THE ANATOMICAL BASIS 11 



mammals, amphibia and fishes, the number of rods much exceeding 

 that of cones. In birds, on the other hand, cones are much in excess of 

 rods. In most reptiles (lizards, snakes, tortoises), only cones are found. 

 There are vertebrates possessing only rods, e.g. amongst fishes, rays 

 and dog-fish ; amongst mammals, hedgehog, bat, mole and night-ape 

 {Nyctipithecus felinus). There are also animals of nocturnal habits 

 possessing only rods. Owls, mice, and rell-mice have only a few 

 rudimentary cones ; rats also possess a few cones. 



Hess^ found rods in fowls and pigeons, though they are few in the 

 posterior and superior parts of the retina, which are most used in pecking. 

 As regards nocturnal birds Schultze (1866), Krause (1894), CTreeft' (1900) 

 and Piper (1905) give contradictory statements. Hess found a "not 

 inconsiderable number " of yellow or brown oil globules in the retinae 

 of the owl and hawk, and these are present only in cones. They are 

 fairly uniformly distributed. 



Coloured oil globules are found in the cones of birds and reptiles ; 

 similar colourless bodies are found in fishes and amphibia. The globules 

 are more deeply coloured, yellow or brown, in night birds and in tortoises 

 than in day birds. They are absent in the crocodile. 



Research has shown that at any rate in lower animals stimulation 

 of the retina by light is accompanied by structural, chemical, and 

 electrical changes^. 



Structural Changes. The chief structural changes are the photo- 

 tropic reaction of the pigment epithelium and the contraction of the 

 cones. To these may be added changes in the Nissl granules of the 

 ganglion cells^. 



When the frog's eye is exposed to light the pigment granules wander 

 into the cell processes between the rods and cones. This light effect 

 is complete after 5 to 10 minutes' exposure. The retreat of the granules 

 to the complete dark position takes one to two hours. The light effect 

 is limited to the area stimulated, so that an " epithelial optogram " 

 can be produced (Klihne). The violet end of the spectrum is more 

 strongly " retinomotor " than the red end (Angelucci^, Engelmann^), 

 and red light causes little reaction. Light on one eye causes wandering 



1 Vergleichende Physiol, d. Gesichtsinnes, Jena, 1912. 



2 Garten, in Graefe-Saemisch Handb. d. ges. Augenheilkuiide, Theil i. Brl. iii. Cap. xii. 

 Anhang, 1907-8. 



3 Birch-Hirschfeld, Arch. f. Ophlh. L. 166, 1900; LXiii. 1, 8.5, 1906. 

 « Arch. f. Anat. u. Physiol. 353, 1878. 



'^ Arch J. d. ges. Physiol, xxxv. 498, 1885. 



