FISHES 



281 



o retina without blood-vessels {in the ad^dt) and, icith few excejttions, 

 •provided only with rods ; 



an optic nerve jnovided with myelinated nerve fibres and, in some 

 species, an axial core of ependymal cells resembling the arrangement in 

 lampreys ; 



a cartilaginous orbit within ivhich the globe is supported by an optic 

 pedicle, also of cartilage. 



THE GLOBE is iisiially large in the sharks, smaller in the upward- 

 looking Batoidei, and varies with the depth of the habitat — in general, 

 the deeper the habitat, the larger the eye. as is exemplified in the 

 enormous eyes of some deep-sea sharks {Etmopterus) ; the dorsal eyes 

 of rays are generally small. ^ The cornea is more highly curved than is 

 seen in other fishes, and is usually oval in shape with the long axis 

 horizontal ; it contains all the layers characteristic of the mammalian 

 cornea with a thick epithelium derived from the skin. Bowman's and 

 Descemet's membranes, the latter with an endothelium, and a neatly 

 laminated substantia propria which, how^ever, tends to become con- 

 siderably thinner centrally (Strampelli, 1934 ; Loewenthal, 1938). It 

 is pigmented peripherally in some species, particularly in its upper 

 part, probably as a protection against light (e.g., Torpedo),?i,nd receives 

 a rich nerve-supply (Shearer, 1898). The sclera varies considerably in 

 thickness, being very thick in the largest sharks; the fibrous outer half 

 is supported by a firm and complete cartilaginous cup on the inner 

 aspect extending from the optic nerve behind to the corneal margin 

 anteriorly (Yatabe, 1932). Sometimes this becomes calcified, and in 

 one shark {Lc^margus) the scleral cartilage sends large processes into 

 the choroid. 



The uveal tract presents features both interesting and distinctive ; 

 it is the only vascularized tissue within the globe of the adult ( Virchow, 

 1890). The vascular part of the choroid is typical in structure, the 

 choriocapillaris being supplied by an artery which enters on the 

 temporal side of the globe and drained by two main veins, one ventral 

 and one dorsal. On its outer aspect is a heavily vascularized epichoroid 

 of connective tissue, sometimes cavernous in its structure, particularly 

 marked near the posterior pole so that the optic nerve has an mtra- 

 choroidal course of several millimetres. Between these two layers the 

 centre of the choroid is occupied by the tapetum lucidum, a structure 

 carried forwards in a much less marked form onto the anterior surface 

 of the iris. 



The TAPETUM LUCIDUM of Selachians is a remarkable structure 

 and is found in all forms except some benthonic sharks (Lamargus) 



Torpedo 



Lcemargus 



1 The dorso-lateial eyes of the eagle -ray, Myliobutis, are, however, quite large. 



