746 



THE SENSES 



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 





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. 



possibly of the rods is also said to occur, as has been already incidentally 

 mentioned. Under the influence of the stimulus of light the outer parts of 

 the cones, which in an eye protected from light extend to the pigment layer, 

 are retracted. In is even thought by some that the contraction is under 

 the control of the nervous system. Finally, according to the careful re- 

 searches of Dewar and McKendrick, and of Holmgren, it appears that the 

 stimulus of light is able to produce an action current in the retina. Mc- 

 Kendrick believes that this is the electrical, expression of those chemical 

 changes in the retina of which we have already spoken. 



Color Sensations. When a ray of sunlight enters the eye it produces a 

 sensation of white light. But if the ray first passes through a prism, then it 



