THE PERCEPTION OF FORM 



like projection on the anterior surface, thus increasing its refractivity 

 in accommodation for near objects (Figs. 786 to 790). 



Compared with the sauropsidan plan, such a mechanism is 

 inefficient ; with a large lens much deformation cannot occur, and if the 

 lens loses its plasticity any deformation is impossible. In a small eye 

 with a large lens accommodation is therefore negligible and when the 



653 



Fig. 786. — Accommodatiox in Mammals. 



The condition of relaxation for distance vision, D ; and accommodation 



for near vision, A'. 



Figs. 787 to 790. — The Lexs Capsule in Mammals. 



Fig. 788. — Capuchin monkey. 



Fig. 789.— Sheep. 



Fig. 790.— Man. 



Diagrams, in which the thickness of the capsule is greatly magnified, 

 showing the relative thicknesses in different regions (E. F. Fincham). 



lens becomes sclerosed with age (as in man) it gradually fails. On the 

 whole the efficiency of mammalian accommodation is therefore poor. 

 In most of the lower Mammals the ciliary muscle is vestigial and some- 

 times absent ; the more primitive Mammals have therefore no accom- 

 modation. Among RODENTS, accommodation is known only among 

 squirrels (Sciurida^) and in them the range is insignificant (1 to 1-5 D). 

 No UNGULATE appears to have any accommodation, and the range in 

 CARNIVORES is small (1 to 35 D) ^ with the excejition of the otter, 



1 The range of accommodation in the dog has been reported as TO D, in the 

 wolf as 2-75 D (v. Hess and Heine, 1898), in the cat as varying from T75 D (v. Hess 

 and Heine, 1898) to 3 D (Marg et al., 1954-5) or 3-5 D (Hartridge and Yamado, 1922). 



