472 SPECIAL PHYSIOLOGY. 



of sight is more liable to individual differences, and to illusions than 

 any of the other senses. 



Of the luminous rays which pass, through the pupil, on to the retina, 

 a certain number are reflected by the choroid, the retina, and the 

 transparent media of the eye. Some of these reflected rays impinge 

 on the posterior surface of the iris, and are there absorbed by the uvea ; 

 this structure also absorbs certain rays, passing from the borders of 

 the lens, which would otherwise confuse and dazzle the retina. Some 

 of the rays reflected from the bottom of the eye, however, pass out 

 through the pupil, and converge to a focus ; these are few in number, 

 and do not, when the eye is ordinarily examined, afford information as 

 to the condition of the deeper ocular structures. But light may be 

 thrown into the eye in such quantity that, when reflected from its 

 deep parts, it furnishes us with this information ; and an ingenious 

 instrument, named the ophthalmoscope, has been invented, by means of 

 which the interior of the eye can be easily seen. It consists of a cir- 

 cular, slightly concave, mirror, having a focus of about ten inches, 

 perforated in the centre by a small aperture, and fixed in a handle. 

 The pupil of the eye to be examined, is previously dilated by the 

 introduction of a few drops of a solution of atropine between the eye- 

 lids, and the examination is made in a darkened room. A lamp is 

 placed close to, and on a level with, the eye to be examined, care being 

 taken that none of its rays fall directly on the eye. The observer 

 sits near to, and facing, the eye, and holding the mirror steadily in 

 one hand, brings the rays from the lamp to a focus on the retina ; he 

 then takes a convex lens, having a focal length of from two and a half 

 to three inches, in the other hand, and hdlds it about two and a half 

 inches in front of the cornea. The anterior parts of the eye can be 

 generally examined without the aid of the lens ; by its use, however, 

 the optic eminence, the yellow spot with its elevated rim, and the arbo- 

 rescent vessels of the retina, can be distinctly seen, but in an inversed 

 position ; the retina, as thus examined, appears to be of a shining red 

 color. In the healthy condition, the crystalline lens is invisible. Va- 

 rious diseased states of this and the deeper structures are, however, 

 easily distinguishable by the aid of this useful instrument. 



The Organs and Function of Sight in Animals. 



The general structure of the eyeball, and the uses of its several coats and 

 humors, are similar in all the Vertebrata, and correspond with the structure 

 and functions of the same parts in Man. But many modifications in these 

 organs, are observed in the several vertebrate classes. In Mammalia, there 

 are noticed, in the first place, remarkable differences in the size of the eyeballs, 

 which, as a general rule, are larger in proportion to the powers of vision, in 

 any given animal. Some species of nocturnal habits, have very large eyes, as 

 noticed in the lemur, dormouse, aye-aye, &c. 



The Carnivora generally, have eyes of moderate size ; in the seals, the coats 

 of the eyeball are strengthened, to enable it to resist the pressure experienced 

 during submergence. In the insectivorous Cheiroptera, the eyes are usually 

 small, except in some nocturnal species. In the mole, which lives habitimlly 

 in darkness, the eyeball is so extremely minute, that even its existence has 

 been denied ; it possesses, however, the usual parts, and is supplied by a 



