INTRODUCTION AND METHODS 



deep into the gullet of the fish until, with some tearing of the tissue, 

 the end of the tube Hes behind and between the eyes. In this position, 

 the tube acts as a support for the eyes, as did the forefinger in the case 

 of the frog. By using the tube as a lever, either eye may be made to 

 protrude. A cut is then made across the cornea and the lens and 

 retina pressed through as before. 



Larger eyes are first removed from the animal. Thus ox eyes, 

 received from a slaughterhouse, are first cleaned and then cut into 

 posterior and anterior halves by peripheral incisions at about the 

 level of the ora serrata. The anterior half contains the cornea, 

 aqueous humour, iris and lens and is discarded. The posterior half, 

 containing the retina, is full of the jelly-like vitreous humour. This 

 may be spilled out, care being taken not to disturb the retina. With 

 a soft paint brush the retina is then gently coaxed away from the 

 underlying pigment epithelium and heaped up around the optic 

 nerve. The cup is then turned inside-out hke a half-orange skin. The 

 retina hangs down from the optic nerve from which it may be snipped 

 away. This procedure may also be used with human eyes. 



EXTRACTANTS FOR VISUAL PIGMENTS 



The visual pigments, as exemplified by visual purple, can be ex- 

 tracted from retinae by aqueous solutions of certain substances. The 

 first workers used bile salts but in 1931 tansley introduced digito- 

 nin, a rare glucoside found in the seeds of the purple foxglove, 

 digitalis purpurea. Digitonin, because it is colourless, quickly sup- 

 planted the bile salts, and since 1931 has been used in most work. 

 However, it is now possible to obtain certain of the bile salts — for 

 example, sodium cholate and sodium deoxycholate — as colourless 

 powders, and these efficient extractants have the advantage over 

 digitonin of dissolving readily in cold water. Saponin is also eff'ective 

 but the pure-white powder should be used. The yellow powder 

 usually suppHed gives a solution which turns brown in a few hours. 

 Certain synthetic substances, in particular cetyltrimethylammonium 

 bromide (CTAB) or chloride (CTAC), are excellent and much 

 cheaper than digitonin. Extractants, such as the soaps, which 

 require alkaline conditions should be avoided as these give opalescent 

 solutions which cannot be clarified by normal centrifuging. To 

 obtain a crystal clear visual pigment preparation the extracting 

 solution should be acid (pH 4-5). After separation of the extract 

 from the retinal debris, it may then be made alkaUne, but not before. 



9 



