THE PERCEPTION OF FORM 649 



remaining musculature of the iris is insignificant and the relatively- 

 firm lens is pushed forwards with little or no change in shape. In 

 water-snakes a supplementary mechanism exists, for in the transference 

 from aquatic to aerial vision such an animal requires an immense 

 range of accommodation. In the European water-snake, Natrix 

 (Tropidonottis) tessellatus, Beer (1898) found that the lens was unusually 

 soft and readily mouldable, and it would appear that in accommo- 

 dation the powerful sphincter muscle of the pupil moulds the anterior 

 surface into a conical shape as it is thrust forward through the pupil — 

 a mechanism common to other reptiles, particularly the turtle (Figs. 

 783-5). 1 In this case the lens is thus both displaced and deformed. 

 Aquatic snakes such as the marine cobras (Hydrophinse) and river snakes 

 (Homalopsinse), as we have seen, make use of a stenopoeic pupil when 

 the eyes are out of water to achieve the necessary accommodation, a 

 device also seen in the seals. ^ 



(b) A deformation of the lens may be effected by direct muscular 

 pressure upon it or by alteration in the tension of an elastic capsule. 



(i) A direct squeezing of the 2)eri2)hery of the lens by the ciliary body 

 is a mechanism peculiar to Sauropsidans (except snakes) ; it is an 

 entirely original and very effective method adopted by this composite 

 group bearing no resemblance to the accommodative devices seen in 

 Fishes or Amphibians. For this reason a number of novel anatomical 

 features is introduced into the sauropsidan eye by virtue of which a 

 high degree of accommodative efficiency is reached. It is significant 

 that the smooth muscle fibres of the Ichthyopsida give place to striated 

 fibres in Sauropsida and a large muscle-mass is developed divided 

 sometimes into two, sometimes three functional segments. Actual 

 contact between the lens and the ciliary body is, of course, necessary ; 

 for this purpose a large annular pad has Ijeen developed at the periphery 

 of the lens extending it equatorially, and the ciliary body, provided 

 with well-developed ciliary processes, is pushed axially by a deep 

 corneo-scleral sulcus, the deformation of the globe being maintained 

 by a concave ring of dove -tailed ossicles (lacking only in Crocodilians 

 — and snakes). In addition, in order to facilitate its deformation the 

 lens itself is unusually soft so that it is readily mouldable (Figs. 779 to 

 782). 



The optical mechanism of accommodation dej^ends essentially on 

 a deformation of the lens : squeezed laterally by the ciliary processes, 

 steadied posteriorly by the vitreous body, and with the peripheral part 

 of the anterior surface restrained by the contraction of the musculature 

 of the iris, the central area of the anterior surface is bulged forwards 

 in a lenticonus thus increasing the refractivity of the lens and accom- 



' p. 652. 2 p_ 641 



