the Sense of Sight. 169 



Microscopic evidence can be adduced which leaves little doubt 

 that the pigment-granules do actually yield at least some 

 portion of the material out of which the rods are built up. 

 If this is indeed the case, it would go far towards establishing 

 the theory here set out, that the granules endeavour to force 

 their way forward towards the source of light and into the 

 opposing layer,which layer, being composed mainly of sensory 

 cells, is in consequence stimulated. 



In the highest vertebrate eyes these cuticular rods may be 

 of great length, forcing the granule-bearing cells back from 

 the external limiting membrane, while, again, the great 

 thickness of the retinal tissue necessitates considerable diffe- 

 rentiation of its elements for support and, perhaps, on the 

 one hand, for nourishment, and on the other for the removal 

 of waste. 



The Choroidal Epithelium. — Having thus briefly sketched 

 the opposing sensory tissue, we turn to the layer of cells con- 

 taining the granules which seek to reach the surface of the 

 body whenever stimulated by light. The bodies of these 

 cells are forced back from the external membrane of the 

 sensory layer by the cuticular outgrowths just described, but 

 they remain attached to it by fine protoplasmic processes. 

 These protoplasmic strands thus run up among the cuticular 

 rods; or, to describe it in another way, just as the granule- 

 bearing cells, on being arrested by dense epidermal tissue on 

 their way to the surface of other parts of the body, penetrate 

 as far as they can between the cells composing this tissue by 

 means of pseudopodia, so here the pigment-bearing cells 

 penetrate with their pseudopodia between the rods protecting 

 the retina until they are stopped by its limiting membrane. 

 In the case of the eye for certain, and from the darkening of 

 the skin by exposure to sunlight probably in this case also, 

 the granules are stimulated to escape along these pseudopodia 

 by the action of light. In the case of the eye very few 

 apparently succeed in getting away at the tips of these cell- 

 processes through the membrana linn tans externa, and 

 baffled, they have to return to the cells in which they are 

 imprisoned. In the case of the skin, however, their escape in 

 considerable numbers into its outer cell-layers seems not only 

 probable in itself, being quite in accordance with what the 

 microscopic study of skins teaches us, but seems also to be 

 required by the dark colour familiar to us under the name 

 of sun-burning, which persists for some days, or even weeks, 

 after the exposure has ceased *. 



* Dark people, as a rule, burn darkest ; in fair people the granules 

 invading the skin are apparently not coloured, but local immigrations of 

 coloured granules, perhaps from blood-vessels, give rise to freckles. If 



