586 



MANUAL OF PHYSIOLOGY. 



protoplasm of the retinal epithelium is affected by it. Moreover 

 in the pigment cells of the frog's skin we are familiar with a 

 change in shape and in the arrangement of their pigment granules 

 in response to different light stimuli. We know that in the 

 nervous centres nerve impulses are commonly originated by proto- 

 plasm under the influences of slight changes in temperature or 

 nutrition. It would be hardly too much to assume, then, that 

 the retinal epithelium has some important share in the trans- 

 formation of light into a nerve stimulus. The arguments point- 

 ing to the rods and cones as the essential part of the retina apply 

 equally well to the pigmented epithelium, for they are so dove- 

 tailed one into the other that they form but one layer. They are 



FIG. 231. 



Epithelial cells of the retina. a, seen from the outer surface ; b, seen 

 from the side as in a section of the retina ; c, shows some rods projecting 

 into the pigmented protoplasm. 



not known to be connected with the nerve fibres, but even sup- 

 posing they be not in any way connected with the nerves they 

 might still be influenced by the light, and by some kind of motion 

 communicate the effect to the contiguous sensitive nerve termi- 

 nals, which are elaborately adapted to appreciate subtle differen- 

 tiations of stimulus. 



COLOR PERCEPTIONS. 



If a beam of pure white sunlight be allowed to pass through 

 an angular piece of glass or prism, it is decomposed into a num- 

 ber of colors, which may be seen by looking through the prism, 

 or may be thrown on a screen, like that of a camera. These 

 colors, which look like a thin slice of a rainbow, are together 



