Page:The Book of Cats.djvu/88

68 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 furnished with a bright metal-like, lustrous, membrane, 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,