694 



THE SENSES 



The visual purple cannot, however, be absolutely essential to the due pro- 

 duction of visual sensations, as it is absent from the retinal cones, and from 

 the macula lutea and fovea centralis of the human retina, and does not appear 

 to exist at all in the retinae of some animals, e.g., bat, dove, and hen, which 

 are, nevertheless, possessed of good vision. 



However, the fact remains that light falling upon the retina bleaches the 

 visual purple, and this must be considered as one of its effects. It has been 

 found that certain pigments, also sensitive to light, are contained in the inner 

 segments of the cones. These colored bodies are said to be oil globules of 





FIG. 472. Sections of Frog's Retina Showing the Action of Light upon the Pigment 

 Cells and upon the Rods and Cones, (von Gendesen-Stort.) A, From a frog which had 

 been kept in the dark for some hours before death; B, from a frog which had been exposed 

 to light just before being killed. Three pigment cells are shown in each section. In A 

 the pigment is collected toward the nucleated part of the cell, in B it extends nearly to the 

 basis of the rods. In A the rods, outer segments, were colored red (the detached one 

 green); in B they had become bleached. In A the cones, which in the frog are much 

 smaller than the rods, are mostly elongated; in B they are all contracted. 



various colors red, green, and yellow called chromophanes, and are found 

 only in the retinae of animals other than mammals. The rhodopsin at any 

 rate appears to be derived in some way from the retinal pigment, since the 

 color is not renewed after bleaching if the retina be detached from its pig- 

 ment layer. The second change produced by the action of light upon the 

 retina is the movement of the pigment cells. On the stimulation by light 

 the granules of pigment in the cells which overlie the outer part of the rod 

 and cone layer of the retina become diffused into the parts of the cells be- 

 tween the rods and cones, the melanin granules, as they are called, passing 

 down into the processes of the pigment cells. A movement of the cones and 



