The eye, or organ of vision , consists of the eyeball, optic nerve and auxiliary organs (eyelids, lacrimal apparatus, eyeball muscles.
The eyeball is an almost spherical formation with a diameter of about 24 mm, located in the eye socket . It has active mobility as a result of the activity of six eye muscles - four straight (upper, lower, inner and outer) and two oblique (upper and lower). The eyeball is separated from the rest of the orbit by a dense fibrous sheath - the tenon capsule, behind which is the fatty tissue.
The wall of the eyeball consists of three shells: the outer - very dense fibrous sheath, consisting of the cornea and the sclera; middle, vascular, and internal - retinal membrane (retina). Inside the eyeball are the lens and the vitreous body. The chambers of the eyeball are filled with aqueous humor — aqueous humor.
1 - sclera;
2 - choroid;
3 - mesh sheath;
4 - optic nerve;
5 - posterior long ciliary artery ;
6 - vorticose vein;
7 - lower rectus muscle;
8 - iris;
9 - cornea;
10 - conjunctiva;
11 - the lens;
12 - ciliary body;
13 - upper rectus muscle.)
The sclera, consisting of dense collagen fibers, is opaque, poor in blood vessels. The front part of the sclera is covered with conjunctiva. At the interface of the sclera with the transparent cornea there is a shallow groove about 1 mm wide, the so-called limb. The cornea is a clear, avascular formation, optically acting like a strong convex glass. It has a very high sensitivity due to the large number of nerve endings, located mainly in its surface layers.
The choroid, or uveal tract, consists of the iris, ciliary, or ciliary, body and the choroid itself. The iris, or iris, is the most anterior part of the choroid. In the center of the iris there is a round hole - the pupil, through which the rays of light penetrate into the eyeball and reach the retina. Depending on the intensity of the luminous flux, the pupil is able to change its size: in bright light it is narrower, in weak light and in darkness it is wider. The size of the pupil changes as a result of the interaction of smooth muscle fibers - the sphincter and dilator, enclosed in the iris and innervated by the parasympathetic and sympathetic nerves. In a number of diseases, pupil dilation occurs — mydriasis or contraction — miosis (see pupillary reflexes ). The iris contains a different amount of pigment, on which its color (eye color) depends. Behind the iris is located the ciliary (ciliary) body, which includes the ciliary, or accommodative, muscle (see Accommodation of the eye ). On the inner surface of the ciliary (ciliary) body, in the front third of it, are located the ciliary processes. They attach the fibers of the ciliary girdle (cinnamon ligaments ), on which the lens is suspended. The ciliary body produces intraocular fluid. Posteriorly, the ciliary body passes into the choroid proper (choroid), which consists mainly of vessels of various calibers. Numerous pigment-rich choroidal cells prevent the penetration of light through the sclera, eliminating light scattering.
The innermost shell of the eyeball - the retina consists of highly differentiated nerve elements. Microscopically, it distinguishes 10 layers. The outermost layer is light-perceiving, it faces the choroid and consists of neuroepithelial cells - rods and cones that perceive light and colors. The following layers are formed by nerve-conductive cells and nerve fibers. Nerve fibers form the optic nerve. The area of highest vision in the retina is the so-called yellow spot with a central fossa containing only cones.
Strongly refracting the rays of light transparent elastic lens has the shape of a biconvex lens. The lens has no vessels and nerves. The absence of the lens in the eye [after cataract removal (see) or resorption of the injured lens] is called aphakia.
The vitreous body, which fills most of the cavity of the eyeball, is a transparent gelatinous mass consisting of thin, delicate fibrils and containing up to 99% of water. The vitreous body also refracts the rays of light.
In the cavity of the eyeball, a small space bounded by the posterior surface of the cornea, the anterior surface of the iris and the central part of the anterior surface of the lens is called the anterior chamber of the eye; it is made of clear watery moisture. The periphery of the anterior chamber of the eye, the so-called angle of the anterior chamber, is important in the circulation of intraocular fluid. The space bounded by the posterior surface of the iris, the peripheral part of the lens and the inner surface of the ciliary body is called the posterior chamber; it is also made of watery moisture, which is the source of nutrition for the avascular tissues of the eye - the cornea, lens and vitreous body.
Conjunctiva - the connective (mucous) membrane of the eye in the form of a thin transparent film covers the back surface of the eyelids and the front of the eyeball over the sclera to the cornea. When the eyelids are open, the conjunctiva forms a sort of bag with a wide slit - the eye slit. At the inner corner of the eye is the lacrimal caruncle. Conjunctiva does not interfere with the mobility of the eyeball, transparent, smooth; in the area of the cartilage, the meibomian glands shine through it. Possessing a rich neurovascular apparatus, the conjunctiva reacts to any irritations (conjunctival reflex).
Fig. 1. Stratified squamous epithelium of the conjunctiva of the eyeball at the limbus.
Fig. 2. Multilayer cylindrical epithelium of the conjunctiva of the transitional fold with goblet cells.
Fig. 3. Conjunctiva cartilage.