EYE 81 EYE EYE, the organ of sight. The principle on which the human eye is constructed is that of the camera obscura, a dark chamber with a small opening for the admission of light, a quantity of black matter for the absorption of superabun- dant rays, and a nervous expansion on that wall which receives the rays of light. For protection it is deeply sunk EYE — CROSS-SECTION OF HUMAN EYE A. Optic nerve B. Retina C. Choroid D. Sclera E. Vitreous humor F. Lens G. Aqueous humor H. Cornea I. Iris J. Fovea centralis K. Arteria centrall retinae L. Orbital fat M. Levator palpebrae superioris muscle N. Rectus superior muscle O. Rectus Inferior P. Eyelids Q. Eyelashes in a fatty cushion within a bone cavity. The human eye is nearly globular, but the anterior part formed by the cornea is part of a smaller sphere, and slightly protuberant, in the proportion of 20 to 19. In the globe itself the chief consti- tuents are: (1) The retina, the expan- sion of the optic nerve; (2) the trans- parent refracting media (the vitreous body or humor, the crystalline lens, the aqueous humor, the iris, and the pupil) ; (3) the tunica sclerotica, forming a dense tunic inclosing the first two. It is opaque except in front, where it becomes (4) the cornea, perfectly transparent, to allow the light to enter (5) the choroid membrane, lying between the retina and sclerotica, and containing a layer of dark pigment. The vitreous humor is imme- diately within the cup formed by the retina, and gives the support ^ inside which the sclerotica does outside; it forms four-fifths of the whole globe, and its perfect fluidity allows for the expan- sion and contraction of the pupil and of the lens itself to or from the cornea. The crystalline lens is divided into three equal parts by three lines, which radiate from the center to one-third of the surface; each one of these layers consists of hun- dreds of concentric layers, connected by finely serrated edges. This beautiful dove-tailing of fibers is not peculiar to man; the best example is the lens of the common codfish. The eyes of the vertebrata are essen- tially like those of man. The eyes of insects are of two kinds: compound eyes and simple eyes or stemmata. The com- pound eyes are immovable. They consist of vastly numerous lenses; thus in the dragon-fly there are 12,000. Spiders have compound eyes; the higher members of the class have ocelli; many of the lower parasitic species are blind. The eyes of Crustacea vary greatly, from a sessile median eye-speck to two distinct eyes placed on movable peduncles. The centipedes have many simple eyes; in lulus these are so near as almost to make two compound eyes. Of mollusks, the cephalopoda have large eyes, the gaster- opoda possess them, as do the pectens among the conchifera, though in most other genera of the class, and in brachi- opoda, they seem wanting. The animals of lower organization are destitute of eyes. hi Architecture. — (1) The circular aperture in the top of a dome or cupola. (2) The circle in the center of a volute scroll. (3) A circular or oval window. In Nautical Parlance. (1) A circular loop in a shroud or rope. A worked circle or grommet in a hank, rope, or sail. (2) The loop of a block-strap. (3) The hole in the shank of an anchor to receive the EYE — CROSS-SECTION OF HUMAN EYE A. Lachrymal gland C. Nasal sac B. Lachrymal canal D. Nasal duct ring. The foremost part of the bows of a vessel, on which formerly eyes used to be painted. The term is also applied to the hawse-holes. The strands of a rope's end opened and divided into two parts and laid over each other, marled, par- celed, and sewed together, and so form- ing an eye, is called a Flemish eye.
