1897- LIGHT-SENSATIONS OF EYELESS ANIMALS. 177 



After having established the facts concerning the sensitiveness to 

 light of the skin in certain animals, the next question that arises is : 

 Are there any specific sense-organs functionally associated with this 

 sensation ? Its answer, contained in Appendix 3, p, 98, is : no such 

 organs can be found. The skin is richly provided with simple sensory 

 cells, and the author is driven to assume either that there are subtle 

 differences, not evident in their microscopic appearances, between these 

 cells, some being tactile, others sensitive to light and so on, or that one 

 and the same apparently simple sensory cell may serve for different 

 sensations. Dr. Nagel is an advocate of the latter hypothesis. The 

 infusorian is a simple cell and is capable apparently of very diverse 

 sensations ; at least it reacts to many different stimuli. And, again, 

 any cell has at least two different and opposite reactions, viz., 

 expansion and contraction, and these, with their degrees, might make 

 it possible for one and the same cell to be the end-organ of more 

 than one sensation. 



We may then briefly sum up Dr. Nagel's conclusions. The whole 

 skin is a possible retina ; cells sensitive to light are there, and the 

 nerves connecting these cells with the central nervous system ; all 

 that is wanted to form an eye proper is a dioptric apparatus, i.e., 

 primarily, a lens to throw an image on the skin. Distinct vision of 

 the external world would result as soon as there were a sufficient 

 number of the sensory cells to give a connected translation of the 

 image with its various lights and shades into sensation. 



Beyond these suggestions as to the probable explanation of this 

 curious function and of its relation to specialised vision. Dr. Nagel 

 does not go, and indeed it does not seem possible to go further along 

 recognised lines. As above stated, however. Dr. Nagel follows in 

 the beaten track, and practically neglects one important factor in the 

 problem. I refer to the pigment. 



The complete absence of the pigment in the siphons of Psammobia 

 already noted is especially emphasised by Dr. Nagel, as tending to 

 strengthen the ordinary view that pigment is not necessary to sight, 

 a fact which was already known from the absence of pigment in the 

 eyes of albinos. Nevertheless, Dr. Nagel, who may be taken to 

 be expressing the generally accepted doctrine, is obhged to admit 

 that the pigment has some very intimate association with light- 

 sensation. It is almost univerally present in eyes, being absent only 

 in albinos, which must be considered as abnormal (Missbildungen). 

 What this relationship is no one has hitherto been able to explain. 

 And on this subject Dr. Nagel confines himself to the oft repeated 

 suggestion, that the pigment forms sheaths round the sensory cells. 

 In this way, the dazzling of the light is prevented, and each end 

 organ is illuminated by itself, the distinctness of the image being thus 

 assured. I should not, of course, wish to deny that this ensheathing 

 of the rods and cones for the isolation of the end-organs is one of the 

 functions of the pigment. There are certain facts, however, which 



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