The structure and functions of the human visual organs. Eyeball and auxiliary apparatus. Passage of light through the eye. Eye protection devices. Structure and functions of the layers of the retina Structure of the eyeball

Lens and vitreous body. Their combination is called a diopter apparatus. Under normal conditions, light rays are refracted from the visual target by the cornea and lens, so that the rays are focused on the retina. The refractive power of the cornea (the main refractive element of the eye) is 43 diopters. The convexity of the lens can vary, and its refractive power varies between 13 and 26 diopters. Thanks to this, the lens provides accommodation of the eyeball to objects located at close or far distances. When, for example, light rays from a distant object enter a normal eye (with a relaxed ciliary muscle), the target appears in focus on the retina. If the eye is directed towards a nearby object, they focus behind the retina (that is, the image on it blurs) until accommodation occurs. The ciliary muscle contracts, weakening the tension of the fibers of the girdle; The curvature of the lens increases, and as a result, the image is focused on the retina.

The cornea and lens together make up a convex lens. Rays of light from an object pass through the nodal point of the lens and form an inverted image on the retina, as in a camera. The retina can be compared to photographic film in that both record visual images. However, the retina is much more complex. It processes a continuous sequence of images, and also sends messages to the brain about the movements of visual objects, threatening signs, periodic changes in light and darkness, and other visual data about the external environment.

Although the optical axis of the human eye passes through the nodal point of the lens and the point of the retina between the fovea and the optic disc (Fig. 35.2), the oculomotor system orients the eyeball to a region of the object called the fixation point. From this point, a ray of light goes through the nodal point and is focused in the central fovea; thus it runs along the visual axis. Rays from other parts of the object are focused in the area of ​​the retina around the central fovea (Fig. 35.5).

The focusing of rays on the retina depends not only on the lens, but also on the iris. The iris acts as the camera's diaphragm and regulates not only the amount of light entering the eye, but, more importantly, the depth of the visual field and the spherical aberration of the lens. As the diameter of the pupil decreases, the depth of the visual field increases and light rays are directed through the central part of the pupil, where spherical aberration is minimal. Changes in pupil diameter occur automatically (i.e., reflexively) when the eye adjusts (accommodates) to examine close objects. Therefore, during reading or other eye activities involving the discrimination of small objects, the image quality is improved by the optical system of the eye.

Another factor that affects image quality is light scattering. It is minimized by limiting the light beam, as well as its absorption by the pigment of the choroid and the pigment layer of the retina. In this respect, the eye again resembles a camera. There, light scattering is also prevented by limiting the beam of rays and its absorption by black paint covering the inner surface of the chamber.

Focusing of the image is disrupted if the size of the pupil does not correspond to the refractive power of the diopter. With myopia (myopia), images of distant objects are focused in front of the retina, without reaching it (Fig. 35.6). The defect is corrected using concave lenses. Conversely, with hypermetropia (farsightedness), images of distant objects are focused behind the retina. To eliminate the problem, convex lenses are needed (Fig. 35.6). True, the image can be temporarily focused due to accommodation, but this causes the ciliary muscles to become tired and the eyes to become tired. With astigmatism, an asymmetry occurs between the radii of curvature of the surfaces of the cornea or lens (and sometimes the retina) in different planes. For correction, lenses with specially selected radii of curvature are used.

The elasticity of the lens gradually decreases with age. The efficiency of his accommodation decreases when viewing close objects (presbyopia). At a young age, the refractive power of the lens can vary over a wide range, up to 14 diopters. By the age of 40, this range is halved, and after 50 years - to 2 diopters and below. Presbyopia is corrected with convex lenses.

The human eye is a remarkable achievement of evolution and an excellent optical instrument. The sensitivity threshold of the eye is close to the theoretical limit due to the quantum properties of light, in particular the diffraction of light. The range of intensities perceived by the eye is, the focus can move quickly from a very short distance to infinity.
The eye is a lens system that forms an inverted real image on a light-sensitive surface. The eyeball is approximately spherical in shape with a diameter of about 2.3 cm. Its outer shell is an almost fibrous opaque layer called sclera. Light enters the eye through the cornea, which is the transparent membrane on the outer surface of the eyeball. In the center of the cornea there is a colored ring - iris (iris) with pupil in the middle. They act like a diaphragm, regulating the amount of light entering the eye.
Lens is a lens consisting of a fibrous transparent material. Its shape and therefore focal length can be changed using ciliary muscles eyeball. The space between the cornea and the lens is filled with watery fluid and is called front camera. Behind the lens is a clear jelly-like substance called vitreous.
The inner surface of the eyeball is covered retina, which contains numerous nerve cells - visual receptors: rods and cones, which respond to visual stimulation by generating biopotentials. The most sensitive area of ​​the retina is yellow spot, which contains the largest number of visual receptors. The central part of the retina contains only densely packed cones. The eye rotates to examine the object being studied.

Rice. 1. Human eye

Refraction in the eye

The eye is the optical equivalent of a conventional photographic camera. It has a lens system, an aperture system (pupil) and a retina on which the image is captured.

The lens system of the eye is formed from four refractive media: the cornea, the aqueous chamber, the lens, and the glass body. Their refractive indices do not differ significantly. They are 1.38 for the cornea, 1.33 for the aqueous chamber, 1.40 for the lens and 1.34 for the vitreous (Fig. 2).

Rice. 2. The eye as a system of refractive media (numbers are refractive indices)

Light is refracted in these four refractive surfaces: 1) between the air and the anterior surface of the cornea; 2) between the posterior surface of the cornea and the water chamber; 3) between the water chamber and the anterior surface of the lens; 4) between the posterior surface of the lens and the vitreous body.
The strongest refraction occurs on the anterior surface of the cornea. The cornea has a small radius of curvature, and the refractive index of the cornea differs most from the refractive index of air.
The refractive power of the lens is less than that of the cornea. It accounts for about one-third of the total refractive power of the eye's lens systems. The reason for this difference is that the fluids surrounding the lens have refractive indices that are not significantly different from the refractive index of the lens. If the lens is removed from the eye, surrounded by air, it has a refractive index almost six times greater than in the eye.

The lens performs a very important function. Its curvature can be changed, which provides fine focusing on objects located at different distances from the eye.

Reduced eye

A reduced eye is a simplified model of a real eye. It schematically represents the optical system of a normal human eye. The reduced eye is represented by a single lens (one refractive medium). In a reduced eye, all the refractive surfaces of the real eye are summed algebraically to form a single refractive surface.
The reduced eye allows for simple calculations. The total refractive power of the media is almost 59 diopters when the lens is accommodated for vision of distant objects. The central point of the reduced eye lies 17 millimeters in front of the retina. A ray from any point on the object enters the reduced eye and passes through the central point without refraction. Just as a glass lens forms an image on a piece of paper, the lens system of the eye forms an image on the retina. This is a reduced, real, inverted image of an object. The brain forms the perception of an object in an upright position and in real size.

Accommodation

To see an object clearly, it is necessary that after the rays are refracted, an image is formed on the retina. Changing the refractive power of the eye to focus near and distant objects is called accommodation.
The farthest point to which the eye focuses is called farthest point visions - infinity. In this case, parallel rays entering the eye are focused onto the retina.
An object is visible in detail when it is placed as close to the eye as possible. Minimum clear vision distance – about 7 cm with normal vision. In this case, the accommodation apparatus is in the most tense state.
A point located at a distance of 25 cm, called dot best vision, since in this case all the details of the object in question are visible without maximum strain on the accommodation apparatus, as a result of which the eye may not get tired for a long time.
If the eye is focused on an object at a near point, it must adjust its focal length and increase its refractive power. This process occurs through changes in the shape of the lens. When an object is brought closer to the eye, the shape of the lens changes from a moderately convex lens shape to a convex lens shape.
The lens is formed by a fibrous jelly-like substance. It is surrounded by a strong flexible capsule and has special ligaments running from the edge of the lens to the outer surface of the eyeball. These ligaments are constantly tense. The shape of the lens changes ciliary muscle. The contraction of this muscle reduces the tension of the lens capsule, it becomes more convex and, due to the natural elasticity of the capsule, takes on a spherical shape. Conversely, when the ciliary muscle is completely relaxed, the refractive power of the lens is weakest. On the other hand, when the ciliary muscle is in its maximum contracted state, the refractive power of the lens becomes greatest. This process is controlled by the central nervous system.

Rice. 3. Accommodation in a normal eye

Presbyopia

The refractive power of the lens can increase from 20 diopters to 34 diopters in children. The average accommodation is 14 diopters. As a result, the total refractive power of the eye is almost 59 diopters when the eye is accommodated for distance vision, and 73 diopters at maximum accommodation.
As a person ages, the lens becomes thicker and less elastic. Consequently, the ability of a lens to change its shape decreases with age. The power of accommodation decreases from 14 diopters in a child to less than 2 diopters between the ages of 45 and 50 years and becomes 0 at the age of 70 years. Therefore, the lens almost does not accommodate. This disturbance of accommodation is called senile farsightedness. The eyes are always focused at a constant distance. They cannot accommodate both near and far vision. Therefore, to see clearly at various distances, an old person must wear bifocals with the upper segment focused for distance vision and the lower segment focused for near vision.

Refraction errors

Emmetropia . It is believed that the eye will be normal (emmetropic) if parallel light rays from distant objects are focused into the retina when the ciliary muscle is completely relaxed. Such an eye clearly sees distant objects when the ciliary muscle is relaxed, that is, without accommodation. When focusing objects at close distances, the ciliary muscle contracts in the eye, providing a suitable degree of accommodation.

Rice. 4. Refraction of parallel light rays in the human eye.

Hypermetropia (hyperopia). Hypermetropia is also known as farsightedness. It is caused either by the small size of the eyeball or by the weak refractive power of the eye's lens system. Under such conditions, parallel light rays are not refracted sufficiently by the lens system of the eye for the focus (and therefore the image) to be on the retina. To overcome this anomaly, the ciliary muscle must contract, increasing the optical power of the eye. Consequently, a farsighted person is able to focus distant objects on the retina using the mechanism of accommodation. There is not enough accommodation power to see closer objects.
With a small reserve of accommodation, a farsighted person is often unable to accommodate the eye sufficiently to focus not only close, but even distant objects.
To correct farsightedness, it is necessary to increase the refractive power of the eye. To do this, convex lenses are used, which add refractive power to the power of the eye's optical system.

Myopia . In myopia (or nearsightedness), parallel light rays from distant objects are focused in front of the retina, despite the fact that the ciliary muscle is completely relaxed. This happens due to the eyeball being too long, as well as due to the refractive power of the optical system of the eye being too high.
There is no mechanism by which the eye can reduce the refractive power of its lens less than is possible with complete relaxation of the ciliary muscle. The process of accommodation leads to deterioration of vision. Therefore, a person with myopia cannot focus distant objects on the retina. The image can only focus if the object is close enough to the eye. Therefore, a person with myopia has limited range of clear vision.
It is known that rays passing through a concave lens are refracted. If the refractive power of the eye is too great, as in myopia, it can sometimes be neutralized by a concave lens. Using laser technology, it is also possible to correct excessive corneal convexity.

Astigmatism . In an astigmatic eye, the refractive surface of the cornea is not spherical, but ellipsoidal. This occurs due to too much curvature of the cornea in one of its planes. As a result, light rays passing through the cornea in one plane are not refracted as much as rays passing through it in another plane. They do not gather in a common focus. Astigmatism cannot be compensated by the eye using accommodation, but it can be corrected using a cylindrical lens that will correct an error in one of the planes.

Correction of optical anomalies with contact lenses

Recently, plastic contact lenses have been used to correct various vision anomalies. They are placed against the front surface of the cornea and are secured by a thin layer of tears that fills the space between the contact lens and the cornea. Hard contact lenses are made of hard plastic. Their sizes are 1 mm in thickness and 1 cm in diameter. There are also soft contact lenses.
Contact lenses replace the cornea as the outer surface of the eye and almost completely cancel out the portion of the eye's refractive power that normally occurs on the front surface of the cornea. When using contact lenses, the anterior surface of the cornea does not play a significant role in the refraction of the eye. The front surface of the contact lens begins to play the main role. This is especially important in individuals with abnormally formed corneas.
Another feature of contact lenses is that, by rotating with the eye, they provide a wider area of ​​clear vision than regular glasses. They are also more convenient to use for artists, athletes, etc.

Visual acuity

The human eye's ability to see fine details clearly is limited. The normal eye can distinguish different point light sources located at a distance of 25 arc seconds. That is, when light rays from two separate points enter the eye at an angle of more than 25 seconds between them, they are visible as two points. Beams with smaller angular separation cannot be distinguished. This means that a person with normal visual acuity can distinguish two points of light at a distance of 10 meters if they are 2 millimeters apart.

Rice. 7. Maximum visual acuity for two point light sources.

The presence of this limit is provided for by the structure of the retina. The average diameter of the receptors in the retina is almost 1.5 micrometers. A person can normally distinguish two separate dots if the distance between them in the retina is 2 micrometers. Thus, in order to distinguish between two small objects, they must excite two different cones. At least there will be 1 unexcited cone between them.

The eye is the only human organ that has optically transparent tissues, which are otherwise called the optical media of the eye. It is thanks to them that rays of light pass into the eye and a person gets the opportunity to see. Let's try to understand in the most primitive form the structure of the optical apparatus of the organ of vision.

The eye has a spherical shape. It is surrounded by the tunica albuginea and the cornea. The tunica albuginea consists of dense, bundles of interwoven fibers; it is white and opaque. In the front part of the eyeball, the cornea is “inserted” into the tunica albuginea in much the same way as a watch glass into a frame. It has a spherical shape and, most importantly, is completely transparent. Rays of light falling on the eye first pass through the cornea, which strongly refracts them.

After the cornea, the light beam passes through the anterior chamber of the eye - a space filled with colorless transparent liquid. Its depth is on average 3 millimeters. The back wall of the anterior chamber is the iris, which gives color to the eye; in its center there is a round hole - the pupil. When examining the eye, it appears black to us. Thanks to the muscles embedded in the iris, the pupil can change its width: narrow in the light and expand in the dark. This is like a camera diaphragm, which automatically protects the eye from the entry of a large amount of light in bright light and, conversely, in low light, expanding, helping the eye to catch even weak light rays. After passing through the pupil, the light beam hits a peculiar formation called the lens. It is easy to imagine - it is a lenticular body, reminiscent of an ordinary magnifying glass. Light can pass freely through the lens, but at the same time it is refracted in the same way as, according to the laws of physics, a light ray passing through a prism is refracted, i.e. it is deflected towards the base.

We can imagine the lens as two prisms joined at the base. The lens has another extremely interesting feature: it can change its curvature. Thin threads called zonules of cinnamon are attached along the edge of the lens, which at their other end are fused with the ciliary muscle located behind the root of the iris. The lens tends to take on a spherical shape, but this is prevented by stretched ligaments. When the ciliary muscle contracts, the ligaments relax and the lens becomes more convex. A change in the curvature of the lens does not remain without an effect on vision, since the rays of light in connection with this change the degree of refraction. This property of the lens to change its curvature, as we will see below, is very important for the visual act.

After the lens, light passes through the vitreous body, which fills the entire cavity of the eyeball. The vitreous body consists of thin fibers, between which there is a colorless transparent liquid with high viscosity; this liquid resembles molten glass. This is where its name comes from - the vitreous body.

Rays of light, passing through the cornea, anterior chamber, lens and vitreous body, fall on the light-sensitive retina (retina), which is the most complex of all the membranes of the eye. The outer part of the retina has a layer of cells that, under a microscope, look like rods and cones. The central part of the retina contains predominantly cones, which play a major role in the process of clearest, distinct vision and color sensation. Further from the center of the retina, rods begin to appear, the number of which increases towards the peripheral areas of the retina. Cones, on the contrary, the further from the center, the fewer they become. Scientists estimate that the human retina contains 7 million cones and 130 million rods. Unlike cones, which operate in light, rods begin to “work” in low light and in the dark. Rods are very sensitive to even small amounts of light and therefore enable a person to navigate in the dark.

How does the process of vision occur? Rays of light hitting the retina cause a complex photochemical process, which results in irritation of the rods and cones. This irritation is transmitted through the retina to the layer of nerve fibers that make up the optic nerve. The optic nerve passes through a special opening into the cranial cavity. Here, the visual fibers travel a long and complex path and ultimately end in the occipital cortex. This area is the highest visual center, in which a visual image is recreated that exactly corresponds to the object in question.

Vision is the channel through which a person receives approximately 70% of all data about the world that surrounds him. And this is possible only for the reason that human vision is one of the most complex and amazing visual systems on our planet. If there were no vision, we would all most likely simply live in the dark.

The human eye has a perfect structure and provides vision not only in color, but also in three dimensions and with the highest sharpness. It has the ability to instantly change focus to a variety of distances, regulate the volume of incoming light, distinguish between a huge number of colors and an even greater number of shades, correct spherical and chromatic aberrations, etc. The eye brain is connected to six levels of the retina, in which the data goes through a compression stage even before information is sent to the brain.

But how does our vision work? How do we transform color reflected from objects into an image by enhancing color? If you think about this seriously, you can conclude that the structure of the human visual system is “thought out” to the smallest detail by the Nature that created it. If you prefer to believe that the Creator or some Higher Power is responsible for the creation of man, then you can attribute this credit to them. But let's not understand, but continue talking about the structure of vision.

Huge amount of details

The structure of the eye and its physiology can frankly be called truly ideal. Think for yourself: both eyes are located in the bony sockets of the skull, which protect them from all kinds of damage, but they protrude from them in such a way as to ensure the widest possible horizontal vision.

The distance the eyes are from each other provides spatial depth. And the eyeballs themselves, as is known for certain, have a spherical shape, due to which they are able to rotate in four directions: left, right, up and down. But each of us takes all this for granted - few people imagine what would happen if our eyes were square or triangular or their movement was chaotic - this would make vision limited, chaotic and ineffective.

So, the structure of the eye is extremely complex, but this is precisely what makes the work of about four dozen of its different components possible. And even if at least one of these elements were missing, the process of vision would cease to be carried out as it should be carried out.

To see how complex the eye is, we invite you to pay attention to the figure below.

Let's talk about how the process of visual perception is implemented in practice, what elements of the visual system are involved in this, and what each of them is responsible for.

Passage of light

As light approaches the eye, light rays collide with the cornea (otherwise known as the cornea). The transparency of the cornea allows light to pass through it into the inner surface of the eye. Transparency, by the way, is the most important characteristic of the cornea, and it remains transparent due to the fact that a special protein it contains inhibits the development of blood vessels - a process that occurs in almost every tissue of the human body. If the cornea were not transparent, the remaining components of the visual system would have no significance.

Among other things, the cornea prevents debris, dust and any chemical elements from entering the internal cavities of the eye. And the curvature of the cornea allows it to refract light and help the lens focus light rays on the retina.

After light has passed through the cornea, it passes through a small hole located in the middle of the iris. The iris is a round diaphragm that is located in front of the lens just behind the cornea. The iris is also the element that gives the eye color, and the color depends on the predominant pigment in the iris. The central hole in the iris is the pupil familiar to each of us. The size of this hole can be changed to control the amount of light entering the eye.

The size of the pupil will be changed directly by the iris, and this is due to its unique structure, because it consists of two different types of muscle tissue (even there are muscles here!). The first muscle is a circular compressor - it is located in the iris in a circular manner. When the light is bright, it contracts, as a result of which the pupil contracts, as if being pulled inward by a muscle. The second muscle is an extension muscle - it is located radially, i.e. along the radius of the iris, which can be compared to the spokes of a wheel. In dark lighting, this second muscle contracts, and the iris opens the pupil.

Many still experience some difficulties when they try to explain how the formation of the above-mentioned elements of the human visual system occurs, because in any other intermediate form, i.e. at any evolutionary stage they simply would not be able to work, but man sees from the very beginning of his existence. Mystery…

Focusing

Bypassing the above stages, light begins to pass through the lens located behind the iris. The lens is an optical element shaped like a convex oblong ball. The lens is absolutely smooth and transparent, there are no blood vessels in it, and it itself is located in an elastic sac.

Passing through the lens, light is refracted, after which it is focused on the fovea of ​​the retina - the most sensitive place containing the maximum number of photoreceptors.

It is important to note that the unique structure and composition provide the cornea and lens with a high refractive power, guaranteeing a short focal length. And how amazing it is that such a complex system fits in just one eyeball (just think what a person could look like if, for example, a meter was required to focus light rays coming from objects!).

No less interesting is the fact that the combined refractive power of these two elements (cornea and lens) is in excellent correlation with the eyeball, and this can be safely called another proof that the visual system is created simply unsurpassed, because the process of focusing is too complex to talk about it as something that happened only through step-by-step mutations - evolutionary stages.

If we are talking about objects located close to the eye (as a rule, a distance of less than 6 meters is considered close), then everything is even more curious, because in this situation the refraction of light rays turns out to be even stronger. This is ensured by an increase in the curvature of the lens. The lens is connected through ciliary bands to the ciliary muscle, which, when contracted, allows the lens to take on a more convex shape, thereby increasing its refractive power.

And here again we cannot fail to mention the complex structure of the lens: it consists of many threads, which consist of cells connected to each other, and thin belts connect it with the ciliary body. Focusing is carried out under the control of the brain extremely quickly and completely “automatically” - it is impossible for a person to carry out such a process consciously.

Meaning of "camera film"

Focusing results in focusing the image on the retina, which is a multi-layered light-sensitive tissue covering the back of the eyeball. The retina contains approximately 137,000,000 photoreceptors (for comparison, we can cite modern digital cameras, which have no more than 10,000,000 such sensory elements). Such a huge number of photoreceptors is due to the fact that they are located extremely densely - approximately 400,000 per 1 mm².

It would not be out of place here to cite the words of microbiologist Alan L. Gillen, who speaks in his book “The Body by Design” about the retina of the eye as a masterpiece of engineering design. He believes that the retina is the most amazing element of the eye, comparable to photographic film. The light-sensitive retina, located on the back of the eyeball, is much thinner than cellophane (its thickness is no more than 0.2 mm) and much more sensitive than any human-made photographic film. The cells of this unique layer are capable of processing up to 10 billion photons, while the most sensitive camera can process only a few thousand. But what’s even more amazing is that the human eye can detect a few photons even in the dark.

In total, the retina consists of 10 layers of photoreceptor cells, 6 layers of which are layers of light-sensitive cells. 2 types of photoreceptors have a special shape, which is why they are called cones and rods. Rods are extremely sensitive to light and provide the eye with black-and-white perception and night vision. Cones, in turn, are not so sensitive to light, but are able to distinguish colors - optimal operation of the cones is noted in the daytime.

Thanks to the work of photoreceptors, light rays are transformed into complexes of electrical impulses and sent to the brain at incredibly high speed, and these impulses themselves travel over a million nerve fibers in a fraction of a second.

The communication of photoreceptor cells in the retina is very complex. Cones and rods are not directly connected to the brain. Having received the signal, they redirect it to bipolar cells, and they redirect the signals they have already processed to ganglion cells, more than a million axons (neurites along which nerve impulses are transmitted) which form a single optic nerve, through which data enters the brain.

Two layers of interneurons, before visual data is sent to the brain, facilitate parallel processing of this information by six layers of perception located in the retina. This is necessary so that images are recognized as quickly as possible.

Brain perception

After the processed visual information enters the brain, it begins to sort, process and analyze it, and also forms a complete image from individual data. Of course, there is still a lot that is unknown about the workings of the human brain, but even what the scientific world can provide today is enough to be amazed.

With the help of two eyes, two “pictures” of the world that surrounds a person are formed - one for each retina. Both “pictures” are transmitted to the brain, and in reality the person sees two images at the same time. But how?

But the point is this: the retinal point of one eye exactly corresponds to the retinal point of the other, and this suggests that both images, entering the brain, can overlap each other and be combined together to obtain a single image. The information received by the photoreceptors in each eye converges in the visual cortex, where a single image appears.

Due to the fact that the two eyes may have different projections, some inconsistencies may be observed, but the brain compares and connects the images in such a way that a person does not perceive any inconsistencies. Moreover, these inconsistencies can be used to obtain a sense of spatial depth.

As you know, due to the refraction of light, visual images entering the brain are initially very small and upside down, but “at the output” we get the image that we are used to seeing.

In addition, in the retina, the image is divided by the brain in two vertically - through a line that passes through the retinal fossa. The left parts of the images received by both eyes are redirected to , and the right parts are redirected to the left. Thus, each of the hemispheres of the viewing person receives data from only one part of what he sees. And again - “at the output” we get a solid image without any traces of connection.

The separation of images and extremely complex optical pathways make it so that the brain sees separately from each of its hemispheres using each of the eyes. This allows you to speed up the processing of the flow of incoming information, and also provides vision with one eye if suddenly a person for some reason stops seeing with the other.

We can conclude that the brain, in the process of processing visual information, removes “blind” spots, distortions due to micro-movements of the eyes, blinks, angle of view, etc., offering its owner an adequate holistic image of what is being observed.

Another important element of the visual system is. There is no way to downplay the importance of this issue, because... In order to be able to use our vision properly at all, we must be able to turn our eyes, raise them, lower them, in short, move our eyes.

In total, there are 6 external muscles that connect to the outer surface of the eyeball. These muscles include 4 rectus muscles (inferior, superior, lateral and middle) and 2 obliques (inferior and superior).

At the moment when any of the muscles contracts, the muscle that is opposite to it relaxes - this ensures smooth eye movement (otherwise all eye movements would be jerky).

When you turn both eyes, the movement of all 12 muscles (6 muscles in each eye) automatically changes. And it is noteworthy that this process is continuous and very well coordinated.

According to the famous ophthalmologist Peter Janey, the control and coordination of the communication of organs and tissues with the central nervous system through the nerves (this is called innervation) of all 12 eye muscles is one of the very complex processes occurring in the brain. If we add to this the accuracy of gaze redirection, the smoothness and evenness of movements, the speed with which the eye can rotate (and it amounts to a total of up to 700° per second), and combine all this, we will actually get a mobile eye that is phenomenal in terms of performance. system. And the fact that a person has two eyes makes it even more complex - with synchronous eye movements, the same muscular innervation is necessary.

The muscles that rotate the eyes are different from the skeletal muscles because... they are made up of many different fibers, and they are controlled by an even larger number of neurons, otherwise the accuracy of movements would become impossible. These muscles can also be called unique because they are able to contract quickly and practically do not get tired.

Considering that the eye is one of the most important organs of the human body, it needs continuous care. It is precisely for this purpose that an “integrated cleaning system,” so to speak, is provided, which consists of eyebrows, eyelids, eyelashes and tear glands.

The lacrimal glands regularly produce a sticky fluid that moves slowly down the outer surface of the eyeball. This liquid washes away various debris (dust, etc.) from the cornea, after which it enters the internal lacrimal canal and then flows down the nasal canal, being eliminated from the body.

Tears contain a very strong antibacterial substance that destroys viruses and bacteria. The eyelids act as windshield wipers - they clean and moisturize the eyes through involuntary blinking at intervals of 10-15 seconds. Along with the eyelids, eyelashes also work, preventing any debris, dirt, germs, etc. from entering the eye.

If the eyelids did not fulfill their function, a person's eyes would gradually dry out and become covered with scars. If there were no tear ducts, the eyes would constantly be filled with tear fluid. If a person did not blink, debris would get into his eyes and he could even go blind. The entire “cleaning system” must include the work of all elements without exception, otherwise it would simply cease to function.

Eyes as an indicator of condition

A person’s eyes are capable of transmitting a lot of information during his interaction with other people and the world around him. The eyes can radiate love, burn with anger, reflect joy, fear or anxiety, or fatigue. The eyes show where a person is looking, whether he is interested in something or not.

For example, when people roll their eyes while talking to someone, this can be interpreted very differently from a normal upward gaze. Big eyes in children evoke delight and tenderness among those around them. And the state of the pupils reflects the state of consciousness in which a person is at a given moment in time. Eyes are an indicator of life and death, if we speak in a global sense. This is probably why they are called the “mirror” of the soul.

Instead of a conclusion

In this lesson we looked at the structure of the human visual system. Naturally, we missed a lot of details (this topic itself is very voluminous and it is problematic to fit it into the framework of one lesson), but we still tried to convey the material so that you have a clear idea of ​​HOW a person sees.

You couldn't help but notice that both the complexity and capabilities of the eye allow this organ to surpass even the most modern technologies and scientific developments many times over. The eye is a clear demonstration of the complexity of engineering in a huge number of nuances.

But knowing about the structure of vision is, of course, good and useful, but the most important thing is to know how vision can be restored. The fact is that a person’s lifestyle, the conditions in which he lives, and some other factors (stress, genetics, bad habits, diseases and much more) - all this often contributes to the fact that vision can deteriorate over the years, i.e. .e. the visual system begins to malfunction.

But deterioration of vision in most cases is not an irreversible process - knowing certain techniques, this process can be reversed, and vision can be made, if not the same as that of a baby (although this is sometimes possible), then as good as possible for each individual person. Therefore, the next lesson in our course on vision development will be devoted to methods of vision restoration.

Look at the root!

Test your knowledge

If you want to test your knowledge on the topic of this lesson, you can take a short test consisting of several questions. For each question, only 1 option can be correct. After you select one of the options, the system automatically moves on to the next question. The points you receive are affected by the correctness of your answers and the time spent on completion. Please note that the questions are different each time and the options are mixed.

Vision is a biological process that determines the perception of the shape, size, color of objects around us, and orientation among them. This is possible thanks to the function of the visual analyzer, which includes the perceptive apparatus - the eye.

Vision function not only in the perception of light rays. We use it to assess distance, volume of objects, and visual perception of the surrounding reality.

Human eye - photo

Currently, of all the human senses, the greatest load falls on the organs of vision. This is due to reading, writing, watching television and other types of information and work.

Structure of the human eye

The organ of vision consists of the eyeball and auxiliary apparatus located in the orbit - the recess of the bones of the facial skull.

The structure of the eyeball

The eyeball has the appearance of a spherical body and consists of three membranes:

• External - fibrous;
• middle - vascular;
• internal - mesh.

Outer fibrous membrane in the posterior section it forms the albuginea, or sclera, and in the front it passes into the cornea, permeable to light.

Middle choroid so called because it is rich in blood vessels. Located under the sclera. The anterior part of this shell forms iris, or iris. It is called so because of its color (rainbow color). The iris contains pupil- a round hole that can change its size depending on the intensity of lighting through an innate reflex. To do this, there are muscles in the iris that constrict and dilate the pupil.

The iris acts as a diaphragm that regulates the amount of light entering the light-sensitive apparatus and protects it from destruction by adjusting the organ of vision to the intensity of light and darkness. The choroid forms fluid - the moisture of the chambers of the eye.

Inner retina, or retina- adjacent to the back of the middle (choroid) membrane. Consists of two leaves: outer and inner. The outer leaf contains pigment, the inner leaf contains photosensitive elements.

The retina lines the bottom of the eye. If you look at it from the side of the pupil, you can see a whitish round spot at the bottom. This is where the optic nerve exits. There are no photosensitive elements and therefore light rays are not perceived, it is called blind spot. To the side of it is yellow spot (macula). This is the place of greatest visual acuity.

In the inner layer of the retina there are light-sensitive elements - visual cells. Their ends have the shape of rods and cones. Sticks contain visual pigment - rhodopsin, cones- iodopsin. Rods perceive light in twilight conditions, and cones perceive colors in fairly bright lighting.

Sequence of light passing through the eye

Let us consider the path of light rays through that part of the eye that makes up its optical apparatus. First, the light passes through the cornea, the aqueous humor of the anterior chamber of the eye (between the cornea and the pupil), the pupil, the lens (in the form of a biconvex lens), the vitreous body (a thick transparent medium) and finally hits the retina.

In cases where light rays, having passed through the optical media of the eye, are not focused on the retina, vision anomalies develop:

• If in front of it - myopia;
• if behind - farsightedness.

To correct myopia, biconcave glasses are used, and farsightedness, biconvex glasses are used.

As already noted, the retina contains rods and cones. When light hits them, it causes irritation: complex photochemical, electrical, ionic and enzymatic processes occur, which cause nervous excitation - a signal. It enters the subcortical (quadrigeminal, visual thalamus, etc.) vision centers along the optic nerve. Then it is sent to the cortex of the occipital lobes of the brain, where it is perceived as a visual sensation.

The entire complex of the nervous system, including light receptors, optic nerves, and vision centers in the brain, makes up the visual analyzer.

The structure of the auxiliary apparatus of the eye

In addition to the eyeball, the eye also includes an auxiliary apparatus. It consists of the eyelids, six muscles that move the eyeball. The back surface of the eyelids is covered by a membrane - the conjunctiva, which partially extends onto the eyeball. In addition, the auxiliary organs of the eye include the lacrimal apparatus. It consists of the lacrimal gland, lacrimal canaliculi, sac and nasolacrimal duct.

The lacrimal gland secretes a secretion - tears containing lysozyme, which has a detrimental effect on microorganisms. It is located in the fossa of the frontal bone. Its 5-12 tubules open into the gap between the conjunctiva and the eyeball in the outer corner of the eye. Having moistened the surface of the eyeball, tears flow to the inner corner of the eye (to the nose). Here they collect in the openings of the lacrimal canaliculi, through which they enter the lacrimal sac, also located at the inner corner of the eye.

From the sac, along the nasolacrimal duct, tears are directed into the nasal cavity, under the inferior concha (which is why you can sometimes notice how tears flow from the nose while crying).

Vision hygiene

Knowledge of the pathways for the outflow of tears from the places of formation - the lacrimal glands - allows you to correctly perform such a hygienic skill as “wiping” the eyes. In this case, the movement of the hands with a clean napkin (preferably sterile) should be directed from the outer corner of the eye to the inner one, “wipe the eyes towards the nose”, towards the natural flow of tears, and not against it, thus helping to remove the foreign body (dust) on the surface of the eyeball.

The organ of vision must be protected from foreign bodies and damage. When working where particles, splinters of materials, or shavings are formed, you should use safety glasses.

If your vision deteriorates, do not hesitate and contact an ophthalmologist and follow his recommendations to avoid further development of the disease. The intensity of workplace lighting should depend on the type of work being performed: the more subtle movements are performed, the more intense the lighting should be. It should be neither bright nor weak, but exactly the one that requires the least visual strain and contributes to efficient work.

How to maintain visual acuity

Lighting standards have been developed depending on the purpose of the room and the type of activity. The amount of light is determined using a special device - a lux meter. The correctness of lighting is monitored by the health service and the administration of institutions and enterprises.

It should be remembered that bright light especially contributes to the deterioration of visual acuity. Therefore, you should avoid looking without sunglasses towards bright light sources, both artificial and natural.

To prevent vision deterioration due to high eye strain, you need to follow certain rules:

• When reading and writing, uniform, sufficient lighting is necessary, which does not cause fatigue;
• the distance from the eyes to the subject of reading, writing or small objects with which you are busy should be about 30-35cm;
• the objects you work with must be placed comfortably for the eyes;
• Watch TV shows no closer than 1.5 meters from the screen. In this case, it is necessary to illuminate the room using a hidden light source.

Of no small importance for maintaining normal vision is a fortified diet in general, and especially vitamin A, which is abundant in animal products, carrots, and pumpkin.

A measured lifestyle, including proper alternation of work and rest, nutrition, excluding bad habits, including smoking and drinking alcoholic beverages, greatly contributes to the preservation of vision and health in general.

The hygienic requirements for preserving the organ of vision are so extensive and varied that the above cannot be limited to. They may vary depending on your work activity, they should be checked with your doctor and followed.