648 



THE EYE IN EVOLUTION 



of objects at a considerable range of distances from the eye to fall 

 upon the visual layer so that an efficient and active mechanism may 

 not be required. But it would seem that in anurans the lens is pulled 

 forward indirectly by two protractor lentis muscles, a dorsal and a 

 ventral. Unlike the analogous muscles of Fishes, they are mesodermal 

 in origin although still smooth in type. They arise at the margin of 

 the cornea, traverse the iris and are inserted into the large median 

 ciliary processes ; these they pull forwards thus drawing the lens in 

 the same direction through its anchorage by the zonule. In urodeles 

 a single ventral protractor lentis muscle inserted into the single mid- 

 ventral ciliary process acts similarly, and less 

 effectively. 



In OPHIDIANS the mechanism for moving 

 the lens forwards is entirely different (Beer, 

 1898 ; Leplat, 1921 ; Kallmann, 1932 ; 

 Michel, 1932-33). The interpretation sug- 

 gested by Beer (1898) is as follows (Fig. 778). 

 In snakes we have already seen that the 

 ciliary (mesodermal) musculature has 

 migrated to the iris around the root of which 

 it forms a sphincteric ring ; when these 

 fibres contract they constrict the globe at 

 the corneo -scleral junction, thus increasing 

 the pressure in the vitreous chamber so that 

 the lens is pushed bodily forward into the 

 pupillary aperture, sometimes as far as half- 

 way towards the cornea. At the same time 

 the constriction in the circumference of the globe in the ciliary region 

 may be compensated by a slight forward bulging of the cornea. Beer 

 (1898) demonstrated this indirect pressure -effect through the vitreous 

 by removing the posterior half of the globe ; electrical stimulation of 

 the ciliary muscle then led to a backward displacement of the lens. In 

 the intact eye an accommodative range sufficient to overcome their + 8 

 or -f 10 D of hypermetropia is readily available to snakes (up to 

 — 17 D) (Kallmann, 1932). The mechanism resembles that seen in 

 Cephalopods.^ 



In tree-snakes with a horizontal slit-shaped puj^il, a nasal aphakic area 

 and a temporal fovea ^ the lens moves nasally as well as forwards ; this has 

 the oj:»tical effect of directing incident light upon the fovea and corresponds to 

 the nasal movemeTit of the lens of those teleostean Fishes with a partially 

 aphakic pnpil when accommodation is relaxed for near objects.^ 



In terrestrial snakes the main effector of the accommodative effort 

 is the mesodermal sphincter at the root of the iris ; the action of the 



1 p. 590. 2 p_ 388. 3 p 304. 



Fig. 778. — Accommodation 



IN Snakes. 



The eye in relaxation for 

 distance vision, D ; the 

 condition in accommoda- 

 tion for near vision, N. Note 

 that both the lens and the 

 cornea have moved forward. 



