ANATOMY AND PHYSIOLOGY OF THE RETINA. 153 



gradually lose tlieir lustre, as they lost their purple colour at 

 the end of the first stage. Their refractive index approaches 

 that of the other layers of the retina, which now, in the third 

 stage, becomes quite transparent. The opacity of the retina 

 in the fourth stage is due, not to changes in the layer of rods, 

 but to coagulations of albuminous substances which take 

 place in the other layers. 



These peculiarities of the living retina are found in all 

 animals which possess a fairly developed layer of rods. 

 Among vertebrata, in addition to amphibia, I could find 

 them in cartilaginous and teleostean fishes (of which I 

 examined a great number of species and individuals at 

 Viareggio this autumn) and in mammals. I must here men- 

 tion that the purple colour of the first stage is generally 

 longer and better preserved in cold-blooded animals and in 

 those which have the largest rods (amphibia and cartila- 

 ginous fishes) than in warm-blooded animals and kinds 

 with very delicate rods (teleostean fishes and roost mam- 

 mals), in which the retina often passes with extraordinary 

 rapidity from the purple tint to the second stage. 



In the two remaining vertebrate classes, birds and rep- 

 tiles, the study of the living retina is surrounded by peculiar 

 difficulties, as we know it already shows a brilliant and per- 

 manent coloration, caused by the presence of variegated fat- 

 drops. This coloration naturally renders it extremely 

 difficult to determine any change of tint in the first stage. 

 But in the pigeon I have succeeded in showing an undoubted 

 and distinct paling of the central red part of the retina in 

 the first ten seconds, so that I must conclude that the intense 

 red colour of the pigeon's retina must be due, not only to the 

 red fat-drops, but also to an optical change occurring in the 

 external segments of the rods ; the characteristic satin-like 

 lustre of the second stage is also always very distinct in the 

 retina of birds. On the other hand, in the retina of tVie 

 lizard (the only reptile which I have hitherto examined) I 

 have found no indication of the appearances which charac- 

 terise the first and second stages ; I will not decide whether 

 they were obscured by the intensely yellow colour of the 

 retina, which is due to fat-drops, or whether the extremely 

 rudimentary development of the outer segment of the rods 

 in this retina may be insufficient to give rise to the optical 

 phenomena described. 



The same characteristic purple colour may be also shown 

 in the eye of invertebrate animals. Already in 1842 Krohn 

 stated that the large r9ds of the retina of cephalopoda had 

 a purple-red colour in the fresh state. The same is known 



