332 THE BIOLOGY OF THE FROG chap. 



of the cones which has been mentioned above, there is a 

 bleaching of the pigment in the outer ends of the rods ; 

 this takes place more rapidly in the visual purple than in the 

 green pigment. After a short exposure in the dark both pig- 

 ments are restored, even in the extirpated eye. It is prob- 

 able that the chemical changes produced in the retinal 

 pigments through the influence of hght, play some part in 

 the stimulation of the sensitive cells. Light also affects the 

 cells of the outer pigmented layer of the retina, causing the 

 pigment to stream out into the processes which extend 

 between the rods and cones. With intense illumination the 

 pigment may spread as far as the inner limiting membrane, 

 in from ten to fifteen minutes, which is about the time which 

 it takes the visual purple to disappear under the same 

 conditions (Boll, Angellucci). 



The amount of light falling upon the retina is regulated by 

 the iris which acts as a sort of screen or curtain, admitting 

 the rays only through its central aperture, the pupil. The 

 iris lies directly in front of the lens. Its outer margin is 

 joined to the ciliary body behind and the juncture of the 

 sclerotic and cornea in front. It contains numerous black 

 pigment cells and several golden ones which gives it its 

 bright yellow spots. Near the margin of the pupil there is a 

 ring of smooth muscle cells forming a sphincter pupillcd which 

 causes the pupil to contract. Strong light causes a contrac- 

 tion of the pupil and consequently a diminution of the 

 amount of light that falls on the retina, thus checking the 

 excessive stimulation of the sensitive part of the eye. This 

 effect is produced, at least in part, through the direct action 

 of the light upon the muscle cells of the iris, since it occurs in 

 eyes that have been removed from the body (Steinach, Guth). 



The optical arrangements of the eye of the frog are such 

 as to throw images of objects upon the retina. The sensi- 



