476 ELECTRO-PHYSIOLOGY CHAP. 



leading off, and may after previous, strong illumination fail 

 altogether, while the isolated retina would still yield a vigorous 

 current of action. 



These differences in the photo-electrical variations of the 

 eyeball and isolated retina appear to derive solely from unavoid- 

 able alteration of the latter during preparation, since careful 

 division of the eye into anterior and posterior halves, with removal 

 of the lens, never produces the middle, negative phase, while this 

 never fails to appear on tearing the retina, or letting out the 

 vitreous humour. The same occurs in the uninjured eyeball, 

 when excitability diminishes gradually in a stale preparation 

 (accumulation of C0 2 ). The positive initial phase of the 

 diphasic variation becomes gradually smaller, and at last fails 

 altogether. Holmgren was the first to observe the differences 

 in the photo-electrical variations of the retinae of various animals. 

 In reptiles (Vipera Berus), birds (fowl), and mammals (rabbit, 

 dog), the uninjured eyeball in situ invariably exhibits first a 

 negative and then a positive variation of the current of rest, 

 instead of the diphasic in both cases positive variation of the 

 frog's eye at the commencement and end of illumination. Since 

 (supra) a similar reaction takes place in dying or fatigued frogs' 

 eyes, the effect might presumably be due to the lower resistance 

 of warm-blooded eyes, 011 the assumption that if investigated 

 under conditions as nearly as possible normal, they would exhibit 

 the same characteristics of photo-electrical variation as the frog's 

 eye. Against this, however, we have on the one hand the fact 

 that in certain cases, e.g. the pigeon (where the long rods and 

 cones are very enduring), the isolated retina lends itself readily 

 to experiment, and plainly shows at least the initial negative 

 phase (especially if the temperature of the chamber is raised 

 a little, Kiihne and Steiner) : on the other hand, the reaction 

 of reptiles' and still more of fishes' eyes may be urged as a 

 plausible objection. 



The results of illuminating the fish's eye were nil for 

 Holmgren, and very unsatisfactory to Dewar and M'Kendrick. 

 Klihne and Steiner, on the contrary, obtained successful effects 

 from the uninjured eyeball, and still more from the isolated 

 retina of several species of fish (Perca fluv., Esox Indus, Leuciscus 

 and Cyprinus barbus). 



While the current of rest gives the same reaction as in the 



