296 



THE EYE IN EVOLUTION 



Karsten, 1923). Not only does this layer cover most of the anterior 

 surface of the iris, but in a few Teleosteans it appears to form a thick 

 stratum, in part fibrillar, in part cellular, on the inner aspect of the 

 scleral cornea — the supplementary layer of Rochon-Duvigneaud 



Fig. 326. — The Angle of the Anterior Chamber of the Trout. 



Showing the immense thickening in the periphery of the cornea and the 

 annular ligament filling up the angle of the anterior chamber and binding the 

 iris to the cornea. The dermal layer of the cornea (as often occurs) has been 

 lost ( X 84) (Norman Ashton). 



11 



(1943) (goby-fishes, Gobius niger, Periojjhthalmus ; the soles, Soleidse, 

 etc.). Various views have been put forward as to the nature of this 

 structure which may add another layer to the already complex cornea 

 Qobius and appears topographically to be continuous with the choroid ; a 



secretory function has been suggested, but 

 r its exact significance must await further 



study (see Ballowitz, 1913 ; Kolmer, 1913 ; 

 Remotti, 1929 ; Schaffer, 1929 ; Baecker, 

 1931). 



The uveal tract shows several distinctive 

 characteristics (Fig. 327). The choroid has 

 the essential vertebrate structure of a chorio- 

 capillaris and a heavily pigmented vascular 

 layer, but is noteworthy for three features — 

 the argentea, the choroidal gland and the 

 falciform process. In the majority of pelagic 

 forms there is a layer of guanine-laden cells 

 interspersed with chromatophores — the 

 argentea — jacketing the outside of the 

 choroid with a silvery coat which is continued 

 forwards over the anterior surface of the iris 

 giving it its metallic appearance. In view of 

 the fact that it is obscured from the retina by 

 pigment, this layer can have no visual value ; 



Fig. 327. — Section through 

 THE Equatorial Segment 

 OF THE Eye of the 

 Trout. 



Showing the thick choroid, 

 ch, the retina, r, and the 

 scleral cartilage, s (Feulgen ; 

 X 6; 'Katharine Tansley). 



