474 



THE EYE IN EVOLUTION 



The lens, suspended freely from the ciliary processes, is usually 

 relatively small and lenticular in shape in diurnal species, the anterior 

 surface being usually the more convex in Garni vora, the posterior in 

 Herbivora and Primates (Figs. 613 to 616) ; it approaches rotundity 

 and is larger in nocturnal species, especially in the small-eyed lower 

 forms, and is round in aquatic species such as the Cetaceans and 

 Pinnipedes. Among Sirenians (sea-cows such as the manatee and 

 dugong) the lens is lenticular in shape but, to suit the optics of an 



Figs. 613 to 616. — The Lenses of Placentals. 



Fig. 613. 



Fig. 614.— Seal. 



Fig. 615.— Dog. Fig. 616.— Primate. 



aquatic environment, approximated closely to the cornea so that the 



anterior chamber is very shallow. In the tree-shrew, Tupaia, and in 



most squirrels (Sciurida?, except the nocturnal flying squirrels), the 



lens is tinted yellow (Merker, 1928 ; Walls, 1931). With regard to its 



structure, the same general plan of a series of radial lamellae is apparent 



throughout the whole vertebrate phylum, with only minor modifications 



(Rabl, 1899) (Fig. 617). The sutural arrangements are usually 



simpler than in man, being made up of two lines having a vertical 



direction anteriorly and a horizontal posteriorly {e.g., rabbit). Tliis 



forms a transient stage in the development of the 



lens of Primates but eventually in these the lines 



branch into a tri-radiate form resembling the letter 



Y standing in the erect position anteriorly and the 



inverted position posteriorly (Figs. 618 to 620). 



In all adult Mammals, the subcapsular epithelium 



ends at the equator, but in many of the lower 



species it extends farther back. The cajjsule is 



always present and in some animals it is very 



„ .,„ ^ ^ thick, showina; definite striations into layers ; 

 Fig. 617.— The Ra- , . , , i . ^ ■ • i 1 



DiA. t.amell^ of thus ni the horse at the anterior pole it is about 



THE lkns of the Q'S miu. tlilck aud is made up of 26 layers. The 



Cham •. (after ^ , . . • , . , ^ -r 



Rabl), local variations m thickness are not umiorm : 



