630 



PHYSIOLOGY 



optogram, and an inverted picture of the window with its cross-bars 

 is obtained on the retina. 



If a retina, which has been bleached by exposure to light, be 

 replaced on the pigment layer lining the choroid, in a short time 

 the colour will be restored. On examining sections through the retina 

 it is found that, in one which has been exposed to light, the cells of 

 the layer of pigmented epithelium send up fine processes full of pig- 

 mented granules between the outer limbs of the rods. In an eye which 

 has been kept in the dark, on the other hand, the cells of the pigment 

 layer are quite flat, so that the front part of the retina, including the 

 rods and cones, can be removed without any difficulty (Fig. 290). 



B A 



FIG. 290. Sections of the frog's retina. 



A, kept in the dark ; B, after exposure to light, showing retraction of 

 the cones, and protrusion of the pigmented epithelium between the outer 

 limbs of the rods. (ENGELMANN.) 



Thus the function of the pigmented epithelium is to supply visual 

 purple to the outer limbs of the rods as fast as the pigment already 

 there is bleached by light. It might be thought that this chemical 

 change was the active agent in producing excitation of the optic nerve 

 fibres ; but the fact that in the fovea centralis,* the region of most 

 distinct vision, we find only cones which contain no visual purple 

 indicates that this chemical process is not essential for the conversion 

 of light-waves into a nervous impulse. When light falls upon the 

 retina the cones are retracted, and lie close upon the external limiting 

 membrane ; whereas in an eye that has been kept in the dark they 

 extend down between the rods as far as the pigmented layer. 



The falling of light on the retina is also accompanied by an electrical 

 change, which may be regarded as analogous to the current of action in 

 nerve. It is, however, much more complicated than the latter. The 



* According to Edridge Green visual purple diffuses into the fovea centralis, 

 and plays an essential part in vision as a sensitiser of the cones. 



