ANIMAL AND VEGETABLE 



221 



I take it, that there is such a thing as memory of the eye, 

 or persistence of vision. 



Comparing the lens of the eye with that of a camera, the 

 iris is the diaphragm to regulate the aperture, and the rays 

 or points of light admitted by the lens are thrown, although 

 not directly, upon a layer of pigment cells which form 

 the outer or choroidal surface of the retina. 



It should also be noted that posteriorly to the iris is a 

 layer of pigment cells, a continuation forwards of the 

 pigment layer of the retina. 



Fig. 119. PIOMENTED EPITHELIUM OF THE HUMAN RETINA. (Ma* 

 Schullze.) 



a, cells seen from the outer surface with clear lines of intercellular 

 substance between ; b, two cells seen in profile with fine offsets extending 

 inwards ; c, a cell still in connection with the outer ends of the rods. 



In colour these pigment cells appear to be dark brown, 

 and, like the macula lutea, apart from the fovea centralis, 

 non-actinic. 



It will be seen, from b and c, that fine offsets or nerve- 

 fibres extend inwards from these cells, and, presumably, 

 either make connection with or influence the rods and 

 cones in their immediate vicinity ; these rods and cones 



