THE RETINA. 323 



first, the internal limiting membrane, 1, a thin structure- 

 less layer. Next comes the nerve-fibre layer, 2, formed by 

 radiating fibres of the optic nerve ; third, the nerve-cell 

 layer, 3; fourth, the inner molecular layer, 4, consisting 

 partly of very fine nerve-fibrils, and largely of connective 

 tissue ; fifth, the inner granular layer, 5, composed of nu- 

 cleated cells, with a small amount of protoplasm at each 

 end, and a nucleolus. These granules, or at any rate the 

 majority of them, have an inner process running to the in- 

 ner molecular layer, and an outer running to, 6, the outer 

 molecular layer, which is thinner than the inner. Then 

 comes, seventh, the rod and cone fibre layer, 7, or outer 

 granular layer, composed of thick and thin fibres on each 

 of which is a conspicuous nucleus with a nncleolus. Next 

 is the thin external limiting membrane, 8, perforated by 

 apertures through which the rods and cones, 9, of the 

 ninth layer join the fibres of the seventh. Outside of all, 

 next the choroid, is the pigmentary layer, 10. The nerve- 

 fibres are believed to be continuous with the rods and cones. 

 Light entering the eye passes through the transparent retina 

 until it reaches the rods and cones and excites these, and 

 they stimulate the nerves. 



The action of the light is probably in the first instance 

 chemical. The rods are stained by a purple substance, 

 which is bleached by light and regenerated in the dark. In 

 the healthy eye the purple is reproduced nearly as fast as de- 

 stroyed, by the pigment-cells of the retina. Parts of the retina 

 which contain none of this visionpurple can see, but they may 

 possess uncolored substances which are changed by light. 



Describe the microscopic structure of the retina. On what con. 

 stituents of the retina does light first act, in producing a sensation? 



With what are the rods of the retina stained? How is the pig- 

 ment reproduced? 



