EYE.
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EYE. there arc two very distinct kinds — 'simple' and 'compound' eyes. Simple eyes correspond in structure to a greater or less degree with the eye of man, while compound eyes are of a totallj dif- ferent kind. The latter are found only among arthropods, where thej reach a high degree of development, the so-called compound eyes of - me mollusks and sen-urchins being really much lesss complex. In arthropods the optic nerve bears two noticeable swellings — the optic ganglion, really a part of the brain, and the retinal gang- lion, from which radiates the nerve fibres, enter- ing the retina] cells. The retinal cells aw grouped in clusters of four to seven, known as 'ret inula',' which are more or less heavily pigmented distally. Each retinula is the basal part of a single eye, the upper portion of which consists of a crystal cone (wanting in the eyes of many insects), and of hypodermal elements covered with the chiti- nous cuticle developed as a cornea. These single eyes are crowded together, though separated from each other by pigment-cells, cm a strongly convex basal membrane, thus forming a more or less convex compound eye. Rays of light falling on the eye are absorbed without giving rise to a vis- ual stimulus, except such as are directly parallel to the long axis of the single eyes. Each of these eyes therefore forms an image of that which lies directly before it, and the whole compound eye thus forms a mosaic, probably with sharp out- lines, but wholly lacking perspective. Image-Forming Eyes. Such are first found in the animal kingdom in the group of Cubomedusfe, where, especially in the genus Charybdea, each individual possesses several quite complex eyes provided with lens, vitreous body, and retina. Many worms have very well-formed and often large eyes. It is among mollusks that we find the best-developed eyes among invertebrates. The eyes of dibranchiate cephalopods, as the squid, have a very complex structure, all of the parts essential to good sight in man's eye being pres- ent. There is, however, one very important dif- ference between the cephalopod and the verte- brate eye, and that is that in the former the retinal-cell rods lie inside the limiting mem- brane, and are thus turned toward the light, while in vertebrates these visual rods are turned away from the light. In some mollusks, with much simpler eyes, the retinal-cell rods are turned away from the light, as in vertebrates. Eye in Vertebrates. Passing now to the con- sideration of the vertebrate eye, we find that the structure is in all eases essentially similar to that of the human eye, though many cases of degenerate eyes are known, associated with some peculiarities of habit, as, in the hagfish, with a parasitic mode of life; or, as in the cave salaman- ders and fishes, with living in the dark. (See Cave Animals.) In fishes the eyes have little power of movement, the cornea is very flat, and the lens is globular; the eyes are thus accom- modated, when at rest, for seeing near objects. The sclerotic is frequently calcified or ossified, and there is no ciliary muscle. In amphibians the eyes are somewhat simpler than in fishes, but the ciliary muscle is present as in all higher ertebrates. In both fishes and amphibians we find examples of angular pupils. In reptiles the eye shows slight advance in structure, in respect to some special peculiarities ; thus, in lizards there is a ring of bony sclerotic plates and a curious structure, the 'peeten' (also present in snakes, crocodilians, and especially in birds), the function of which is in dispute, some saying it neernod solely with flic nutrition of the re that it aids in accommodation. In birds i he eyeball is n spherica I (as in other vertebrates), but i- i o that it is much deeper than high. This i, mi In mammals the sclerotic is entirely fibrous, the externa] surface of i he lens i . le I ban the internal, and there i no peeten. The pupil be round., t iM nsvei I, or ha i e the form of a vertical slit. In aquatic mammals the cornea is flattened as in fishes. Bibliography. Lang, Text Book of Gom tive Anatomy (New York, 1896); Wiedersheim, Comparative Anatomy of Vertebrates (New York, 1880) ; Carriere, Die Sehorgane der Thiere (Mun- ich and Leipzig, 1885). EYE, Diseases of the. The disea es of the- eye enumerated by the surgeon are very numi i ous, owing to the variety of the tissues and parts of which the eye is formed. Nearly all its parts are liable to inflammation and its consequences. (See Ophthalmia; Retinitis.) The eyelids are liable to various diseases, as growths of several kinds, most of which the surgeon may rem and inflammation (see Blepharitis ; Stye) ; they may be misdirected inward or outward, entropion and ectropion (qq.v.) : and the upper eyelid may droop (ptosis) from paralysis of the motor oculi nerve, increased weight of the lid, or atrophy or loss of the eyeball. The eyelashes may grow in upon the eye (trichiasis, q.v. ), and produce se- rious results. The duct whose function is to con- vey the tears to the nose is liable to inflammation and obstruction. (See Lachrymal Organs.) The cornea is liable to opacity in various degrees. ( See Cornea.) (For inflammation of the mucous membrane covering the eyeball and eyelids, see Conjunctivitis.) The pupil may be closed as the result of iritis (q.v.), or of operations for cataracts. (For opacities of the crystalline lens, see Cataract. For an account of inflammation of the optic nerve, see Optic Neuritis.) An im- portant disease of the eye is glaucoma (q.v.). Various affections of vision may arise from pe- culiar or altered conditions of refraction, changes in the nerves, or in the action of the muscles moving the eyeball. (See Sight, Defects of.) The parts between the eye and its bony orbit may be the seat of inflammation, abscess, or tumor, making the eye protrude. The movements of the eyeballs may be affected from paralysis of the motor nerves, or from contraction or weak- ness of the muscles, causing squinting. (See Strabismus.) The eye may lose all feeling from paralysis of the fifth pair of nerves. The whole of the same side of the face, nostril, and mouth will be in the same condition, and the eye becomes inflamed and disorganized, because in- juries to the eye by foreign bodies are not noticed. Substances thrown against the eye may injure it. If a caustic alkaline substance has got into the eye, weak vinegar or milk is the best thing to introduce until the physician arrives. If it is oil of vitriol (sulphuric acid) that has been the cause of the injury, a weak solution of soda may be used in the first place to neutralize the acid. In gunpowder explosions near the eye. besides the burn, the particles are driven into the sur- face of it, and will cause permanent bluish stains over the white of the eye, unless they are care- fully removed at the time. When chips of glass,
