652 THE EYE IN EVOLUTION 



range is seen in aquatic birds such as the cormorant, Phalacrocorax ; in 

 it the lens is very soft and plastic, the sphincter of the iris extremely 

 powerful, and the compression and moulding of the lens to form a 

 marked lenticonus has been said to provide an accommodative excur- 

 sion of up to 50 D in vision under water (v. Hess, 1912). 



Figs. 783 to 785. — Accommodation in Chelonians. 



D 



Fig. 783. 



Fig. 784. 



Fig. 785. 



The lens at rest for distance vision (D, Fig. 783 ; Fig. 784) ; deformed 

 into an anterior lenticonus for near vision (A'^, Fig. 783 ; Fig. 785). 



(ii) A deformation of the lens by a variatiori in the elasticity of the 

 capsule is a mechanism peculiar to mammals and has no analogy 

 elsewhere in the vertebrate phylum. According to the most generally 

 accepted hypothesis of Helmholtz (1855) and Fincham (1925), the 

 plastic lens retains its characteristically flattened shape owing to the 

 moulding effect of the elastic capsule stretched by the pull of the 

 zonule. The capsule varies considerably in thickness, being thinnest 

 at the posterior and anterior poles. When the ciliary muscle contracts 

 on accommodation, the ciliary body approaches the lens, the zonule 

 slackens and the capsule relaxes allowing the plastic lens to assume a 

 more spherical shape— the shape, in fact, which it assumes when removed 

 from the eye. Since the posterior pole is restrained by the support of 

 the vitreous body and the capsule is relatively thick and tough in the 

 peripheral region, the greatest bulging occurs in the form of a conus- 



