FISHES 295 



(2) An intermediate layer between the dermal and scleral portions 

 corresponding topographically to the episcleral tissue. It consists of 

 very loose lamellar tissue, so loose that it readily allows the superficial 

 layer to be peeled from the deeper and occasionally permits some 

 degree of movement of the globe under the dermal cornea (the eel, 

 AnguiUa) (Hein, 1913). It is interesting that on luxation of the eye 

 the scleral cornea readily splits from the dermal so that the latter may 

 remain in place and be left behind (Rochon-Duvigneaud. 1916) (cf. 

 Fig. 324). 



(3) A scleral layer consisting of dense lamella' of substantia 

 propria structurally continuous with the sclera itself. 



(4) Descemet's membrane and its endothelium of extreme 

 delicacy. In some species, indeed, the endothelium and Descemet's 

 membrane appear to be absent in the central area of the cornea (carp, 

 Cyprinus — Smelser and Chen, 1954) (Fig. 325). 



So far this arrangement somewhat resembles that seen in lamprej's,^ and 

 appears to be more primitive than the typically vertebrate selachian cornea. 

 In some species, however, there is an apparently separate layer of coarse fibres 

 on the inner aspect of the finely lamellar scleral layer — the autochthonous 

 LAYEK of Leuckart (1876). It thickens greatly towards the periphery and termin- 

 ates abruptly at the scleral margin, but is probably merely a modified portion 

 of the scleral cornea. 



An interesting phenomenon is the occurrence of yellow pigmentation in the 

 corneae of many Fishes due to the presence of xanthophores in the ei^ithelium. 

 In the bull-head. Coitus, for example, there is a pigmented process running over 

 the cornea like a yellow waterfall (Walls and Judd, 1933), while the entire 

 cornea of the carp, Cyprinus, and the jDike, Esox (Schiefferdecker, 1887) is yellow. 

 It is interesting that Soemmerring (1818) in describing this appearance originally, 

 attributed it to a yellow aqueous humour. The pigment must act as a light - 

 filter as does yellow pigmentation in the lens.- 



A regular feature of the teleost cornea is an accumulation of cells, 

 apparently continuous with the endothelium, which fills the angle of 

 the anterior chamber and is reflected over the surface of the iris to 

 form a massive axnular ligament (Angelucci, 1881 ; Lauber, 1901), 

 the " vesiculo -hyaline tissue of the angle " of Rochon-Duvigneaud 

 (1943) (Fig. 326) ; from it the tensor choroidete muscle is probably 

 derived. The annular ligament, somewhat reminiscent of the endothelial 

 proliferation seen in Cyclostomes and Chondrosteans, is elaborately 

 developed in Teleosteans. It is composed of large polyhedral epithelioid 

 cells (Giacomelli. 1935) ; it may be vascularized (the mud-skipper, 

 Periojihthalmus^) or contain melanophores (the cod, Gadus) and is 

 sometimes rich in lymphatic sinuses which, however, cannot be 

 considered homologous with the canal of Schlemm (Franz, 1910 ; 



1 p. 265. 



^ Compare the yellow pigmentation in the cornea of the bow-fin, Ajyiia, in the 

 lenses of the lamprev and of diurnal snakes and squirrels, or yellow oil-globules in some 

 retinal cones (p. 656). » p. 326, Fig. 386. 



