THE TELEOST EYE 579 



then atrophying — e.g., minnows) — occasionally, jrom preformed car- 

 tilage {Salmo, Pagellus, Crenilabrus) . Both ossicles, only one, or none 

 may occur within one family (herrings), and both are lacking in many 

 small-eyed and bottom forms. Conversely, they may be enormous and 

 joined to form a complete ring in large, large-eyed, swift swimmers 

 (tuna, swordfish). 



The cornea is also variable in make-up. Topographically, it is usually 

 broad, and it tends to depart from a circular outline in the direction 

 of a horizontally elliptical one, and to have its center shifted more or 

 less nasally. These tendencies are more pronounced in swift swimmers 

 than in slow ones, in marine forms than in freshwater species, and are 

 obviously purposed to enlarge the binocular field with a minimal sacrifice 

 of periscopy in the horizontal plane. Anatomically, the only constant 

 feature of the teleost cornea is the portion of the substantia propria 

 which is directly continuous with the cartilaginous and fibrous layers 

 of the sclera, and which may be designated the 'scleral' portion of the 

 cornea. External to this, and ordinarily fused with it, is an additional 

 mass of tendinous substantia propria representing the dermis of the skin 

 (see Fig. 151, p. 451), and bearing externally the (usually) thin corneal 

 epithelium. This 'dermal' part of the cornea and the scleral layer are 

 jointly homologous with the entire cornea in the chondrosteans and 

 elasmobranchs; but, unlike those 'more primitive' fishes, teleosts have 

 preserved a visible distinctness of the two layers, which moreover usually 

 expresses itself in a ready separability of them. Although this situation 

 would seem to be truly primitive — placing the teleost cornea between 

 those of the lampreys and the elasmobranchs — interestingly enough it is 

 particularly in some of the lower (physostome) teleosts that the sub- 

 stantia propria has become most nearly homogeneous and is no longer 

 easily peeled apart into dermal and scleral laminae (e.g., salmonids, 

 minnows, pikes).* The customary easy separability of the cornea has 

 an evolutionary aspect as well as an immediate mechanical one, for it has 

 led many times to the production of a 'spectacle' through a reversion to 

 the primitive cyclostome situation in which the skin was not joined to 

 the dural capsule of the eyeball. 



*The homogeneity may be owing to the dermal propria's having actually disappeared from 

 between the epithelium and the scleral propria. This appearance is given, for example, by 

 the goldfish, where the dermal propria is either absent or consists at most of but a single 

 layer of collagenous fibers continuous with such a layer in the conjunrtiva. It may be that 

 variations in the elaboration of the general head dermis can reflea in the laminations of 

 the cornea. 



