334 ANATOMY OF VERTEBRATES. 



innermost layer is the f membrana pictaj sen ' RuyschianaJ 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 is laid upon the central surface, not 

 the periphery of the choroid. 1 



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

 branes is well shown in the eye of the Sword-fish Xiphias, fig. 216, 

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

 vex border of the bony sclerotic. 2 The membrana argentea upon 

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

 fishes. The pupil, z, is large and usually round : in many Pla- 

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

 bodied Skates and Pleuroiiectida?, that grovel at the bottom and 

 receive the rays of light from above, a fringed process descends 

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

 of admitted light by being let down or drawn up like a blind. 



The muscular structure of the iris is very feebly developed 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 (Sclache) 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 the iris :' 3 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 cap- 

 sule, fig. 216, /, Z, fig. 219, t /,jf, is illustrated in most Osseous Fishes 

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

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

 also of adipose tissue, fig. 216, which fills the wide interspace be- 

 tween the sclerotic and the choroid. In the fibrous 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 Orthagoriscus, as in the Plagiostomes, the 

 capsule is strengthened by a single hollow, cartilaginous, perforated 

 spheroid. This varies in thickness at different parts, being usually 

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

 rior aperture is closed by the cornea ?z, which is essentially a 

 modified portion of the corium o, adhering to, as it passes over, 

 the usually thickened borders of that aperture. In the eye of the 

 may be traced an accession to the cornea from the outer 



1 xx. vol. iii. p. 147, prep. no. 1669. 2 Ib. prep. no. 1661. 



3 Ib. prep. no. 1670, A. 4 Ib. prep. no. 1661. 



