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Last reviewed: 20.11.2021
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The eye (oculus; Greek ophthalmos) consists of the eyeball and the optic nerve with its membranes. The eyeball (bulbus oculi) is round, with poles anterior and polus posterior in it. The anterior pole corresponds to the most prominent point of the cornea, the posterior pole is lateral to the exit point from the eyeball of the optic nerve. The line connecting these points is called the external axis of the eyeball (axis bulbi externus). It is approximately 24 mm and is in the plane of the meridian of the eyeball. The inner axis of the eyeball (axis bulbi internus), extending from the posterior surface of the cornea to the retina, is 21.75 mm. In the presence of a longer internal axis, the rays of light after refraction in the eyeball collect in focus in front of the retina. A good vision of objects is possible only at a close distance - nearsightedness, myopia (from the Greek myops - a screwing eye). The focal length of myopic is shorter than the inner axis of the eyeball.
If the inner axis of the eyeball is relatively short, then the light rays after refraction gather in focus behind the retina. Vision in the distance is better than near, it is hyperopia, hypermetropia (from the Greek metron - measure, ops - genus, opos - vision). The focal length of farsighted persons is longer than the length of the inner axis of the eyeball.
The vertical size of the eyeball is 23.5 mm, the transverse dimension is 23.8 mm. These two sizes are in the plane of the equator.
Isolate the optic axis (axis opticus) of the eyeball - the distance from its front pole to the central fossa of the retina - the points of the best vision.
The eyeball consists of membranes that surround the nucleus of the eye (watery moisture in the anterior and posterior chambers, the lens, the vitreous body). There are three shells: external fibrotic, medium vascular and internal photosensitive.
[Fibrous membrane of the eyeball
Fibrous membrane of the eyeball (tunica fibrosa bubi) performs a protective function. The front part of it is transparent and is called the cornea, and the large posterior part due to the whitish color is called the gallbladder, or sclera. The border between the cornea and sclera is a shallow circular scurvy furrow (sulcus sclerae).
The cornea (cornea) is one of the transparent media of the eye and is devoid of blood vessels. It has the form of a watch glass, convex in front and concave behind. The diameter of the cornea is 12 mm, the thickness is about 1 mm. Peripheral margin - the limb of the cornea (hmbus crenella) is inserted into the anterior part of the sclera, into which the cornea passes.
Sclera (sclera) consists of a dense fibrous connective tissue. In the back part of it there are numerous holes through which the bundles of fibers of the optic nerve come out and pass the vessels. The thickness of the sclera at the exit point of the optic nerve is about 1 mm, and in the region of the eyeball's equator and in the anterior section is 0.4-0.6 mm. On the border with the cornea in the thickness of the sclera there is a narrow circular channel filled with venous blood, the venous sinus of the sclera (sinus venosus sclerae), or the helmet canal.
The vascular membrane of the eyeball (tunica vasculosa bulbi oculi) is rich in blood vessels and pigment. It is directly from the inside to the sclera, with which it is firmly fused at the exit point from the optic nerve of the optic nerve and at the border of the sclera with the cornea. In the choroid, three parts are distinguished: the vasculature itself, the ciliary body and the iris.
Actually the choroid
(chroidea) lining the large posterior part of the sclera, which is spliced loose, and restricts from within the so-called circumvascular space (spatium perichoroideale) that exists between the shells.
In the composition of the vascular membrane itself, three layers of lamina are distinguished : supervascular, vascular and vascular-capillary. The supervascular plate is attached to the sclera. It is formed by a loose fibrous connective tissue with a large number of elastic fibers, fibroblasts and pigment cells. The vascular plate consists of intertwining arteries and veins located in a loose fibrous connective tissue. Bundles of smooth myocytes and pigment cells are also present in this plate. The vascular capillary plate is formed by capillaries of different diameter, between which there are flattened fibroblasts.
Between the choroid and the retina there is a so-called basal complex with a thickness of 1-4 μm. The outer (elastic) layer of this complex consists of thin elastic fibers, coming here from the vascular capillary plate. The middle (fibrous) layer of the basal complex is formed predominantly by collagen fibers. The inner layer adjacent to the retina is a basal plate.
The ciliated body (corpus ciliare) is the middle thickened section of the vascular membrane located behind the iris in the form of a circular cushion in the area of the corneal transition into the sclera.
The ciliary body is distinguished by its posterior part - the ciliary circle and the anterior part - the ciliary crown. The cervix (orbiculus ciliaris) has the appearance of a thickened circular strip 4 mm wide, which passes into the vascular wall proper. The anterior part of the ciliary body forms about 70 radially oriented thickened at the ends of the folds of up to 3 mm each - the cilia processus ciliares. These processes consist mainly of blood vessels and form a ciliary crown (corona ciliaris).
From the ciliary processes, freely protruding into the cavity of the posterior chamber of the eye, the connective tissue fibers forming the ciliary band (zonula ciliaris) or the zinnum ligament depart. These fibers are intertwined in the capsule of the lens along its entire circumference. Between the fibers of the ciliary band there are narrow slits filled with watery moisture, emerging from the capillaries of the ciliary processes.
In the thickness of the ciliary body lies the ciliary muscle (m. Ciliaris), consisting of intricately intertwining bundles of smooth muscle cells. With the contraction of the muscle, the accommodation of the eye takes place - an adaptation to a clear vision of objects located at different distances. In the ciliary muscle, meridional, circular and radial bundles of undistorted (smooth) muscle cells are isolated. The meridional (longitudinal) muscle bundles (fibrae meridionales, s. Fibrae longitudinales) of this muscle originate from the edge of the cornea and from the sclera and weave into the anterior part of the vascular membrane proper. With the contraction of these muscle bundles, the vascular membrane shifts anteriorly, as a result, the tension of the ciliary band on which the lens is strengthened decreases. The capsule of the lens at the same time relaxes, the lens changes its curvature, becomes more convex, and its refractive power increases. Circular "fibers" (fibrae circulares), beginning with meridional "fibers", are located inward from the latter in a circular direction. With their contraction, they narrow the ciliary body, bringing it closer to the lens, which also helps relax the lens capsule. Radial "fibers" (fibrae radiales) begin from the cornea and sclera in the region of the iris-corneal angle. These smooth muscle bundles are located between the meridional and circular bundles of the ciliary muscle, bringing their bundles closer together with their contraction. Present in the thickness of the ciliary body, elastic fibers spread the ciliary body while relaxing its muscles.
Iris is the most anterior part of the choroid, visible through a transparent cornea. It has the form of a disk with a thickness of about 0.4 mm, placed in the frontal plane. In the center of the iris there is a round hole - the pupil (pupilla). The diameter of the pupil is unstable.
The inner shell of the eyeball
The inner (sensitive) shell of the eyeball (tunica interna, s. Sensoria bulbi), or the retina, fits snugly from the inside to the choroid along its entire length, from the point of exit of the optic nerve to the edge of the pupil. In the retina developing from the wall of the anterior cerebral bladder, two layers (leaf) are isolated: the outer pigment part (pars pigmentosa), and the intricate internal photosensitive, called the nerve part (pars nervosa). Accordingly, the functions vschlyayut large posterior visual part of the retina (pars optica retinae), containing sensitive elements - rod-shaped and cone-shaped visual cells (rods and cones), and smaller - the "blind" part of the retina, devoid of rods and cones.
The inner part of the eyeball is filled with watery moisture, which is in the anterior and posterior chambers of the eyeball. Together with the cornea, all these formations are light-refracting media of the eyeball. The anterior chamber of the eyeball (camera anterior bulbi) containing watery humor (humor aquosus) is located between the cornea in front and the anterior surface of the iris behind. On the circumference, where the edges of the cornea and iris meet, the chamber is bounded by a comb-like ligament (lig. Pectinatum iridis). Between the bundles of fibers of this ligament there are cracks confined by flat cells - spaces of the iris-angular angle (spatia anguli iridocornealis, fountain spaces). Through these spaces, watery moisture from the anterior chamber flows into the venous sinus of the sclera (sinus venosus sclerae, the helmet canal), and from it enters the anterior ciliary veins.
Through the aperture of the pupil, the anterior chamber communicates with the posterior chamber of the eyeball (camera posterior bulbi), which is located behind the iris and is bounded from behind by the lens. The posterior chamber communicates with the spaces between the fibers of the ciliary band connecting the lens capsule (capsule) with the ciliary body. The spaces of the belt (spatia zonularia) have the appearance of a circular slit (the petite channel) passing along the periphery of the lens. They, as well as the back chamber, are filled with watery moisture, which is formed with the participation of numerous blood vessels and capillaries, lying in the thickness of the ciliated body.
Located behind the eyeballs, the lens has the shape of a biconvex lens, which has a large light refractive power. The front surface of the lens (facies anterior lentis) and the most prominent point of it - the front pole (polus anterior) are facing toward the posterior chamber of the eyeball. The more convex posterior surface (facies posterior) and the posterior pole of the lens (polus posterior lentis) adhere to the anterior surface of the vitreous.
The vitreous body (corpus vitreum), which is covered on the periphery by the membrane, is located in the vitreous bulbi of the eyeball behind the lens, where it is densely attached to the inner surface of the retina. The lens is pressed into the anterior part of the vitreous humor, which in this place has a depression called the fossa hyaloidea. The vitreous humor is a jelly-like mass, transparent, devoid of blood vessels and nerves. The refractive power of the vitreous is close to the refractive index of aqueous humor filling the eye chamber.