EFFECT OF LIGHT ON THE RETINA 287 



are, in the eye, resynthesised into rhodopsin. Removed from the 

 retina, the pigment bleaches at a rate which varies in velocity with 

 the intensity and wave-len»th of the incident light and remains in 

 its bleached state. That is, the rods or their adnexa are essential 

 for the restoration process. (3) Other pigments, such as visual 

 yellow and pigmented oil drops (amphibians, reptiles and birds) 

 have been described, but their nature, distribution and function 

 are not yet clear. It has been suggested that these red, yellow and 

 green globules act as colour filters in much the same way as the 

 coloured starch grains do in the Lumiere process for colour 

 photography. 



The Retina. (2) Effect of Light on it. 



When light falls on the sensitive surface of the retina physical 

 and chemical changes occur. 



(1) PhysicaJ. » 



(a) Pigment moves inwards (Boll and Kuhne, 1877). 



(6) Cones retract (Angelucci, 1882). 



(c) Outer parts of rods swell. 



{d) Ganglionic chromatin is decreased. 



{e) Electrical changes occur. 



(2) Chemical. 



(f) Retina develops an increase in H+ concentration. 

 {g) Rhodopsin and fucsin, two pigments, are bleached, 



and a physiological change is produced, whereby we see. Of 

 these changes, («) and {b), and probably (c) and [d] as well, are 

 secondary products. They only occur if the nervous mechanism 

 is intact, and may be produced by stimulation of either optic 

 nerve by any means, i.e. they are caused by retinomotor impulses 

 from the brain. That leaves three factors to be reckoned with in 

 any explanation of the mechanism of the retina, viz. the electro- 

 motive force developed, the alteration in hydrogen ion concentra- 

 tion, and the bleaching of the pigments. 



(1) Electromotive Force. Du Bois Reymond in 1840 noticed 

 that alteration in the nature and intensity of illumination altered 

 the potential of the retina as a whole. His work has been con- 

 firmed and amplified by Holmgren (1880) and later workers. In 

 general, illumination causes a negative variation in the electro- 

 motive force of any point of the retina, i.e. a current would flow 

 in the retina from an ilhmiinated point to the rest of the structure. 

 In other words, the ilhuninated part becomes zincative just like 

 the injured part of any cell, the contracting part of a muscle or the 

 momentary site of the impulse in a nerve. This is not strange when 



