ACCOMMODATION IN FISHES 265 



there was no way in which pressure could be conveniently brought to 

 bear upon the equator of the lens to change the radii of curvature of its 

 surfaces. 



Consequences of Lens Movement — The very fact that the fish lens 

 is never required to change its shape affords one advantage, however, for 

 the lens is enabled to be firm and thus to have a relatively high index of 

 refraction — reaching, in one silurid, a value of 1.72+, which is rather 

 higher than that of most optical glass. The axis of the fish eyeball can 

 consequently be its shortest diameter, thus economizing a bit upon 

 space in the head. But the shiftings of the lens during accommodation 

 introduce a complication whose existence is often neglected : the aqueous 

 humor being incompressible, the lens can move only if the aqueous is free 

 to get out of its way. In the lampreys, the deformation of the globe in 

 accommodation results in no actual change in volume of the anterior 

 chamber, for the lens remains always in contact with the cornea. In the 

 teleosts, when the lens is drawn backward by the retractor lentis, the 

 aqueous in the posterior chamber is free to flow through the pupil, if need 

 be, to keep internal pressures balanced. But in the elasmobranchs the for- 

 ward movement of the accommodating lens tends to seal off the anterior 

 chamber by pressing the lens against the pupil margin, for there is no 

 canal of Schlemm. These fishes have consequently had to leave unclosed 

 a small portion of the embryonic fissure, at the root of the iris, so that the 

 aqueous can flow readily between the anterior and posterior chambers. 



Amphibians — The amphibians are not completely emancipated from 

 the water, and a few frogs and salamanders never leave that medium for 

 a moment; but the eyes of amphibious amphibians have undergone whole- 

 sale modification, for vision through air, both as regards the structure of 

 the globe and the production of protective adnexa. The cornea comes 

 into its own here as the principal refracting structure, the lens becoming 

 merely adjuvant to the accommodatory adjustment of the location of the 

 image. The anterior chamber is deepened by the regular dome-shape 

 taken on by the cornea, so that the eyeball is practically spherical (Fig. 

 106) ; and the lens lies much deeper in the eye than in fishes, since it no 

 longer needs to protrude through the pupil. Closer responsiveness of the 

 iris to the intensity of illumination is thus permitted, and the photome- 

 chanical changes of the retina here begin their phylogenetic degeneration 

 (see Table II, p. 150). 



