For further details regarding these different layers, we must refer to Todd and Bow man's Physiological Anatomy, vol. ii. pp. The choroid coat is a dark-colored vascular membrane, which is brought into view on the removal of the sclerotic. Its outer surface, which is nearly black, is loosely con nected with the sclerotic by connective tissue, in which are contained certain nerves and vessels—termed the ciliary nerves and vessels—which go to the iris. Its inner sur face is soft, villous, and dark-colored. In front, it is attached to the membrane of the vitreous humor by means of the ciliary processes, which consist of about sixty or seventy radiating folds. These are alternately long and short, and each of them is termi tinted by a small free interior extremity; andthey are lodged in corresponding folds in the membrane of the vitreous humor. In other parts, it is loosely connected with the retina. The choroid is composed of minute ramifications of vessels—especially of veins, which, from their whirl-like arrangement, are termed visa con nective tissue, and of pigment cells, which usually approximate to the hexagonal form, and are about , of an inch in diameter. In albinos, this pigment is absent, and hence their eyes have a pink appearance, which is due to the unconcealed blood in the capillaries of the choroid and iris.
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 membranes. It is a thin flat mem branous 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) chamber, 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 center of the iris, and varies in size according to the action of the muscular fibers of the iris, so as to admit more or less light into the interior of the eye ball; its diameter varying, under these circumstances, from about id of an inch. It is muscular in its structure, one set of fibers being arranged circularly round the pupil, and, when necessary, effecting its contraction, while another set lie in a radiat ing direction from within outwards, and by their action dilate the pupil. These fibers are of the unstriped or, involuntary variety. The nerves which are coilerned in these movements will be presently noticed.
The varieties of color in the eyes of different individuals, and of different kinds of animals, mainly 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, although continuous with the optic nerve— of which it is usually regarded as a cuplike expansion—differs very materially from it. in structure. Before noticing the elaborate composition of this part of the eye, which has only been revealed by recent microscopical investigation, we shall briefly mention those points regarding it which can be established by ordinary examination. It is delicate semi-trausparent sheet of nervous matter, lying immediately behind the vitre ous humor, and extending from the optic nerve nearly as far as the lens. On examin ing 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 (limbus luteus), of about Ath of an inch in diameter, called, after its discoverer, the yellow spot of Sommeriny. 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 our most eminent English micro scopists, after several examinations, regard it as a small mound or projection of the retina towards the vitreous humor, with a minute aperture in the summit. The only mammals in which it exists are man and the monkey. Its use is unknown, but vision is remarkably perfect at this spot—a circumstance which, however, may possibly be accounted for by the fact, that it is singularly 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) about of an inch, and as it extends anteriorly, it soon diminishes to of an inch, the following layers from without inwards may be distinguished in all parts of it: 1. The layer of rods and cones, frequently termed, from its discoverer, the membrane of Jacob; 2. The granular layer; 3. The layer of gray nerve substance; 4. The expansion of the optic nerve; and, 5. The limitary membrane. Details regarding the nature of these various layers are given in Milker's Manual of Human histology, and in Todd and Bowman, op. cit.
It now remains for us to describe the transparent media which occupy the interior of the globe, and through which the rays of light must pass before they can reach the retina, and form on it the images of external objects. We shall consider them in the order in which the rays of light strike them.