NERVOUS CONSTITUENTS OF THE RETINA. 205 



Mammals, in the Bat, the Hedgehog, and Mole ; * whilst the bacillar 

 layer of the retina of many Lizards, Snakes, and Tortoises, and pro- 

 bably indeed of all Keptiles, is altogether destitute of rods, and is 

 therefore c.cclusircb/ composed of cones.^ In Birds the number of 

 cones is in general far greater than that of the rods, whilst in Mammals 

 the reverse obtains. In the retina of Man and the Monkeys, as is well 

 known, it is only at the yellow spot that the cones exceed the rods 

 in number ; at the centre of this acutely perceptive area the rods en- 

 tirely fail. The retina of Birds resembles consequently the macula 

 lutea of Man throughout its whole extent, in the relative proportion 

 of the cones to the rods, and this similarity is still further increased 

 by the circumstance that the yellow oil globules in the outer segments 

 of the cones of Birds correspond to the presence of yellow pigment 

 in the most sensitive area of the human retina. It is very remarkable 

 that the number of the cones in the dusk or night-flying Owls is re- 

 markably less, so that in these birds the rods again predominate, whilst 

 at the same time the intensity of the yellow pigment in the cones is con- 

 siderably smaller than in the day Birds, and the red pigment is wholly 

 absent. J In Mammals that roam by night or in the twilight the pro- 

 portionate number of the cones likewise diminishes, or they altogether 

 fail, as in the Bat and other genera already mentioned. The cones 

 in the Rat, Mouse, Dormouse, Guinea-pig, though present, are quite 

 rudimentary as compared with those of Man, the Pig, Ruminants, 

 and the Dog. Cats have distinct but slender cones ; those of Rabbits 

 are not so well marked. 



It is also a remarkable circumstance that the absolute length of the 

 outer segments of' the rods of most nocturnal animals is very con- 

 siderable. || With the length of the outer segments, the number of 



* Archiv fitr Mikroskop. Anatomic, Band ii. , p. 198, Band iii. ; p. 238. 



t Archiv fur Mikroskop. Anatomie, Band ii. , p. 209. The statement made 

 by Krause, that in Lacerta agilis both rods and cones are present, is erroneous. 

 Hulke, as I have shown, has not been able to distinguish the rods from the 

 cones in Reptiles, so that his statements must be received with caution. 



See my statements in the A rchiv f. Mikrosk. A nat. , Band ii. , p. 208, the 

 accuracy of which, after repeated re-investigation, I must still maintain, 

 notwithstanding the opposition of W. Krause (Membrana fenestrata, p. 29). 



Max Schultze, Archiv fur Mikroskop. Anatomie, Band ii., p. 197. For 

 the conflicting statements of W. Krause, see his Anatomy of the Rabbit, 

 p. 129, and Membrana fenestrata, p. 30. 



|| Max Schultze, Archiv fur Mikroskop. Anatomie, Band ii., p. 199, 

 Taf. xiv., fig. 7 (Rat) ; p. 208, Taf. ix., figs. 10 and 11 (Owl) ; Band iii., 

 p. 243. W. Krause, Membrana fenestrata, p. 31. 



