EPITHELIAL CELLS OF THE KETINA. 585 



terminals as the waves of sound excite the endings of the auditory 

 nerve. But we know that light has a very distinct action on 

 many chemical combinations, such as reducing salts of silver and 

 gold, etc. We, therefore, imagine that the light waves set up, in 

 the outer layer of the retina, certain intermolecular motions or 

 chemical interchanges, the result of which is that the nerve fibres 

 are stimulated to activity and transmit an impulse to the brain. 

 In the outer layer of the retina the light may be said to produce 

 a change in the retina which in some respects may be compared 

 to that which occurs on the sensitive photographic plate, when ex- 

 posed, 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 con- 

 dition 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 co-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 pigmented 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 



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