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Eye
Medical expert of the article
Last reviewed: 04.07.2025
The eye (oculus; Greek ophthalmos) consists of the eyeball and the optic nerve with its membranes. The eyeball (bulbus oculi) is round, it has poles - anterior and posterior (polus anterior et polus posterior). The anterior pole corresponds to the most protruding point of the cornea, the posterior pole is located lateral to the place where the optic nerve exits the eyeball. The line connecting these points is called the external axis of the eyeball (axis bulbi externus). It is approximately 24 mm and is located in the plane of the meridian of the eyeball. The internal axis of the eyeball (axis bulbi internus), passing from the back surface of the cornea to the retina, is 21.75 mm. With a longer internal axis, the rays of light after refraction in the eyeball are collected in focus in front of the retina. Good vision of objects is possible only at a close distance - myopia (from the Greek myops - squinting eye). The focal length of the nearsighted is shorter than the inner axis of the eyeball.
If the inner axis of the eyeball is relatively short, then the rays of light after refraction are collected in focus behind the retina. In this case, vision into the distance is better than near vision - this is farsightedness, hypermetropia (from the Greek metron - measure, ops - genus, opos - vision). The focal length of farsighted people is greater than the length of the inner axis of the eyeball.
The vertical size of the eyeball is 23.5 mm, the transverse size is 23.8 mm. These two dimensions are in the plane of the equator.
The visual axis (axis opticus) of the eyeball is distinguished - the distance from its anterior pole to the central fovea of the retina - the point of best vision.
The eyeball consists of membranes that surround the core of the eye (aqueous humor in the anterior and posterior chambers, the lens, and the vitreous body). There are three membranes: the outer fibrous membrane, the middle vascular membrane, and the inner photosensitive membrane.
Fibrous membrane of the eyeball
The fibrous membrane of the eyeball (tunica fibrosa bubi) performs a protective function. Its front part is transparent and is called the cornea, and the large back part, due to its whitish color, is called the white membrane, or sclera. The border between the cornea and the sclera is a shallow circular groove of the sclera (sulcus sclerae).
The cornea is one of the transparent media of the eye and is devoid of blood vessels. It has the appearance of a watch glass, convex in front and concave in the back. The diameter of the cornea is 12 mm, the thickness is about 1 mm. The peripheral edge - the limbus of the cornea (hmbus sclerae) is inserted into the anterior part of the sclera, into which the cornea passes.
The sclera consists of dense fibrous connective tissue. In its posterior part there are numerous openings through which bundles of optic nerve fibers emerge and vessels pass. The thickness of the sclera at the point where the optic nerve emerges is about 1 mm, and in the area of the equator of the eyeball and in the anterior section - 0.4-0.6 mm. At the border with the cornea in the thickness of the sclera lies a narrow circular canal filled with venous blood - the venous sinus of the sclera (sinus venosus sclerae), or Schlemm's canal.
The vascular tunic of the eyeball (tunica vasculosa bulbi oculi) is rich in blood vessels and pigment. It is directly adjacent to the sclera on the inside, with which it is firmly fused at the point where the optic nerve exits the eyeball and at the border of the sclera with the cornea. Three parts are distinguished in the vascular tunic: the vascular tunic proper, the ciliary body, and the iris.
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The choroid proper
(chroidea) lines the large posterior part of the sclera, with which it is loosely fused, and limits from the inside the so-called perivascular space (spatium perichoroideale) that exists between the membranes.
The choroid proper is composed of three layers of the plate: supravascular, vascular, and vascular-capillary. The supravascular plate is adjacent to the sclera. It is formed by loose fibrous connective tissue with a large number of elastic fibers, fibroblasts, and pigment cells. The vascular plate consists of intertwined arteries and veins located in loose fibrous connective tissue. This plate also contains bundles of smooth myocytes and pigment cells. The vascular-capillary plate is formed by capillaries of different diameters, between which flattened fibroblasts are found.
Between the choroid and the retina there is the so-called basal complex, 1-4 µm thick. 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 mainly by collagen fibers. The inner layer, adjacent to the retina, is the basal plate.
The ciliary body (corpus ciliare) is the middle thickened section of the vascular membrane, located behind the iris in the form of a circular ridge in the area where the cornea transitions to the sclera.
The ciliary body has a posterior part, the ciliary circle, and an anterior part, the ciliary crown. The ciliary circle (orbiculus ciliaris) looks like a thickened circular strip 4 mm wide, which passes into the vascular tunic proper. The anterior part of the ciliary body forms about 70 radially oriented folds up to 3 mm long, thickened at the ends, each - ciliary processes (processus ciliares). These processes consist mainly of blood vessels and form the ciliary crown (corona ciliaris).
Connective tissue fibers extend from the ciliary processes, which protrude freely into the cavity of the posterior chamber of the eye, forming the ciliary belt (zonula ciliaris), or Zinn's ligament. These fibers are woven into the lens capsule along its entire circumference. Between the fibers of the ciliary belt there are narrow slits filled with aqueous humor secreted from the capillaries of the ciliary processes.
The ciliary muscle (m. ciliaris) is located in the thickness of the ciliary body. It consists of complexly intertwined bundles of smooth muscle cells. When the muscle contracts, the eye accommodates - adapts to clearly seeing objects at different distances. The ciliary muscle has meridional, circular and radial bundles of unstriated (smooth) muscle cells. The meridional (longitudinal) muscle bundles - "fibers" (fibrae meridionales, s. fibrae longitudinales) of this muscle originate from the edge of the cornea and from the sclera and are woven into the anterior part of the choroid. When these muscle bundles contract, the choroid shifts forward, resulting in a decrease in the tension of the ciliary belt, on which the lens is attached. The lens capsule relaxes, the lens changes its curvature, becomes more convex, and its refractive power increases. The circular "fibers" (fibrae circulares), which begin together with the meridional "fibers", are located medially from the latter in the circular direction. When they contract, they narrow the ciliary body, bringing it closer to the lens, which also helps to relax the lens capsule. The radial "fibers" (fibrae radiales) begin from the cornea and sclera in the area of the iridocorneal angle. These smooth muscle bundles are located between the meridional and circular bundles of the ciliary muscle, bringing their bundles closer together when they contract. The elastic fibers present in the thickness of the ciliary body straighten the ciliary body when its muscle relaxes.
The iris is the most anterior part of the vascular tunic, visible through the transparent cornea. It has the appearance of a disk about 0.4 mm thick, placed in the frontal plane. In the center of the iris there is a round opening - the pupil (рupilla). The diameter of the pupil is not constant.
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The inner lining of the eyeball
The inner (sensitive) membrane of the eyeball (tunica interna, s. sensoria bulbi), or retina, is tightly adjacent to the choroid on the inner side along its entire length - from the exit of the optic nerve to the edge of the pupil. In the retina, developing from the wall of the anterior cerebral vesicle, two layers (sheets) are distinguished: the outer pigment part (pars pigmentosa), and the complexly structured inner light-sensitive part, called the nervous part (pars nervosa). Accordingly, the functions distinguish the larger posterior visual part of the retina (pars optica retinae), containing sensitive elements - rod-shaped and cone-shaped visual cells (rods and cones), and the smaller - "blind" part of the retina, devoid of rods and cones.
The interior of the eyeball is filled with aqueous humor, which is located in the anterior and posterior chambers of the eyeball. Together with the cornea, all these structures are the light-refracting media of the eyeball. The anterior chamber of the eyeball (camera anterior bulbi), containing aqueous humor (humor aquosus), is located between the cornea in front and the anterior surface of the iris in the back. Along the circumference, where the edges of the cornea and iris converge, the chamber is limited by the pectineal ligament (lig. pectinatum iridis). Between the bundles of fibers of this ligament are slits limited by flat cells - the spaces of the iridocorneal angle (spatia anguli iridocornealis, fountain spaces). Through these spaces, aqueous humor from the anterior chamber flows into the venous sinus of the sclera (sinus venosus sclerae, Schlemm's canal), and from there it enters the anterior ciliary veins.
Through the pupillary opening, the anterior chamber communicates with the posterior chamber of the eyeball (camera posterior bulbi), which is located behind the iris and is limited at the back by the lens. The posterior chamber communicates with the spaces between the fibers of the ciliary zonule, which connect the bag (capsule) of the lens with the ciliary body. The spaces of the zonule (spatia zonularia) have the appearance of a circular slit (Petit's canal), passing along the periphery of the lens. They, like the posterior chamber, are filled with aqueous humor, which is formed with the participation of numerous blood vessels and capillaries located in the thickness of the ciliary body.
The crystalline lens, located behind the chambers of the eyeball, has the shape of a biconvex lens with high light-refracting power. The anterior surface of the lens (facies anterior lentis) and its most protruding point, the anterior pole (polus anterior), face the posterior chamber of the eyeball. The more convex posterior surface (facies posterior) and the posterior pole of the lens (polus posterior lentis) are adjacent to the anterior surface of the vitreous body.
The vitreous body (corpus vitreum), covered at the periphery by a membrane, is located in the vitreous chamber of the eyeball (camera vitrea bulbi) behind the lens, where it is tightly adjacent to the inner surface of the retina. The lens is as if pressed into the anterior part of the vitreous body, which in this place has a depression called the vitreous pit (fossa hyaloidea). The vitreous body is a jelly-like mass, transparent, devoid of vessels and nerves. The refractive power of the vitreous body is close to the refractive index of the aqueous humor filling the chambers of the eye.
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