Page:The American Cyclopædia (1879) Volume VII.djvu/55

 EYE when the accurate fitting of the latter allows it to be moved by the muscles acting in sym- pathy with the sound eye. Without here treat- ing 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 raya 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 inte- rior of optical instruments; the iris, 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 tho retina, though the mental perception is of an erect image. Not only spherical but FIG. 3. Illustration of the change in the form of the lens when adjusted a to distant, b to near objects. 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 physiolo- gy and defects of vision will be more properly treated in the article VISION ; for recent obser- vations by Kolliker on the structure of the dif- ferent layers of the retina, the reader is re- ferred to the works of Dr. Carpenter on the principles of human and comparative physiol- ogy. The pupil is diminished by the action >f muscles deriving their nervous influence from the third pair, but is dilated through the influence of the cervical portion of the sympa- thetic 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 311 VOL. vii. 4 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 sen- sibility of the eye is derived from the ophthal- mic branch of the fifth pair; by the ophthal- mic 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 be- comes 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 con- structed to suit the circumstances of the life of the animal ; of large size in ruminants and rodents, it is small in moles, bats, and ceta- ceans, and in the latter flattened anteriorly as in fishes. The eyes are generally placed later- ally, but in the nocturnal species they are di- rected forward as in man ; the lachrymal ca- runcle at the inner angle has in man only a rudiment of a nictitating membrane, which is more developed in some mammals, but re- markably 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, remark- ably brilliant at night, from the depth of the organ. In animals and man destitute of the usual coloring matter of the surface, or in albi- nos, 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 Sommering is said not to exist in any mam- mals below the quadrumana; the tear gland is found in all except cetacea. In birds the sclerotic becomes more or less strengthened by cartilage, and in the neighborhood of the cornea is provided with a series of bony plates, arranged in a circle, and overlapping each other ; but the chief peculiarity consists in the pecten, folded like a comb or fan, and projected forward toward the lens ; it is vascular like the choroid, though not connected with it, and is dark with pigment ; its use is not satisfac- torily ascertained. Many species of reptiles have osseous pieces in the sclerotic ; snakes have no movable lids; the chameleon has a single circular lid. In fishes the eyes are gen- erally large, the sclerotic thick, and in some (as the tunny) osseous anteriorly; they have neither lids, except the most rudimentary, nor lachrymal glands ; the cornea is very flat, and