

NERVOUS CONSTITUENTS OF THE RETINA. 263 



Perosmic acid applied to the fresh retina of Birds and Reptiles 

 hrings out the lenticular body with extraordinary distinctness, pre- 

 serving the sharpness of its outline, and at the same time scarcely 

 altering its colour. No structure similar to this can be discovered in 

 the cones and rods of Man, either in the fresh state, or when acted 

 on by perosmic acid. It is particularly worthy of notice that in some 

 animals the lenticular body of the rods is composed of two segments 

 that react differently to this acid, and present also different refractive 

 powers.* In the rods of Birds a small anterior segment often be- 

 comes separated from the point of the ellipsoidal lens, appearing in the 

 form of a short pointed process, possessing a strong lustre t (fig. 358, s) ; 

 and in Tritons the posterior segment presents a spherical concavity in 

 front, in which the anterior segment lies (fig. 358, 5 c). We may 

 reasonably suppose that we have here to do with arrangements 

 which refract in a very definite way the rays of light passing to 

 the outer segment. 



The peculiar " twin or double cones,' 1 first described by Hannover, 

 are quite enigmatical from a physiological point of view. { They have 

 not up to the present time been discovered in Mammals or in Man, 

 but they exist in Birds and Reptiles, Amphibia and Fishes. In 

 Fishes, where they attain the largest size, and are the most numerous, 

 and where consequently they are most easily examined, they consist 

 of two apparently exactly similar coalesced cones, the outer segments of 

 which, however, as well as the cone fibres, are separate and distinct, 

 so that it might almost be said they were cones multiplying by longi- 

 tudinal fission. It is different in other classes of animals ; for, as I 

 have shown, there are essential points of distinction between- the two 

 halves of the twin-cones, which must possess some physiological signi- 

 fication. In Birds, Tortoises, Lizards, and in the Frog, in which each 

 ordinary cone contains a coloured or colourless globule, such a 

 globule is found in only one of the halves of the twin-cones, the other 

 half possessing merely the ellipsoidal lenticular body, which in many 



* Max Sclmltze, Archiv fiir Mikroskop. Anatomic, Band v. , pp. 401, 403, 

 figs. 2 and 17. 



f I formerly stated that this pointed body, as seen in swollen internal 

 segments, might possibly be a persistent and resistant axis fibre. Archiv 

 fur Mikroskop. A natomie, Band iii. , p. 245, fig. 6. 



J Hannover, Rechcrchcs microscopiqucs, etc., 1841. A more minute de- 

 scription of them, by Max Schultze, will be found in the Archiv fur 

 Mikroskop. Anatomic, Band iii., p. 231. 



Hannover, indeed, believed he had found them in both Man and 

 Mammals, but his observations were incorrect. 



