ACCOMMODATION 



XVI. 29 A^V^UiVli\lUJL»/\ 1 IKJ1S 483 



shape of the eye is fixed by the sclerotic plates, the lens becomes more 

 curved and hence accommodated for near vision; contraction of the iris 

 sphincter assists in the process. Crampton's muscle is so arranged as 

 to pull on the cornea, shortening its radius and further assisting in 



lb) 



(3) 



tenacular Ligament: v / / 

 Cramp tons m. \/' 



scleral ossicle 



lens 



Bruckes m. 



cdiary 

 body 



zonule 



Fig. 293. The mechanism of accommodation in a bird's eye; 



the positions during near vision are shown dotted, b. The lens of 



the cormorant's eye at rest (full line) and fully accommodated 



(dotted line). (From Pumphrey, after Franz and Hess.) 



■cornea- 



scleral ossicle 



pecten 



Fig. 294. Diagrams of right eye of man and left eye of swan to show the difference 

 in shape. The position of the image plane in man is shown dotted; it lies behind 

 the retina except near the centre. The arrows point forward. (From Pumphrey.) 



accommodation. This double method of active accommodation for 

 near vision is most fully developed in diurnal predators, such as the 

 hawks, less so in night-birds. In aquatic birds Crampton's muscle is 

 reduced, and the cornea is of little importance in image-formation. 

 Special arrangements are found in diving birds, for instance in the 

 cormorants Briicke's muscle is large and there is a very powerful iris 

 muscle, which assists the ciliary muscles to give the great change in 

 shape of the soft lens, allowing accommodation of 40-50 diopters (about 



