RETINA 



185 



passing through the sclerotic and choroid, perforate the 

 retina, and spreads out over its inner surface. Next comes 

 a layer of nerve-cells (g), and then several layers of fibres 

 and nuclei (gr, nc) ; and finally, forming the outer surface of 

 the retina proper, is a layer of delicate, 

 transparent bodies called, from their 

 form, the rods (r) and cones (c) ; these 

 are known from their development to 

 be modified epithelial cells. The whole 

 of these structures are supported by 

 a complex framework of connective- 

 tissue. In close contact with the outer 

 or free ends of the rods and cones is 

 a layer of cells the protoplasm of 

 which is filled with a dense black 

 pigment. It is this pigment-layer 

 (p. ep\ which, as we have seen, is 

 often counted as part of the choroid. 



In spite of its complex structure, the 

 retina is not much more than ^th mm. 

 (_i._th inch) thick, and is perfectly 

 transparent. Hence, when an image 

 is formed on it, the rays of light easily 

 penetrate its whole thickness until 

 they are stopped by the opaque layer 

 of pigment. The rays can thus 

 stimulate the rods and cones, and the 

 stimulus is transmitted through the layers of nuclei and 

 nerve-cells to the fibres of the optic nerve, along which it 

 is conveyed to the brain. Thus the actual organ of sight 

 is not the eye as a whole, but the retina : all the rest is 

 to be looked upon as an accessory apparatus, for focussing 

 and for regulating the admission of light. 



FIG. 58. Vertical section 



of frog's retina. 

 c. cones ; g. layer of 

 nerve - cells ; gr, gr . 

 outer and inner granu- 

 lar layers; tic, nc. 'outer 

 and inner nuclear 

 layers ; n.f. nerve-fibre 

 layer ; /. ep. pigment- 



epitl 

 (Aft. 



:er Howes.) 



