RETINAL CHANGES INVOLVED IN VISION 



565 



in the dark for some time be killed, an eye taken out, bisected, and the 

 retina removed and examined by a weak light, it will be found to have 

 a purplish-red colour. On microscopical examination this colour is seen to 

 be confined to the outer limbs of the rods. After a very short exposure to 

 diffuse daylight the colour disappears. The colouring- matter (rJiodopsin) 

 may be dissolved out by means of a solution of bile salts. The purple-red 

 solution thus formed also bleaches rapidly on exposure to light. By means 

 of this rhodopsin photographs or ' optograms ' of external objects may be 

 taken on the retina. The rabbit's eye is cut out and placed in front of a 

 window. After some time the eye is Bisected and plunged into a 4 per cent. 

 solution of alum, which partially fixes the 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 



I 



FIG. 292. Sections of the frog's retina. 



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

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

 rods. (ENGELMANN.) 



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 pigmented 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. 

 292). 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. 



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

 p'a-ys an essential part in vision as a sensitiser of the cones. 



