68 Tlie Book of Cats, 



night, the structure of its visual organs is adjusted 

 for both. The retina, or expansion of the optic 

 nerve, is most sensitive to the stimulus of light ; 

 hence, a well-marked ciliary muscle contracts the 

 pupil to a mere vertical fissure during the day, 

 while in the dark, the pupil dilates enormously, and 

 lets in as much light as possible. But even this 

 would be insufficient, for Cats have to look for their 

 prey in holes, cellars, and other places where little 

 or no light can penetrate. Hence, the Cat is fur- 

 nished with a bright metal-like, lustrous, mem- 

 brane, called the Tapetum, which lines part of the 

 hollow globe of the eye, and sheds considerable 

 light on the image of an object thrown on the 

 retina. This membrane is, we are told, common to 

 all vertebrated animals, but is especially beautiful 

 and lustrous in nocturnal animals. The herbivora, 

 such as the ox and sheep, have the tapetum of the 

 finest enamelled green colour, provided probably 

 to suit the nature of their food, which is green. 

 The subject, however, of the various colours of the 

 tapetum in different animals is not yet understood. 

 The sensibility of the retina in Cats is so great that 

 neither the contractions of the pupil nor the closing 

 of the eye-lids would alone afford them sufficient 

 protection from the action of the light. Hence, 



