, fig. 216, 

 where its thick base or ' ciliary ligament ' h overlaps the con- 

 vex border of the bony sclerotic.^ The membrana argentea upon 

 the front of the iris gives great brilliancy to the eye, in many 

 fishes. The pupil, i, is large and iisually round : in many Pla- 

 giostomes it is elliptic ; in Galeus it is quadrangular ; in the flat- 

 bodied Skates and Pleuronectidje, that grovel at the bottom and 

 recei's'e the rays of light from above, a fringed process descends 

 from the upper margin of the pupil, and regulates tlie quantities 

 of admitted light by being let down or drawn up like a l)lind. 



The muscular structure of the iris is very feebly de^-eloped in 

 most fishes : it is best seen in the pupillary curtain of the Skate, 

 the plicated anterior border of the uvea forms the so-called 

 ' ciliary zone, or processes,' k : they are the most complicated in 

 the great Shark (Selache) where each process ' consists of two or 

 three minute folds, which, as they run forward, unite into one, 

 and terminate in a point at the circumference of tlie iris :'' but 

 they do not, as yet, project freely inward and forward from the 

 surface of the uvea. 



The subordinate and accessory character of the sclerotic caji- 

 sule, fig. 216, Z, Z, fig. 219, f,f, is illustrated inmost Osseous Fishes 

 by its deviation from the sub-spherical form of the true eveljall 

 which it protects, and by the great quantity of cellular, and often 

 also of adijjose tissue, fig. 216, which fills the wide inters})aee be- 

 tween the sclerotic and the choroid. In the filirous tissue of the 

 sclerotic are usually developed the two cartilaginous or osseous 

 hemispheroid cups already described (p. 115, fig. 81, 17); but in 

 place of these, in the Orthaiioriscus, as in the Plagiostomes, the 

 capsule is strengthened liy a single hollow, cartilaginous, perforated 

 spheroid. This varies in tliickness at ditl'crent parts, being usually 

 thickest behind, and particularly so in the Sturgeon. The ante- 

 rior aperture is closed hj the cornea w, which is essentially a 

 modified ]iortion of the corium o, adlicring to, as it passes over, 

 the usually thickened borders of that aperture. In tlie eve of the 

 Xiphias-^ may be traced an accession to the cornea from the outer 



> XX. vol iii. p. 147, prep, no. lC(i<.). = Ih, jircp. no. KiGl. 



' lb. prop. no. 1C70, A. < Jb. prep. no. lOi'.I. 



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