KETINAL EXCITATION 347 



chloride. If one of the optic nerves had been cut, the consensual 

 reflex reaction did not appear. 



Fresh confirmation of Engelmann's experiments was adduced 

 by Lodato and Pirrone (1900) and, by an ingenious modification, 

 by Chiarini (1904). 



These morphological changes in the retina, which result from 

 the action of light and darkness, are mostly developed in fishes 

 and amphibia (Figs. 168, 169) ; they can be seen, but to a less 

 extent, in the retina of reptiles and birds (Figs. 170-171), and are 

 scarcely appreciable in mammals and man. This is proved by 

 the researches of Angelucci, of van Genderen-Stort, and, more 

 recently, of Chiarini and Garten. 



Lizards exhibit all the changes described in Leuciscus and in 

 the frog, especially the movement of pigment and shortening of 

 the contractile part of the cones, which alone form the mosaic 

 layer, as these reptiles have no rods (Fig. 170). 



FIG. 170. Vertical section of temporal half of retina of Lacerta ar/ilis, fixed in v. Tellyesnieszky's 

 fluid. Magnified 510 diameters. (P. Chiarini.) A, after keeping the animal in the dark for 

 24 hours ; B, after exposing it to direct sunlight for 6 hours. 



In ravens the migration of pigment is even more marked than 

 in lizards, and with prolonged action of sunlight it entirely leaves 

 the body of the cell and wanders into the filiform processes. The 

 layer of retinal cells consists in birds of long, fine rods and cones 

 of varying thickness, which contract on exposure to sunlight in 

 the inner segments only (Fig. 171). 



It is far more difficult to study the slight morphological 

 changes in the mammalian retina. Guinea-pigs and rabbits are 

 of little use for such experiments because their retina contains no 

 cones ; in dogs the cones are very few and extremely slender ; in 

 pigs and monkeys they are more numerous and well-developed. 

 Van Genderen-Stort first demonstrated shortening of the cones 

 under the action of light in the pig ; Chiarini failed to detect any 

 difference in light or darkness in the few slender cones of the dog. 

 Garten's experiments on monkeys (Macacus and Cercopithecus) 

 led to very doubtful results, because the differences of length 

 (measured between the membrana limitans externa and the 

 ellipsoidal base of the cones) in retinae exposed to light, and those 



