THE VISUAL PIGMENTS 



only an hour or two) the concentration of visual pigment in the 

 retina rises to its maximum value. In some cases this period of dark 

 adaptation is useful for another reason. In fish, birds, most amphibia 

 and some reptiles — but not in mammals — the dark brown granules of 

 the pigment epithelium (Fig. 1.2) move forward on Ught adaptation 

 and recede again in the dark. Thus in certain light-adapted retinae 

 the outer Hmbs of the rods and the granules are inextricably mixed. 

 In the dark-adapted condition, however, the retina can be cleanly 

 removed. Movement of the granules is largely conditioned by Ught, 

 but is also partly dependent on temperature. There is often also a 

 strong diurnal rhythm, and on this account, frog retinae can be more 

 cleanly removed in the evening than in the morning. 



Although dark-adaptation of the hving animal is advisable, it is 

 not essential. Eyes obtained from a slaughterhouse have been 

 exposed to light and yet from them satisfactory preparations of visual 

 pigments can often be made. Regeneration of the visual pigment 

 does not seem to cease at once on the death of the animal and, if such 

 eyes are kept in darkness for an hour or two, some replenishment 

 occurs. Ideally, the operator should attend the kiUing and put the 

 eyes immediately into a light-tight box. 



REMOVAL OF THE RETINA 



After killing the animal the next step is to obtain the retinae. The 

 manner in which this is done depends partly on the size of the eye, 

 and no general rules can be given. With frogs and toads, the animals 

 are first decapitated and the heads washed free from blood. A head 

 is then held in the left hand with the forefinger on the palate and the 

 thumb, in opposition, on top of the head. The forefinger provides a 

 backing to the eye. A blade-point of a sharp pair of scissors is then 

 pressed into the side of the eye at the corneal-scleral junction and a 

 single cut made right across the cornea. By applying gentle pressure 

 with the forefinger, first the lens and then the retina appears through 

 the cut. The retina can then be gently hfted away. The retinae of the 

 frog and indeed of most small animals is surprisingly coherent and is 

 little damaged by this procedure. On placing it in water, for example, 

 it resumes its shape, billowing out into a hemisphere — at the base of 

 which can be seen a small hole where it was torn from its connections 

 to the optic nerve. 



A somewhat similar procedure works well with small fish eyes. In 

 these cases, the tapering end of a 10 ml centrifuge tube may be thrust 



8 



