EPITHELIAL CELLS OF THE RETINA. 587 



graphic plate when exposed in a camera. In some respects only, 

 however, because there is this great difference, that, while the 

 chemical change on the sensitive plate persists so as to give rise 

 to a photograph, in the eye, on the other hand, it only lasts 

 during the brief moment during which we can recognize the 

 positive after image. The chemical explosion in muscle may be 

 compared to the explosion of gunpowder, in giving rise to force, 

 but not in the result to the materials. For in muscle as the 

 chemical change which causes the contraction is rapidly repaired, 

 so in the retina a new sensitive plate is at once produced by the 

 restoration of the normal condition of the molecules. 



The idea that the layer of rods and cones undergoes some chem- 

 ical change on exposure to light which suffices to induce excita- 

 tion of the optic nerve, has received great support from the 

 observation that a color of a red or purplish hue exists in the 

 outer part of the rods and that this changes when exposed to the 

 light. But this so-called visual purple cannot have a very in- 

 separable connection with vision, since it is absent where the 

 retina is most sensitive, i.e., the fovea centralis, where there are 

 no rods, and further, frogs with blanched eyes seem to see quite 

 well. 



The peculiar pigmental epithelial cells of the retina have been 

 observed to change their shape slightly, and definitely to alter the 

 position of the pigment granules they contain when exposed to 

 light. When we remember how sensitive to light the protoplasm 

 of many unicellular infusoria is, we cannot be surprised that the 

 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 gran- 

 ules 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 pigmental epithelium, for they are so dove- 



