The accommodative significance of the “ramp” retina of the eye of the stingray

Abstract

A rapid freezing and sectioning method was used to study the lens-retina relationship in three species of stingrays and in the gulf flounder. In each case a vertical asymmetry of the eye was noted. The dorsal portion of the retina is located further away from the lens than the distance found along the geometric axis of the eye. The refractive effect of this asymmetry was calculated using measurements of radii of curvature and refractive indices of the ocular media. The resulting value (about 6 D) closely matched changes in refractive state measured retinoscopically along lines corresponding to the central and dorsal retina. Measurements of changes in refractive state before and after anaesthesia and in response to far and near targets indicate that a dynamic accommodative ability exists, at least in one of the three ray species studied. The consideration of the dorsal asymmetry of the eye as an accommodative mechanism is questioned in view of (a) the existence of dynamic accommodation, (b) the depth of focus which may be provided by the pupil and (c) the apparent limitation of the ventral visual field by the large pectoral fins.