268 ADAPTATIONS TO SPACE AND MOTION 



— particularly the most secretive forms with the most active pupils, 

 whose crude eyes mediate mere light-sense rather than form-sense (e. g., 

 Megalobatrachus japonicus) . The refraction of toads has not been much 

 studied; but land forms might be expected to be emmetropic or myopic 

 in the air, and hypermetropic when in the water during the breeding 

 season. Toad rods are longer than frog rods, helping to reduce the need 

 for accommodation, though toads (Fig. 106b, c) do have more accommo- 

 dation than frogs anyway. Salamandra is known to be emmetropic in air. 



Role of the Vitreous in Ichthyopsidan Accommodation — The 



vitreous humor is an important part of the mechanism of accommodation 

 in the Ichthyopsida, although this is not at first apparent. The original 

 vertebrate eye did not at first possess any semblance of a zonule, and 

 without the jellification of the mass of fluid lying behind the lens, the 

 latter could not be held in place icf. Fig. 103, p. 258). In the vertebrate- 

 like eye of a squid (Fig. Ig, p. 3), where the tough 'epithelial body' 

 serves as a zonule, the Vitreous cavity' behind this lens-holding plate of 

 tissue is filled with watery liquid, not with a jelly. In the lampreys, the 

 elastic cushion of the vitreous keeps the lens propped against the cornea 

 and insures that the position of the resting lens will always be the same 

 at every relaxation of accommodation. 



In the elasmobranchs the gelatinous, discoid zonule, though far less 

 strong than the tissue 'zonule' of a cephalopod, might perhaps restore 

 the lens to position after relaxation of the protractor lentis, even if the 

 vitreous were not jelled. In the teleosts, however, the elasticity of the 

 vitreous is needed to serve as a quick-acting antagonist of the retractor 

 lentis, which must work against it and could not single-handedly replace 

 the lens simply by elongating in relaxation. 



According to this idea, the vitreous — or at least its gel condition — is 

 ordinarily a useless vestige in the higher vertebrates and has only per- 

 sisted because (being transparent) it does not interfere, and affects 

 nothing but the distribution of accommodatory deformation between the 

 anterior and posterior lens surfaces. It returns to usefulness however in 

 the snakes, and in those few amphibious reptiles, birds, and mammals 

 which squeeze the front of the lens with the sphincter iridis, tending to 

 force the lens backward. This tendency must be controlled by the cush- 

 ioning action of the vitreous, and by the zonule fibers acting as check 

 ligaments; else in these animals, the efforts of the iris to increase the 

 refracting power of the lens would be nullified by a decrease in the 

 distance from lens to retina. 



