i897. LIGHT-SENSATIONS OF EYELESS ANIMALS. 179 



their bodies or between them, also indirectly and mechanically 

 stimulate the indifferent nerve cells or fibres in contact with them. It 

 seems to me clear that the specialisation of this process might give rise 

 both to the dermatoptic function and to eyes. 



In the first place, it must be noted that the retinal cells, in the 

 great majority of eyes, are modified epithelial cells. As such it is 

 quite possible that they are not themselves directly continuous with 

 nerve fibrils ; the nerve fibrils may be merely associated with the 

 retinal cells. It would then be these latter that are stimulated by 

 light, and they in their turn stimulate the associated nerve fibrils. 

 This possibility is admitted in the last edition of Quain's Anatomy, 

 vol. iii., pt. iii., p. 155; 1894. I* is now generally acknowledged, as 

 there noted, that this indirect stimulation of the nerves by modified 

 epithelial cells obtains in the specific organs of touch, taste and 

 hearing. It has not, however, been established either for sight or 

 smell, although, as above stated, there are reasons for believing that 

 it holds good for the former. In view of this indirect stimulation of 

 the nerves in three out of our five senses, much of the discussion as to 

 the possibility of undifferentiated sensory cells responding to different 

 stimuli is, at least in the present instance, not quite up to date. At 

 any rate it seems to me that the simpler problem should come first ; 

 what are the changes produced in the epithelium by light ? Now it 

 is well known that the epithelial cells are influenced in a striking way 

 by light, viz., in the movements within and between them of the 

 pigmented granules. Cannot this fact supply us with a clue to the 

 understanding of the sense of light, whether diffuse or concentrated 

 in eyes ? 



When we turn to the eye, we find that one of the few effects 

 of light, about which we have no doubt, is that the pigmented 

 granules move backwards and forwards in enormous quantities 

 between the prolonged outer ends, the rods and cones, of the 

 retinal cells. This process must surely be regarded as a speciali- 

 sation of the ordinary invasion of the epidermis by pigmented matter 

 under the action of light. For not only is the eye a specialised 

 portion of the epidermis, but in both the eye and in the epidermis the 

 pigment travels outwards under the action of light. It seems to me 

 indeed impossible to avoid the conclusion that eyes are only specialised 

 organs for the utilisation of these movements of the pigment under the 

 action of light in the interests of sensation. For if the stimulation of 

 epithelial . cells by waves of sound can be communicated to the 

 associated nerves, awakening the sensation called hearing, there is no 

 reason that I can see why the stimulation of the retinal cells by the 

 movement of granules against their surfaces or within their bodies 

 should not also be communicated to the nerves, awakening the 

 sensation called sight. 



As I have elsewhere maintained, eyes arising by the stimulation 

 of the ordinary sensory nerves of the skin indirectly by the epithelial 



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