SS4 ANATOMY OF VERTEBRATES. 



innermost layer is the 'membrana picta? seu ' Ruyschiana? a, 

 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, h, by the production of all these mem- 

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

 Avhere 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, i, is large and usually round : in many Pla- 

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

 bodied Skates and Pleuronectidre, 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 ( 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 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, 7, /, fig. 219,/,/*, 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 n, which is essentially a 

 modified portion of the corium 0, adhering to, as it passes over, 

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

 Xiphias 4 may be traced an accession to the cornea from the outer 



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



3 lb. prep. no. 1670, A. A lb. prep. no. 1661. 



