Beschreibung:

A vertical cut of the left human eye with its muscles and nerves in their natural positions (Significantly magnified) 1. The first or exterior layer of skin of the eyeball, the white sclera, is a tough, white, nontransparent skin of fibrous tissue, with few blood vessels and nerves. The optic nerve penetrates at the back, and the eye muscles are attached at the front. 2. The cornea is a cartilaginous, transparent skin with very few blood vessels, veined with numerous fluid canals. It forms the window of the eye, as it were, and allows rays of light to enter the interior. It is firmly connected with the sclera and rises up from it as a separate small segment; it is externally covered by the conjunctiva. 3. The choroid is the central layer of skin, very thin and containing a large number of blood vessels; it is also penetrated by the optic nerve at the back, to which it is connected by thin threads. At the front, it is connected to the transitional points of sclera and cornea by the gray, circular fibres of the ciliary muscle. 4. Ciliary muscle or choroid. The frontal section diverts from the sclera towards the axis and forms the so-called ciliary processes, of which there are between 70 and 80. In their totality, they form a regularly shaped ring of folds whose peaks reach the margin of the iris. Its interior surface is coated with a thick layer of black pigment, which belongs to the retina. The entire anterior part with the ciliary muscle, which is situated to the left of the lens, is called the ciliary body. 5. The iris is attached at its outer margin to the ciliary body and the cornea. The junction forms a canal (Schlemm's canal) that is connected by fine crevices to the anterior chamber. The iris is vertically suspended behind the cornea and in front of the lens, like a disk with a hole in the center (the pupil). When viewed through the cornea from the outside, the iris appears as a colored ring (brown, yellowish, with bright rays) surrounding the pupil. Its posterior part, which is deep black, is called the uvea. The space in front of the iris is the anterior chamber, and the space behind it, i.e., before the lens, the posterior chamber of the eye. Both chambers are filled with the aqueous humor. The third or innermost layer of skin is formed by the membrane that is most important for vision, the retina (6). It is the skin-like extension of the optic nerve (7) and terminates at the ora serrata, and consists of ten complexly arranged layers; its function is to transport the image we see via the optic nerve to the brain. The light-measuring apparatus, which fills the hollow space at the core of the eyeball, surrounded by the described three layers of skin, is formed by completely transparent, partly solid and partly liquid objects and substances: the aqueous humor of the anterior and posterior chambers, the crystalline lens (8), and the vitreous body (9). The aqueous humor contained in the chambers of the eye (they are represented here as hollow cavities) is a colorless, thin liquid (lymphatic fluid emitted by the iris and the ciliary body). The lens (8), contained entirely in the very elastic lens capsule, resembles a strongly convex magnifying glass. It sits in a dent of the vitreous body, close behind the iris, and is surrounded by the ciliary body and the ora serrata. The lens mass, made up of fibrous cells, has the consistency of a solid, elastic jelly, and takes on a yellowish, cloudy hue with old age. Its curvature, especially that of its anterior side, can be increased by the activity of the choroid's ciliary muscle (4). The vitreous body (9), which forms the shape of a clear ball, fills the space behind the lens, which is surrounded by the retina, and contains the lens, to which it is connected by the zonular fibers of Zinn, in a dish-shaped dent in its anterior side. Through the center of the vitreous body runs a canal that contains the central retinal artery and vein (Hyaloid canal). The vitreous body consists of