ANIMAL MECHANISM. 131 



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 explana- 

 tion, 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 carnivora, ruminantia, pachydermata, cetacea, 

 owls, crocodiles, snakes, rays and sharks : it is wanting 

 in apes, glires, chiroptera, hedgehogs and moles ; in birds, 

 with the exception of owls, and in osseous fishes. But the 

 gnawers or glires, bats, the hedgehog and mole, are ani- 

 mals that obtain their food more by night than during the 

 day ; and many of them conduct themselves in the deep- 

 est 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 animals sensations of 

 external objects in the dark. We have in favor 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 them- 

 selves 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 they are of little or no use to the 

 animal. The genera that see in the dark, have undoubt- 

 edly so irritable a retina, that they can only see during a 

 very feeble light ; whereas in those animals whose eyes 

 are organized equally for daylight and nocturnal dark- 

 ness, the retina possesses less irritability. Hence, 

 although these are without a tapetum, it does not follow 

 that this organic part does not afford a mean 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. witlfthose amphibia and 

 fishes which are provided with this shining envelope ; the 

 second occurs in carnivorous and ruminating animals, it 

 is more extended in the ruminating than in the carnivo- 

 rous tribes. But it always extends so far as to encompass 

 the posterior extremity of the internal occular axis. All 



