EYE REFRACTION AND ACCOMMODATION 109 



case, refractive measurements were made after several minutes of dark 

 adaptation, to permit sufficient pupil dilation. 



With skates and rays, Verrier simply removed a portion of one pectoral 

 fin. Franz assumed that the eye in such an instance would appear to lie at 

 a position three-fourths the actual distance between it and the aquarium 

 wall. This factor slightly affected consideration of the distance at which 

 retinoscopy was performed when neutrality of the retinoscopic reflex was 

 achieved. In both studies an average refractive error of 9 or 10 diopters 

 of hyperopia was found. 



Verrier (1930) did not verify experimentally whether an elasmobranch 

 accommodative ability exists. She noted, however, that a refractive error 

 of hyperopia precludes a mechanism involving movement of the lens to- 

 ward the retina, such as that described by Beer (1894) for teleosts. 



It is commonly assumed that change in lens shape is not a practical 

 mechanism in aquatic vertebrates because the lens, as the principal refrac- 

 tive structure, has a high refractive index and lacks flexibility (Walls 1942). 

 Reduction in hyperopia can be accomplished only by increasing the dis- 

 tance between the lens and retina. Verrier and Rochon-Duvigneaud (1943) 

 denied the existence of an intraocular lens muscle in elasmobranchs. The 

 additional lack of ciliary muscle fibers that might squeeze the globe and 

 elongate the optic axis of the eye (a mechanism suggested by Verrier (1928, 

 1934, 1947) for teleosts) led both Verrier and Rochon-Duvigneaud to 

 declare that elasmobranchs cannot accommodate. 



The only published report indicating that elasmobranchs can accom- 

 modate is that of Franz (1931). Franz electrically stimulated the eyes of 

 a number of species after removing the cornea and iris. He observed for- 

 ward motion of the lens in a ray and in a torpedo but reported little or 

 no success in the case of sharks. Franz believed that the observed lens 

 motion was a result of contraction of ectodermal muscle fibers in a 

 ventral papillary extension of the ciliary body. The question of whether 

 this papilla (the protractor lentis muscle or pseudo-campanule) does in 

 fact contain muscle fibers or whether it is merely a part of the non- 

 contractile suspensory apparatus of the lens is not clear. As mentioned 

 above, Verrier and Rochon-Duvigneaud believed the latter. A histological 

 study by Wang (1968) also disputed the contractile nature of the pseudo- 

 campanule. This controversy may be a result of the inherent difficulty of 

 identifying contractile tissue of ectodermal origin. 



The Concept of the "Ramp" Retina 



The term accommodation as used in the above context refers to a dynamic 

 change in the refractive state of the eye along a specific axis. An additional 

 accommodative concept appears in the literature in respect to rays and 

 horses (Franz 1934; Walls 1942; Duke-Elder, 1958). The term ramp retina 

 refers to a static accommodative mechanism consisting of variation of the 

 distance between lens and retina. An animal possessing such an eye would 

 theoretically accommodate for the viewing of near targets by moving the 



