There are 2 poles in the eyeball: posterior and anterior. The distance between them averages 24 mm. It is the largest size of the 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, lens and vitreous body. From all sides, the core of the eyeball is surrounded by the following three shells of the eye: fibrous (outer), vascular (middle) and reticular (inner). 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 the most convex part of it. In appearance, it is a concave-convex, somewhat elongated lens, which is turned back by a concave surface. About 0.5mm is approx.corneal thickness. Its horizontal diameter is 11-12 mm. As for the vertical one, its size is 10.5-11 mm.
The cornea is the transparent membrane of the eye. It incorporates a transparent connective tissue stroma, as well as corneal bodies that form its own substance. The posterior and anterior boundary plates adjoin the stroma from the posterior and anterior surfaces. The latter is the main substance of the cornea (modified), while the other is a derivative of the endothelium, which covers its posterior surface, and also lines the entire anterior chamber of the human eye. Stratified epithelium covers the anterior surface of the cornea. It passes without sharp boundaries into the epithelium of the connective sheath. Due to the homogeneity of the tissue, as well as the absence of lymphatic and blood vessels, the cornea, unlike the next layer, which is the white of the eye, is transparent. Now let's move on to the description of the sclera.
Sclera
The white of the eye is called the sclera. This is a larger, posterior section of the outer shell, making up about 1/6 of it. The sclera is the immediate continuation of the cornea. However, it is formed, unlike the latter, by connective tissue fibers (dense) with an admixture of other fibers - elastic. The white shell of the eye, moreover, is opaque. The sclera passes into the cornea gradually. The translucent rim is on the border between them. It is called the edge of the cornea. Now you know what the albuginea iseyes. It is transparent only at the very beginning, near the cornea.
Departments of the sclera
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, here it is covered only by the endothelium. That inner surface of the sclera, which faces the choroid, is also covered by the endothelium. The sclera is not uniform in thickness throughout its entire length. The thinnest section is the place where it is pierced by the fibers of the optic nerve, which exits the eyeball. Here a lattice plate is formed. The sclera is thickest 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 near the equator. Passing to the area of muscle attachment, the sclera thickens again, its length here is about 0.6 mm. Not only optic nerve fibers 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 along its entire length the sinus of the sclera, which runs circularly.
Choroid
So, we have briefly characterized the outer shell of the eye. We now turn to the characteristic of the vascular, which is also called the 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 vascularshell. The second part is the middle one, located on the border between the cornea and the sclera. This is the eyelash body. And finally, the third part (smaller, anterior), translucent through the cornea, is called the iris, or iris.
The choroid itself passes without sharp boundaries in the anterior sections into the ciliary body. The jagged edge of the wall can act as a boundary between them. For almost the entire length, the choroid itself only adjoins the sclera, except for the spot area, as well as the area that corresponds to the optic nerve head. The choroid in the region of the latter has an optic opening through which the fibers of the optic nerve exit to the cribriform plate of the sclera. Its outer surface for the rest of its length is covered with pigment and endothelial cells. It limits the perivascular capillary space together with the inner surface of the sclera.
Other layers of the membrane we are interested in are formed from a layer of large vessels that form the choroid plate. These are mainly veins, but also arteries. Connective tissue elastic fibers, as well as pigment cells, are located between them. The layer of middle vessels lies deeper than this layer. It is less pigmented. Adjacent to it is a network of small capillaries and vessels, forming a vascular-capillary plate. It is especially developed in the region of the yellow spot. The structureless fibrous layer is the deepest zone of the choroid proper. 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 the main plate, which is a continuation of the leaf. The leaf refers 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 a 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. The ciliary muscle is formed by smooth muscle fibers. Among them, circular and meridional fibers are distinguished. The latter are highly developed. They form a muscle that serves to stretch 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 rear part) and the choroid proper (anterior part). This reduces the tension of the lash line.
Ciliary muscle
Circular fibers are involved in the formation of the circular muscle. Its contraction reduces the lumen of the ring, which is formed by the ciliary body. Due to this, the place of fixation to the equator of the lens of the ciliary band approaches. This causes the girdle to relax. In addition, the curvature of the lens increases. It is because of this that the circular part of the ciliary muscle is also called the muscle that compresses the lens.
Ciliary Circle
Thisposterior inner part of the ciliary body. It is arched in shape, has an uneven surface. The ciliary circle continues without sharp boundaries in the choroid proper.
Ciliary whisk
It occupies the front-inner part. Small folds running radially are distinguished in it. These ciliary folds pass anteriorly into the ciliary processes, which are about 70 and which hang freely into the region of the posterior chamber of the apple. The rounded edge is formed in the place where there is a transition to the ciliary corolla of the ciliary circle. This is the site of attachment of the fixing lens of the ciliary band.
Iris
The anterior part is the iris, or the iris. Unlike other departments, it does not adjoin directly to the fibrous sheath. The iris is a continuation of the ciliary body (its anterior section). It is located in the frontal plane and is 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 is found in the cells of the posterior surface of the iris.
Her smooth muscles are in two directions: radial and circular. In the circumference of the pupil lies a circular layer. It forms a muscle that constricts the pupil. The fibers arranged radially form a muscle that expands it.
Frontthe 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 on the outside by an irregular jagged line running circularly. 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 considered all the shells of the eye. We presented fibrous and vascular. Which part of the eye has not yet been considered? The answer is internal, reticular (it is also called the retina). This sheath is represented by nerve cells located in several layers. It lines the inside of the eye. The significance of this shell of the eye is great. It is she who provides a person with vision, since objects are displayed on it. Then information about them is transmitted to the brain through the optic nerve. However, the retina does not see everything equally. The structure of the shell of the eye is such that the macula is characterized by the greatest visual ability.
Macula
It is the central part of the retina. We have all heard since school that there are rods and cones in the retina. But in the macula there are only cones that are responsible for color vision. Without it, we could not distinguish small details, read. The macula has all the conditions for registering light rays in the most detailed way.manner. The retina in this area becomes thinner. This allows light rays to reach the light-sensitive cones directly. There are no retinal vessels that can interfere with clear vision in the macula. Its cells receive nutrition from the choroid, which is deeper. Macula - the central part of the retina of the eye, where the main number of cones (visual cells) is located.
What's inside the shells
Inside the shells are the anterior and posterior chambers (between the lens and the iris). They are filled with liquid inside. Between them are the vitreous body and the lens. The latter in shape is a biconvex lens. The lens, like the cornea, refracts and transmits light rays. This brings the image into focus on the retina. The vitreous body has the consistency of jelly. The fundus is separated from the lens with it.
