How some Animals see in itie Dark. 6$ 



nctrates into the interior of the eye, we cannot help considering 

 it as the means employed for this purpose, by its collecting the 

 light, and illuminating, by its reflection, objects lying in the 

 axis of the eye. Prevost objects to this explanation, that there 

 are many animals whose eyes have no tapetum, although they 

 conduct themselves as if they saw in the dark. This is actually 

 the case. The tapetum occurs in camivora, ruminantia, pachy- 

 dermata, cetacea, owls, crocodiles, snakes, rays and sharks : it is 

 wanting in man, apes, glircs, chiroptera, hedgehogs and moles ; 

 in birds, with exception of owls ; and in osseous fishes. But 

 the gnawers or glires, bats, the hedgehog and mole, are animals 

 that obtain their food more by night than during the day ; and 

 many of them conduct themselves in the deepest darkness, as if 

 they were directed by the sense of sight. But this objection 

 may be obviated, by remarking, that it is probably some other 

 sense than that of vision, which procures for many of these ani- 

 mals sensations of external objects in the dark. We have in 

 favour of this opinion, not only the experiments of Spallanzani 

 on bats, from which it appears that, after these creatures were 

 deprived of the use of their eyes, they conducted themselves as 

 if they still possessed the power of vision, but also the examples 

 of species of that family, in which the eyes are so imperfectly 

 developed, or lie so much concealed behind the outer skin, that 

 it is of little or no use to the animal. The genera that see in 

 the dark, have undoubtedly so irritable a retina, that they can 

 only see during a very feeble light, whereas in those animals 

 whose eyes are organized equally for day light and nocturnal 

 darkness, the retina possesses less irritability. Hence, al- 

 though these are without a tapetum, it does not follow that 

 this organic part does not afford a mead for seeing during a 

 feeble light. 



The tapetum is either spread over the whole choroid, or only 

 over the upper half of it. The first is the case with the cetacea, 

 owls, and with those amphibia and fishes which are provided witli 

 this shining envelope ; the second occurs in carnivorous and ru- 

 minating animals. It is more extended in the ruminating than in 

 the carnivorous tribes. But it always extends so far as to encom- 

 pass the posterior extremity of the internal ocular axis. All the 

 rays of hght from external objects which reach it, arc united on it. 



