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Anatomy of the eye, corneal structure


The eyeball is a sphere with a diameter of 25 mm, which consists of three coats. The outer, fibrous coat consists of an opaque sclera with the thickness of about 1mm, which from the front passes into the cornea. Outside the sclera is covered with a thin transparent mucous coat – the conjunctiva. The average coat is called vascular. From its title it is clear that it contains many blood vessels that nourish the eyeball. It also forms iris and the ciliary body. The inner coat of the eye is the retina.

The eye has also an adnexa, the eyelids and lachrymals. Eye movements are controlled by six muscles – four straight and two oblique. By its structure and functions, eyes can be compared with the optical system, such as the camera. Its main task is to "pass" the right image to the optic nerve. The image on the retina (similar to a film) is formed as the result of refraction of light rays in a system of lenses located in the eye (cornea and crystalline lens) (similar to object glass). Let’s look at how this occurs in detail.

The structure of the anterior segment of the eye

Light entering the eye first passes through the cornea – the transparent lens that has a dome shape (curvature radius of approximately 7.5 mm, thickness in the central part of about 0.5 mm). There are no blood vessels and there are many nerve endings, so the damage or inflammation of the cornea develops so-called corneal syndrome (lacrimation, photophobia and the inability to open the eyes).

The front surface of the cornea is covered with epithelium, which is able to regenerate (restoration) if damaged. Deeper is located stroma, which consists of collagen fibers, and inside the cornea is covered with a layer of cells – the endothelium, which if damaged cannot be restored, leading to development of dystrophies of the cornea, i.e., the violation of its transparency.

So during abdominal operations on eyes (when manipulations are carried out on the inside part of the cornea), this layer always requires special protection with special substances – viscoelastic.

Cornea is a lens, which accounts for 40 diopters of all 60 diopters of the total refractive power of the eye. That is, the cornea is the most powerful lens in the optical system of the eye. This is due to the difference of the refraction index of the air, which is in front of the cornea and the refraction index of its substance.

Place of the cornea transition into the sclera is called limb (limbus cornea). This is a shallow translucent slot, in the upper part it has a width of 1.75 mm, from the sides – 1 mm, at the bottom – 0.5-0.7 mm.

Sclera is opaque outer coat of the eyeball, which in the front part of the eyeball passes into a transparent cornea. To the sclera are attached 6 opticokinetic muscles. There is a small number of nerve endings and blood vessels.

Choroid covers the posterior sclera, it is adjoined by the retina, with which it is closely connected. Choroid is responsible for the blood supply of intraocular structures. In cases of diseases of the retina it is often involved in the pathological process. In the choroid there are no nerve endings, so in case of a disease there is no pain, which is usually a signal of some problems.

Leaving the cornea, light enters a liquid-filled so-called anterior chamber of the eye – the space between the inner surface of the cornea and the iris.

The iris is a part of the choroid of the eye and is a circle, a diaphragm with a hole in the center – pupil, which diameter can vary (with contraction and relaxation of the muscles of the iris), depending on the lighting, adjusting the flow of light entering the eye. The more light, the smaller pupils is. The iris is responsible for eye color (if it is blue – it means that there are few pigment cells, if brown – there are many). It performs the same function as the camera diaphragm.

Periphery of the cornea around the circumference is almost connected to the iris, forming a so-called angle of the anterior chamber, by anatomical elements of which (helmet channel, trabecular and other growths that have a common name – the drainage paths of the eye) happens the outflow of fluid, that constantly circulates in eye, into the venous system.

Behind the iris there is a transparent and flexible crystalline lens – one more lens, which refracts the light. The optical power of the lens is less than the corneal – it is about 18-20 diopters. The lens is held ciliary belt – it has all around threads looking like strings (so-called zonula ciliaris) that are connected to the ciliary muscle, located in the wall of the eye. These muscles can contract and relax. Depending on this, zonula ciliaris can also relax or contract, as a result of what the radius of curvature of the lens changes almost immediately "bringing into focus" – so people can see clearly both near and far.

This ability, called accommodation, with age (after 40 years) is lost because of induration of the lens matter – near vision deteriorates.

Sometimes zonula ciliaris completely or partially detach (due to injury or age) from the place of their attachment and the lens changes its position – takes place its so-called subluxation or dislocation. In the presence of a cataract such lens position may make adjustments to the operation on its removal.

The lens, like the cornea, is in the optical system of the eye, according to its structure it seems to have a grape with a grapeseed – it has a coat – capsular bag, thick substance – core (like the seed) and less dense material (resembles the grape pulp) – lens mass. As it is young the core of the lens is soft, however, to 40-50 years it thickens. The front capsule the lens faces the iris, the back – the vitreous body, and the boundary between them are zonula ciliaris. Such a detailed description of the anatomy of the lens will let us understand how the cataract is removed – the opaque lens, as well as an artificial lens is implanted into the eye.

Around the equator of the lens throughout its circumference is situated the ciliary body, which is part of the choroid. It has shoots that produce intraocular fluid. This fluid enters through the pupil into the anterior chamber of the eye and in the anterior chamber angle is removed into the venous system of the eye. The balance between production and outflow of the fluid is very important because its violation leads to glaucoma development.

The structure of the posterior segment of the eye

Behind the lens is positioned the vitreous body that fills most part of the eye and gives its form, is involved into intraocular metabolism. It doesn’t have any other functions, is in the optical system of the eye, but it actually doesn’t refract any light. In most cases it is a transparent jelly-like structure, but sometimes it can rarefy. On the other hand, there may appear indurate sections in the form of threads or lumps, the presence of which the patient feels as "flies" and floating points. It is believed that such changes often occur in case of myopia and are increased with the development of its stage. In some places the vitreous body is tightly welded with the retina, so in case of indurates, the vitreous body can pull the retina on itself, sometimes causing its detachment.

Some inflammatory diseases of the eye (so-called uveitis) may also cause severe opacities in the vitreous body.

Vitreous is insufficiently studied. In some situations (if due to opacities the patient's vision deteriorates) it can be replaced with a special solution (through a rather complicated operation).

After going through all of these structures the light gets on the retina that plays the role of a film. Retina consists of nine layers of cells and serves to convert light energy into nerve impulses. Retina consists of millions of small nerve cells, photoreceptors (they are sensitive to light). Cell-receptors located in the retina are divided into two types: cones and rods. These cells produce an enzyme – rhodopsin, they convert light energy (photons) into electrical energy of the nerve tissue, i.e. photochemical reaction, and send it to the brain. The rods have high photosensitivity and allow to see in low light, and they are responsible for peripheral vision. Cones, on the contrary, demand for their work more light, but they allow you to see fine details (responsible for central vision), make it possible to distinguish colors. The largest cluster of cones is located in the central fossa (macula), which is responsible for the highest visual acuity. The retina is adjacent to the choroid, but in many areas not tightly. Here it tends to detach at various diseases of the retina.

Nerve impulses are collected from the retina by the optic nerve, which consists of about 1 million nerve fibers. Information is transmitted through the optic nerve into the chiasma, visual tracts in certain areas of occipital parts of the cerebral cortex, where is analyzed the visual image and the picture of the external world that we see. All this forms our visual analyzer or visual system.

Damage, injury or optic nerve compression at any level lead to almost irreversible vision loss even with the normal functioning of other anatomical structures of the eye and transparency of ocular substances.

Doctor using special equipment calculates the ocular fundus through the pupil of the patient, sees a place of the exit of the optic nerve in the form of a disc (OND), which normally is of pale pink color. In the center of OND is visible vascular bundle – a place of the entrance onto the retina of the eye vein and artery. Near the OND is seen so-called macula (ML), or a yellow spot (it was called a corresponding light reflex, which is normally obtained during the check) – the point of the retina responsible for central vision.

Binocular vision

Having two eyes allows us to make our vision of stereoscopic (i.e. to form three-dimensional image). The right side of the retina of each eye passes through the optic nerve the "right side" of the image to the right side of the brain, similarly acts the left side of the retina. Then two images – the left and the right – the brain links together.

Since each eye sees "its" image, in violation of the joint motion of the right and the left eye binocular vision can be damaged. In plain language, you will have double vision or you'll simultaneously see two completely different pictures.

Key functions of the eye:

- Optical system that projects the image;

- A system that receives and "codes" information to the brain;

- "Serving" life-support system.

The structure of the cornea

Knowing the structure of the cornea is particularly useful for those who want to understand how the laser correction is conducted and why it is so, and for those who will have an operation on the cornea.

The cornea is the anterior transparent spherical layer of the eye, with the thickness of 0.5 mm, which has the properties of a biological lens.

Epithelial layer is the surface protective layer, which is restored after damage. Since the cornea has no blood vessels, then for the "oxygen delivery" is responsible the epithelium that takes it away from the lacrimal film that covers the surface of the eye. The epithelium also regulates the flow of fluid within the eye. 

Bowman's membrane is located just below the epithelium, is responsible for the protection and takes part in the nutrition of the cornea. If damaged, it is not restored.

Stroma is the biggest part of the cornea. Most of it are collagen fibers arranged in horizontal layers. It also contains cells that are responsible for recovery.

Dua’s layer is a discovery which was announced in 2013, has made adjustments in the understanding of the functioning of the human eye. Dua’s layer, with the thickness of about 15 microns, is highly durable, which endures the tension of 150-200 kPa, and is located between the stroma and Descemet’s membrane. Dua’s layer and the cornea perform complementary functions in focusing the light that got into the human eye. Dua’s layer name was given in honor of the scientist Harmindera Dua from Nottingham University. According to Professor Dua and his colleagues, the new layer can be involved in disorders such as degenerative eye disease, accumulation of fluid in the cornea and increased intraocular tension, knowing about that now will radically improve the efficiency of keratoplasty.

Descemet’s membrane separates the stroma from the endothelium. It has high elasticity, and is resistant to damage.

The endothelium is responsible for the transparency of the cornea and participates in its nourishing. It is poorly restored. Performs a very important function of an "active pump", which is responsible for no accumulation of extra liquid in the cornea (otherwise there will be a swelling). Thus, endothelium maintains the transparency of the cornea.

The number of endothelial cells throughout life is gradually reduced from 3500 to 2 mm at birth to 1500-2000 cells per 2 mm at the elderly age. Reducing the density of these cells can occur due to various diseases, traumas, surgeries, etc. At the density below 800 cells per mm 2, cornea becomes swollen and loses its transparency. The sixth layer of the cornea is often called the lacrimal film on the surface of the epithelium, which also plays a significant role in the optical properties of the eye.

As a result of diseases such as keratoconus, bullous keratopathy, hereditary corneal dystrophies, and as a result of traumas, bacterial and viral keratitis, the cornea can reduce in thickness and opalesce. The optical properties of the cornea will be inalterably lost.

The only way to restore the transparency of the cornea and get vision in severe cases of corneal lesions is keratoplasty.

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