There are 2 poles in the eyeball: posterior and anterior. The distance between them averages 24 mm. It is the largest eyeball. The bulk of the latter is the inner core. This is transparent content that is surrounded by three shells. It consists of aqueous humor, the lens and the vitreous. On all sides, the core of the eyeball is surrounded by the following three membranes of the eye: fibrous (external), vascular (middle) and reticular (internal). Let's talk about each of them.
Outer shell
The most durable is the outer shell of the eye, fibrous. It is thanks to her that the eyeball is able to maintain its shape.
Cornea
The cornea, or cornea, is its smaller, anterior section. Its size is about 1/6 of the size of the entire shell. The cornea in the eyeball is its most convex part. In appearance, it is a concave-convex, somewhat elongated lens, which faces back with a concave surface. About 0.5 mm is the approximate thickness of the cornea. Its horizontal diameter is 11-12 mm. As for the vertical, its size is 10.5-11 mm.
The cornea is the transparent membrane of the eye. It incorporates a connective tissue transparent stroma, as well as corneal bodies that form its own substance. From the back and front surfaces to the stroma are adjacent the posterior and anterior border plates. The latter is the main substance of the cornea (modified), the other is a derivative of the endothelium, which covers its posterior surface, and also lines the entire front chamber of the human eye. A stratified epithelium covers the anterior surface of the cornea. It passes without sharp boundaries into the epithelium of the connective membrane. Due to the homogeneity of the tissue, as well as the absence of lymphatic and blood vessels, the cornea, in contrast to the next layer, which is the white membrane of the eye, is transparent. We now turn to the description of sclera.
Sclera
The white membrane of the eye is called sclera. This is a larger, posterior part of the outer shell, comprising about 1/6 of it. The sclera is a direct continuation of the cornea. However, it is formed, in contrast to the latter, by connective tissue fibers (dense) mixed with other fibers - elastic. The albuminous membrane of the eye is also opaque. The sclera passes into the cornea gradually. A translucent rim is located on the border between them. It is called the edge of the cornea. Now you know what the white membrane of the eye is. It is transparent only at the very beginning, near the cornea.
Scleral departments
In the anterior section, the outer surface of the sclera is covered with conjunctiva. This is the mucous membrane of the eye. Otherwise, it is called connective tissue. As for the posterior section, only the endothelium covers it here. The inner surface of the sclera, which faces the choroid, also covers the endothelium. Not throughout its entire length, the sclera is the same in thickness. The thinnest area is the place where the fibers of the optic nerve that penetrate the eyeball penetrate it. A trellis plate is formed here. The sclera has the greatest thickness in the circumference of the optic nerve. It is here from 1 to 1.5 mm. Then the thickness decreases, reaching 0.4-0.5 mm at the equator. Passing to the area of muscle attachment, the sclera thickens again, its length here is about 0.6 mm. Not only fibers of the optic nerve pass through it, but also venous and arterial vessels, as well as nerves. They form a series of holes in the sclera, which are called sclera graduates. Near the edge of the cornea, in the depths of its anterior section, lies throughout the entire length of the sclera sinus, which runs circularly.
Vascular membrane
So, we briefly characterized the outer shell of the eye. We now turn to the characteristic of the vascular, which is also called average. It is divided into the following 3 unequal parts. The first of them is a large, posterior one, which lines about two-thirds of the inner surface of the sclera. It is called the choroid itself. The second part is the middle one, located on the border between the cornea and sclera. This is the ciliary body. And finally, the third part (smaller, anterior), visible through the cornea, is called the iris, or iris.
The choroid proper passes without sharp boundaries in the anterior sections into the ciliary body. The serrated edge of the wall can act as a boundary between them. Almost throughout the entire vascular membrane itself is only adjacent to the sclera, except for the area of the spot, as well as the area that corresponds to the optic disc. The vascular membrane in the region of the latter has an optic opening through which the optic nerve fibers exit to the trellis plate. Its outer surface is covered with pigment and endothelial cells for the rest of its length . It limits the vascular capillary space together with the inner surface of the sclera.
Other layers of the membrane of interest to us are formed from a layer of large vessels that form a vascular plate. These are mainly veins, as well as arteries. Connective tissue elastic fibers, as well as pigment cells are located between them. A layer of middle vessels lies deeper than this layer. It is less pigmented. A network of small capillaries and blood vessels adjacent to it forms a vascular-capillary plate. It is especially developed in the area of the macula. The structureless fibrous layer is the deepest zone of the choroid itself. It is called the main plate. In the anterior section, the choroid thickens slightly and passes without sharp boundaries into the ciliary body.
Ciliary body
It is covered from the inner surface with a main plate, which is a continuation of the leaf. The leaf belongs to the choroid itself. The ciliary body in its bulk consists of the ciliary muscle, as well as the stroma of the ciliary body. The latter is represented by connective tissue rich in pigment cells and loose, as well as many vessels.
The following parts are distinguished in the ciliary body: ciliary circle, ciliary corolla and ciliary muscle. The latter occupies its outer section and is adjacent directly to the sclera. Smooth muscle fibers formed ciliary muscle. Among them, circular and meridional fibers are distinguished. The latter are highly developed. They form the muscle, which serves to tension the choroid itself. From the sclera and the angle of the anterior chamber, its fibers begin. Heading backwards, they are gradually lost in the choroid. This muscle, contracting, pulls forward the ciliary body (its posterior part) and the choroid itself (the anterior part). Thus, the tension of the ciliary girdle is reduced.
Ciliary muscle
Circular fibers are involved in the formation of circular muscle. Its contraction reduces the lumen of the ring, which is formed by the ciliary body. Due to this, the fixation point is approaching the equator of the ciliary lens. This causes loosening of the girdle. In addition, the curvature of the lens increases. Due to this, the circular part of the ciliary muscle is also called the muscle that compresses the lens.
Cilia circle
This is the posterior part of the ciliary body. It is curved in shape and has an uneven surface. The ciliary circle continues without sharp boundaries in the choroid itself.
Ciliary whisk
It occupies the anterior-inner part. It distinguishes small folds that run radially. These ciliary folds pass anteriorly into the ciliary processes, which are about 70 and which hang freely in the region of the posterior chamber of the apple. The rounded edge is formed at the place where the transition to the ciliary corolla of the ciliary circle is observed. This is the attachment point of the ciliary girdle fixing lens.
Iris
The front is the iris, or iris. Unlike other departments, it does not directly adhere to the fibrous membrane. The iris is a continuation of the ciliary body (its anterior section). It is located in the frontal plane and somewhat removed from the cornea. A round hole, called the pupil, is in its center. The ciliary edge is the opposite edge that runs along the entire circumference of the iris. The thickness of the latter consists of smooth muscles, blood vessels, connective tissue, as well as many nerve fibers. The pigment that determines the "color" of the eye has cells on the back of the iris.
Her smooth muscles are in two directions: radial and circular. A circular layer lies in the circumference of the pupil. It forms a muscle that narrows the pupil. Fibers located radially form a muscle that expands it.
The front surface of the iris is slightly convex anteriorly. Accordingly, the back is concave. On the front, in the circumference of the pupil, there is an inner small ring of the iris (pupillary girdle). About 1 mm is its width. The small ring is bounded externally by an irregular serrated circular line. It is called the small circle of the iris. The rest of its front surface is about 3-4 mm wide. It belongs to the outer large ring of the iris, or ciliary part.
Retina
We have not yet examined all the shells of the eye. We presented fibrotic and vascular. What shell of the eye has not yet been examined? The answer is internal, reticular (it is also called the retina). This membrane is represented by nerve cells located in several layers. She lines the eye from the inside. Great value of this shell of the eye. It provides the person with vision, since objects are displayed on it. Then information about them is transmitted to the brain through the optic nerve. However, not every retina sees the same way. The structure of the lining of the eye is such that the macula is characterized by the greatest visual ability.
Macula
It represents the central part of the retina. We all heard from school that there are rods and cones in the retina . But in the macula there are only cones that are responsible for color vision. Were it not for us, we could not distinguish between small details, read. The macula has all the conditions for registering light rays in the most detailed way. The retina in this area is thinning. Due to this, light rays can directly hit the photosensitive cones. There are no vessels of the retina that can interfere with clear vision in the macula. Its cells receive nutrition from the choroid located deeper. The macula is the central part of the retina of the eye, where the main number of cones (visual cells) is located.
What is inside the shells
Inside the shells are the anterior and posterior chambers (between the lens and the iris). Inside they are filled with liquid. Between them are the vitreous and the crystalline lens. The latter is a biconvex lens in shape. The lens, like the cornea, refracts and transmits rays of light. Thanks to this, the image is focused on the retina. Vitreous in consistency jelly. The fundus of the eye is separated from the lens with it.