The choroid coot 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. vinous, and dark-eolored. 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 flue retina. The ehoroid is composed of minute ramifications of vessels—especially of veins which, from their whorl-like arrangement, are termed vases vortieosa—of connective tissue, and of pigment-eells, 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 cornea 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 cireu rly round the pupil, and, when necessary, effect ing its contraction; while :mother set lies in a radiating direction from within outward, and by its action dilates the puifil. These fibres are of the unstriped and involuntary variety. The nerves which are concerned in these movements will he presently noticed.
The ciliary muscle is a thin band or ring of inm-striated muscular fibres which lies between the iris and the choroid. Its posterior attachment
is to the anterior margin of the choroid, while anteriorly it is attached by an annular ligament to the outer margin of the iris and to the adjacent portions of the cornea and sclera. liy the con traction of this muscle the ehoroid is drawn for ward, the suspensory ligament of the lens is re laxed, and accommodation of the eye is effected.
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 ehoroid 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 eve, 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. ()n 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 S6mmering,' 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 Air. 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 of an inch. and as it extends anteriorly it soon diminishes to ,,h of an inch, the following layers from without in ward may he distingnished in all parts of it: (1) The layer of rods .and cones, frequently termed, from its discoverer, the membrane of Jacob; (2) the external limiting membrane; (3) the outer nuclear layer; (4) the outer molecular layer: (5) the inner nuclear layer; (6) the inner molecular layer; (7) the layer of ganglion cells; (8) the layer of nerve-fibres; (9) the in ternal limiting membrane.