XI 



RETINA 



185 



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 con- 

 tact 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 

 Jth mm. (y^th inch) thick, and is 

 perfectly transparent. Hence, when 

 an image is formed on it, the rays of 

 light easily penetrate its whole thick- 

 ness until they are stopped by the 

 opaque layer of pigment. The rays 

 can thus stimulate the rods and 

 cones, and the stimulus is trans- 

 mitted through the layers of nuclei 

 and nerve-cells to the fibres of the 

 optic nerve, along which it is con- 

 veyed 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. 



300.) 



c. cones ; g. layer of 

 nerve-cells ; gr, gr'., 

 outer and inner granu- 

 lar layers ; nc, nc', outer 

 and inner nuclear 

 layers ; n. f. nerve-fibre 

 layer : p. ip. pigment- 

 epithelium ; r. reds. 

 (After Howes.) 



