650 



THE EYE IN EVOLUTION 



modating for near vision. At the same time in lizards and Birds the 

 deepening of the corneo -scleral surface deforms the cornea, producing 

 a peripheral flattening and making it more convex at the apex and 

 thus augmenting the increase in total refractivity (Kahmann, 1932-33) ; 

 this mechanism is absent in diving birds in which it would be ineffective 

 since the corneal refraction is eliminated under water. ^ The entire 



Fig. 779. — Accommodation in Saukopsida. 



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



near vision, N. 



Figs. 780 to 782. — To Illustrate the Effect of the Annular Pad in 

 Transmitting an Evenly Distributed Pressure to the Lens by 

 the Ciliary Body. 



Fig. 780, represents a balloon filled with air. If it is compressed directly by 

 a relatively small body such as the fist it is deformed (Fig. 781) ; if, however, 

 the iinpact of the fist is distributed regularly by means of an open hand, a 

 lenticular shape is ensured (Fig. 782). 



process depends on lateral pressure by the ciliary musculature and 

 pressure from the vitreous plays no active role apart from restraining 

 any change in the posterior surface of the lens : in contrast to the 

 events in the eye of the snake, the process takes place without change 

 if the posterior segment of the eye is removed and the vitreous is 

 eliminated (v. Hess, 1912). 



The factor determining the act of accommodation is essentially the ciliary 

 muscle — a well -developed striated muscle running meridionally, a descendant 

 from the minute tensor choroidese of Fishes, but inserted not into the choroid 



1 p. 276. 



