The eye is frequently destroyed by accident or disease; in cases of removal of the organ artificial eyes are used to remedy the deformity; these are made of glass and enamel, and when having the natural size, shape, coloration of iris, form of pupil, projection of cornea, tint of sclerotic, and vascularity, it is often very difficult to detect the real from the artificial organ, especially when the accurate fitting of the latter allows it to be moved by the muscles acting in sympathy with the sound eye.-Without here treating of the laws of refraction, of the aberration of sphericity, and of other optical principles involved in vision, it will be sufficient to say that the rays from an object are first modified by the convex cornea, pass across the aqueous humor through the pupil-opening of the iris, thence through the dense crystalline lens and the vitreous humor, and are by these media of different densities and shapes converged at the proper focal distance on the retina. All rays beyond those necessary for perfect vision are absorbed by the pigment layer of the choroid, which answers the purpose of the black interior of optical instruments; the iri3, like the telescopic diaphragm, shuts off the rays from the circumference of the lens, thus correcting the aberration of sphericity, contracting or dilating the pupil according to the brilliancy or dimness of the illumination of the object, or its distance from the eye; it is well known that the pupil of a cat in a bright light becomes diminished to a vertical slit.

As the rays are crossed in the lens, an inverted image is formed on the retina, though the mental perception is of an erect image. Not only spherical but chromatic aberration is corrected sufficiently for all practical purposes in healthy eyes by the different refractive powers of the media and by the different curves of their surfaces, so that the image on the retina is well defined and free from false colors. The power by which the eye adapts itself instantly to variations in the distance of objects depends upon a change in the curvatures of the crystalline lens, this body becoming more convex, and consequently more highly refractive, in vision for near objects, less so in vision for remote objects. The physiology and defects of vision will be more properly treated in the article Vision; for recent observations by Kolliker on the structure of the different layers of the retina, the reader is referred to the works of Dr. Carpenter on the principles of human and comparative physiology. The pupil is diminished by the action of muscles deriving their nervous influence from the third pair, but is dilated through the influence of the cervical portion of the sympathetic nerve.

The movements of the eyeballs, whenever voluntary, are always harmonious, but not necessarily symmetrical; though one cannot be elevated and the other depressed at the same time, one may be turned outward I and the other inward when the axes of the eyes are turned toward an object on either side of the head. The muscles of the eyeball are moved principally through the third pair of nerves, the motores oculorum, but the superior oblique has a special nerve, the fourth pair, and the external recti the sixth pair; the sensibility of the eye is derived from the ophthalmic branch of the fifth pair; by the ophthalmic or ciliary ganglion the sensory branches of the fifth pair, the motor branches of the third pair, and the sympathetic filaments are united together. The vascular supply of the globe of the eye is derived from the ophthalmic branch of the internal carotid artery.-The complicated eye of the mammal and bird becomes more simple in reptiles and fishes, losing the eyelids, and in the articulates generally losing all that is anterior to the vertebrate crystalline lens, as well as mobility, the latter loss being supplied by the multiplication of the organs or facets.

The mammalian eye is constructed to suit the circumstances of the life of the animal; of large size in ruminants and rodents, it is small in moles, bats, and cetaceans, and in the latter flattened anteriorly as in fishes. The eyes are generally placed laterally, but in the nocturnal species they are directed forward as in man; the lachrymal caruncle at the inner angle has in man only a rudiment of a nictitating membrane, which is more developed in some mammals, but remarkably in birds; the sclerotic is thicker in animals whose eyes vary much from a sphere, especially posteriorly, this membrane in a whale with an eye of the size of an orange being an inch thick behind; the choroid, dark in man, in the carnivora, ruminants, and other orders, reflects vivid metallic colors, remarkably brilliant at night, from the depth of the organ. In animals and man destitute of the usual coloring matter of the surface, or in albinos, the iris is pink, from the color of the blood circulating in its vessels; during foetal life, until the end of the seventh month, the pupil is closed by a membrane.

The foramen of

Fig. 1.-Horizontal Section of the Eyeball.

Scl., sclerotic coat; Cn.. cornea: 7?.. attachments of the tendons of the recti muscles; Ch„ choroid; C.p.. ciliary processes: Cm., ciliary muscle; Jr., iris; Aq„ aqueous humor; Cry., crystalline lens; Vt.. vitreous humor; Rt, retina; Op., optic nerve; M.I., the yellow spot. The section has passed through a ciliary process on the left side, and between two ciliary processes on the right.

Fig. 2.-Muscles of the Eyeball viewed from above and from the inner side.

S. R., superior rectus; Inf. It., inferior rectus; E. R., external rectus; In. R., internal rectus; S ob., superior oblique; Inf. ob., inferior oblique; Ch., chiasma of the optic nerves (II.); III., the third nerve, which supplies all the muscles except the superior oblique and the external rectus.