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 modiiied 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 \Xh mm. 

 (j-^^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 are thus en- 

 abled to 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, 

 t. cones ; g;. layer of 

 nerve - cells ; gr, gy, 

 outer and inner granu- 

 lar layers; nc, «c.'outer 

 and inner nuclear 

 layers ; n.f. nerve-fibre 

 layer ; p. ep. pigment 

 epithelium ; r. rods. 

 (After Howes.) 



