Page:The New International Encyclopædia 1st ed. v. 07.djvu/443

* EYE. 395 EYE. lv distinguishable from one another — viz. (pro- ceeding from the front backward): (1) The conjunctiva] layer of epithelium. It is in this epithelium that particles of iron, stone, etc., for- cibly driven against the eye usually lodge, and it is a highly sensitive membrane. (2) The an- terior elastic lamina forming the anterior boun- dary of the cornea proper; it is not more than one two-thousandth of an inch in thickness, and its function seems to be that of maintaining the exact curvature of the front of the cornea. (3) The cornea proper, on which the thickness and strength of the cornea mainly depend. (4) The posterior elastic lamina, which is an extremely thin membrane ; in which no structure can be detected. It probably contributes, like the an- terior lamina, to the exact maintenance of the cilrvafurc of the cornea, so necessary for correct vision. (5) The posterior endothelium of the aqueous humor, which is probably concerned in the secretion of that fluid. For further details regarding these different layers, the reader is re- ferred to Prudden, A Manual of Practical Normal Histology (New York, 1886). The choroid coat is a dark-colored vascular membrane, which is brought into view on the re- moval of the sclerotic. Its outer surface, which is nearly black, is loosely connected with the sclerotic by connective tissue, in which are con- tained certain nerves and vessels (termed the ciliary nerves and vessels) which go to the iris. Its inner surface is soft, villous, and dark-colored. In front it is attached to the membrane of the vitreous humor by means of the ciliary pro- cesses, which consist of about sixty or seventy radiating folds. These are alternately long and short, and each of them is terminated by a small, free, interior extremity, and they are lodged in corresponding folds in the membrane of the vitre- ous humor. In other parts it is loosely connect- ed with the retina. The choroid is composed of minute ramifications of vessels — especially of veins which, from their whorl-like arrangement, are termed vasa vorticosa — of connective tissue, and of pigment-cells, which usually approximate to the hexagonal form and are about one one- thousandth of an inch in diameter. In albinos this pigment is absent, and hence their eyes have a pink appearance, which is due to the uncon- cealed blood in the capillaries of the choroid and iris. See Albino. The iris may be regarded as a process of the choroid, with which it is continuous, although there are differences of structure in the two mem- branes. It is a thin, flat, membranous curtain, hanging vertically in the aqueous humor in front of the lens, and perforated by the pupil for the transmission of light. It divides the space between the eornea and the lens into an anterior (the larger) and a posterior (the smaller) cham- ber, these two chambers freely communicating through the pupil. The outer and larger border is attached all round to the line of junction of the sclerotic and cornea, while the inner edge forms the boundary of the pupil, which is nearly circular, lies a little to the inner side of the centre of the iris, and varies in size according to the action of the muscular fibres of the iris, so as to admit more or less light into the interior of the eyeball, its diameter varying, under these circumstances, from about one-third to one- twentieth of an inch. It is muscular in its structure, one set of fibres being arranged circu- Voi.. VII.- 26. hnlv round the pupil, and, when nerr :n. elfect- ing its contraction; while another set lies in a radiating direction from within outward, and by its action dilates the pupil. These fibres are of the unstriped and involuntary variety. The nerves which are concerned in these movements will be presenl Ij noticed. The ciliary muscle is a thin band or ring of non-striated muscular fibres which lies between the iris and the choroid, [ts posterior al tachment is to the anterior margin of the choroid, while anteriorly it is attached bj an annular ligament to the outer margin of the iris and to the ad ja cent portions of the cornea and sclera. By the con- traction of this muscle the choroid is drawn for- ward, the suspensory ligament of the lens is re- laxed, and accommodation of the eye is effected. DIAGRAM ILLUSTRATING THE MECHANISM OF ACCOMMODA- TION OF THE EVE. In B the lena is accommodated for near objects ; in A It is accommodated for objects at a distance. The varieties of color in the eyes of different in- dividuals, and of different kinds of animals, main- ly depend upon the color of the pigment which is deposited in cells in the substance of the iris. Within the choroid is the retina, which, al- though continuous with the optic nerve — of which it is usually regarded as a cup-like expansion — differs very materially from it in structure. Be- fore noticing the elaborate microscopical struc- ture of this part of the eye, we shall briefly men- tion those points regarding it which can be es- tablished by ordinary examination. It is a deli- cate semi-transparent sheet of nervous matter, lying immediately behind the vitreous humor, and extending from the optic nerve nearly as far as the lens. On examining the concave inner surface of the retina at the back of the eye, we observe, directly in a line with the axis of the globe, a circular yellow spot called 'mac- ula lutea,' and known, after its discoverer, as 'the yellow spot of Sommering,' of about one- twentieth of an inch in diameter. As there has been much discussion regarding the structure and function of this spot, we may observe that Dr. Todd and Mr. Bowman, two of the most eminent English microscopists, after several ex- aminations, regard it as a small mound or pro- jection of the retina toward the vitreous humor, with a minute depression in the summit. The only mammals in which it exists are man and the monkey. It is the area of most distinct vision — a circumstance which may partly be ac- counted for by the fact that it is singu- larly free from blood-vessels, which curve round it and apparently avoid it. The structure of the retina, as revealed by the microscope, is in the highest degree remarkable. Although its greatest thickness (at the entrance of the optic nerve) is only about ,L of an inch, and as it extends anteriorly it soon diminishes to ^y of an inch, the following layers from without in- ward may be distinguished in all parts of it: ( 1 ) The layer of rods iind cones, frequently termed, from its discoverer, the membrane of