the Sense of Sight, 167 



merely qualitative *. That these granules, in spite of this 

 deficiency, seek the light, we gather from the fact that the 

 outer skin of albinos is, but for the absence of colour, appa- 

 rently normal. The connexion between the granules and 

 integumentary protective structures has already been referred 

 to, and need not here be repeated. I may merely remark 

 that we have almost every gradation between dark-skinned 

 people with coal-black eyes, through fair-skinned people with 

 less deeply pigmented eyes, to albinos with so little pigment 

 in their skins and eyes tiiat, if present at all, it is not at first 

 sight apparent. Perhaps faint traces might be found if the 

 granules of the eyes were specially examined for that purpose. 

 My own, all too limited, observations on this point have so 

 far left me undecided. 



We may conclude, however, that the colour of the granules is 

 not essential, for, without it, they strive to reach, and in the 

 skin succeed in reaching, the surface. Nevertheless the fact 

 that the granules collected in visual organs are, as a rule, 

 deeply coloured, shows that the colour is very useful. Its 

 absorption of the light prevents a diffusion injurious to clear 

 vision, and, perhaps, also increases the vigour of the move- 

 ments of the granule by slightly raising its temperature. 



We have, then, these two tissues to consider : — 



(1) The layer of cells (choroidal or pigment epithelium) 

 containing the usually pigmented granules, which seek to 

 force a way towards the source of the light that stimulates 

 them to action. 



(2) The thick layer of tissue, chiefly nervous and sensory, 

 which blocks the way, i. e. the retina. 



We will, for convenience, take the latter first. 



The Retina. — It will be freely admitted that it is not easy 

 to give a simple morphological definition or description of 

 this complicated structure. While certain of its elements are 

 fairly well understood, we are still far from a complete com- 

 prehension of it as a svhole. In fact, the application of the 

 most recent and approved methods of staining tend, it seems 

 to me, to make it more enigmatical than ever. What follows 

 is therefore put forward purely tentatively. The retina is a 

 many-layered sensory epithelium, in which originally, %. e. 

 when less highly developed, the cells stretched as fine thread- 

 like strands between its limiting membranes, the nuclei being- 

 suspended on these threads at different levels. This is a 



* Pigmented granules lose their colour in the process of being trans- 

 formed into cuticle or slime within epidermal cells. This fact larg^ily 

 explains why the two have not hitherto been associated in tlie manner 



