THE EYE.
A sensibility to the impressions of light is common to all those animals, which in a natural state are exposed to this element ; it appears at least very evidently to exist in some of the most sim ple zoophytes, as the armed polypes (hy dra :) but the power of perceiving the images of external objects is confined to those who are provided with eyes for their reception. Nature has bestowed on some species, even of red-blooded animals, a kind of rudiment of eyes, which have not the power of perceiving light ; as if in compliance with some general model for the bodily structure of such animals. This circumstance at least has been asserted of the blind rat (marmots typhlus) among mammalia ; and of the myxine glutinosa among fishes.
The conjunctiva covering the front of the eye-ball, in the former animal, is co vered with hair, so that the eye, which is exceedingly small, seems to be com pletely useless.
Large animals have small eye-balls in proportion to their size : this is very re markably the case with the whales. Those which are much tinder ground have the globe also very small ; as the mole and shrew : in the former of these instances its existence has been altogether de nied ; and it it is not in fact larger than a pin's head.
The eyes of man and the simix are di rected forwards : in the latter animals, indeed, they are placed nearer to each other than in the human subject. The lemul tarsius has them more closely ap proximated than any other animal. All other Mammalia have these organs se parated by a considerable interval, and directed laterally. The same circum stance obtains in birds, with the excep Lion of the owl, who looks straight for wards. They are placed laterally in all reptiles. Their situation varies much in fishes : they look upwards in the uranos copus : they are both on the same side of the body in the pleuronectes ; but in their direction is lateral.
The form of the globe varies according to the medium in which the organ is to he exerted. In man and the mammalia, it deviates very little from the spherical figure. In fishes it is flattened on its an
terior part ; in birds it is remarkably convex in front, the cornea being some times absolutely hemispherical. The convexity of the crystalline is an inverse ratio to that of the cornea. Thus in fishes it is nearly spherical, and projects through the iris, so as to leave little or no room for aqueous humour : the ceta cea, and those quadrupeds and birds which are much under water, have this part of the same form. The aqueous hu mour, being of the same density with the medium in which these animals are plac ed, would have no power of refracting rays of light which come through that medium : its place is supplied by an in creased sphericity of the lens. In birds these circumstances are reversed : they inhabit generally a somewhat elevated reigon of the atmosphere, and the rays which pass through this thin medium are refracted by the aqueous humour, which exists in great abundance. Man, and the mammalia, which live on the surface of the earth, hold a middle place between these two extremes.
The inner surface of the choroid coat, which in man is black throughout, is co loured very beautifully on the temporal side of the eye in most quadrupeds, and this part is called the tapetum.
The pigmentum nigrum is entirely de ficient in the eye of the white rabbit, white ferret, &c. as well as in the va riety of the human race called the al bino.
The quadrumana alone possess the foramen centrale of the retina, besides man.
Most mammalia possess a membrana nictitans, or third eyelid, behind which the eyeball can be drawn, when offended by any extraneous matter.
Birds are distinguished by having a bony ring, composed of numerous flat and over-lapping thin plates, in the sub stance of the sclerotica, at its anterior part.
Another great peculiarity consists in the marsuplum or pecten, which ap pears as a large folded process of the choroid, coming through the retina of the back of the eye, and running in the substance of the vitreous humour towards the crystalline lens, which it does not quite reach.