The structure and functions of the human visual organs. Eyeball and auxiliary apparatus. Visual analyzer The passage of light rays through the optical structures of the eye

Separate parts of the eye (cornea, lens, vitreous body) have the ability to refract rays passing through them. WITH from the point of view of eye physics represents yourself an optical system capable of collecting and refracting rays.

Refracting the strength of individual parts (lenses in the device re) and the entire optical system of the eye is measured in diopters.

Under One diopter is the refractive power of a lens whose focal length is 1 m. If refractive power increases, focal length increases is working. From here it follows that a lens with a focal a distance of 50 cm will have a refractive power equal to 2 diopters (2 D).

The optical system of the eye is very complex. It is enough to point out that there are only several refractive media, and each medium has its own refractive power and structural features. All this makes it extremely difficult to study the optical system of the eye.

Rice. Construction of an image in the eye (explanation in the text)

The eye is often compared to a camera. The role of the camera is played by the eye cavity, darkened by the choroid; The photosensitive element is the retina. The camera has a hole into which the lens is inserted. Rays of light entering the hole pass through the lens, are refracted and fall on the opposite wall.

The optical system of the eye is a refractive collecting system. It refracts the rays passing through it and again collects them into one point. In this way, a real image of a real object appears. However, the image of the object on the retina is reversed and reduced.

To understand this phenomenon, let's look at the schematic eye. Rice. gives an idea of the path of rays in the eye and obtaining a reverse image of an object on the retina. A ray emanating from the upper point of an object, indicated by the letter a, passing through the lens, is refracted, changes direction and takes the position of the lower point on the retina, indicated in the figure. A 1 A ray from the bottom point of an object, being refracted, falls on the retina as the top point in 1 . Rays from all points fall in the same way. Consequently, a real image of the object is obtained on the retina, but it is reversed and reduced.

Thus, calculations show that the size of the letters of a given book, if when reading it is at a distance of 20 cm from the eye, on the retina will be equal to 0.2 mm. the fact that we see objects not in their inverted image (upside down), but in their natural form, is probably explained by accumulated life experience.

In the first months after birth, a child confuses the top and bottom sides of an object. If such a child is shown a burning candle, the child, trying to grab the flame, will extend his hand not to the upper, but to the lower end of the candle. By controlling the readings of the eye with his hands and other senses throughout his later life, a person begins to see objects as they are, despite their reverse image on the retina.

Accommodation of the eye. A person cannot simultaneously see objects at different distances from the eye equally clearly.

In order to see an object well, it is necessary that the rays emanating from this object are collected on the retina. Only when the rays fall on the retina do we see a clear image of the object.

The adaptation of the eye to obtain distinct images of objects located at different distances is called accommodation.

In order to obtain a clear image in each caseTherefore, it is necessary to change the distance between the refractive lens and the back wall of the camera. This is how the camera works. To get a clear image on the back of the camera, move the lens closer or closer. Accommodation occurs according to this principle in fish. With the help of a special device, their lens moves away or moves closer to the back wall of the eye.

Rice. 2 CHANGE IN THE CURVATURE OF THE LENS DURING ACCOMMODATION 1 - lens; 2 - lens bag; 3 - ciliary processes. The top picture is an increase in the curvature of the lens. The ciliary ligament is relaxed. Bottom picture - the curvature of the lens is reduced, the ciliary ligaments are tense.

However, a clear image can also be obtained if the refractive power of the lens changes, and this is possible when its curvature changes.

According to this principle, accommodation occurs in humans. When seeing objects located at different distances, the curvature of the lens changes and due to this, the point where the rays converge approaches or moves away, hitting the retina each time. When a person examines close objects, the lens becomes more convex, and when viewing distant objects, it becomes flatter.

How does the curvature of the lens change? The lens is in a special transparent bag. The curvature of the lens depends on the degree of tension of the bag. The lens has elasticity, so when the bag is stretched, it becomes flat. When the bag relaxes, the lens, due to its elasticity, acquires a more convex shape (Fig. 2). The change in the tension of the bag occurs with the help of a special circular accommodative muscle, to which the capsule ligaments are attached.

When the accommodative muscles contract, the ligaments of the lens bag weaken and the lens takes on a more convex shape.

The degree of change in the curvature of the lens depends on the degree of contraction of this muscle.

If an object located at a far distance is gradually brought closer to the eye, then at a distance of 65 m accommodation begins. As the object approaches the eye further, the accommodative efforts increase and at a distance of 10 cm they are exhausted. Thus, the point of near vision will be at a distance of 10 cm. With age, the elasticity of the lens gradually decreases, and consequently, the ability to accommodate also changes. The closest point of clear vision for a 10-year-old is at a distance of 7 cm, for a 20-year-old - at a distance of 10 cm, for a 25-year-old - 12.5 cm, for a 35-year-old - 17 cm, for a 45-year-old - 33 cm, in a 60-year-old - 1 m, in a 70-year-old - 5 m, in a 75-year-old, the ability to accommodate is almost lost and the nearest point of clear vision is pushed back to infinity.

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.

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 the 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. Consequently, 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 very front part of the eye is called the cornea. It is transparent (transmits light) and convex (refracts light).

Behind the cornea is Iris, in the center of which there is a hole - the pupil. The iris is made up of muscles that can change the size of the pupil and thus regulate the amount of light entering the eye. The iris contains the pigment melanin, which absorbs harmful ultraviolet rays. If there is a lot of melanin, then the eyes are brown, if the average amount is green, if there is little, they are blue.

The lens is located behind the pupil. This is a transparent capsule filled with liquid. Due to its own elasticity, the lens tends to become convex, while the eye focuses on close objects. When the ciliary muscle relaxes, the ligaments holding the lens tighten and it becomes flat, the eye focuses on distant objects. This property of the eye is called accommodation.

Located behind the lens vitreous, filling the eyeball from the inside. This is the third and final component of the refractive system of the eye (cornea - lens - vitreous).

Behind the vitreous body, on the inner surface of the eyeball, is the retina. It consists of visual receptors - rods and cones. Under the influence of light, receptors are excited and transmit information to the brain. The rods are located mainly on the periphery of the retina, they provide only a black and white image, but they only need low lighting (they can work in the twilight). The visual pigment of the rods is rhodopsin, a derivative of vitamin A. The cones are concentrated in the center of the retina, they produce a color image and require bright light. There are two spots in the retina: the yellow spot (it has the highest concentration of cones, the place of greatest visual acuity) and the blind spot (it has no receptors at all, the optic nerve emerges from this place).

Behind the retina (the innermost layer of the eye) is located choroid(average). It contains blood vessels that supply the eye; in the front part it changes into iris and ciliary muscle.

Behind the choroid is located tunica albuginea, covering the outside of the eye. It performs a protective function; in the front part of the eye it is modified into the cornea.

Choose one, the most correct option. The function of the pupil in the human body is
1) focusing light rays onto the retina
2) regulation of luminous flux
3) transformation of light stimulation into nervous excitation
4) color perception

Answer

Choose one, the most correct option. A black pigment that absorbs light is located in the human organ of vision in
1) blind spot
2) choroid
3) tunica albuginea
4) vitreous body

Answer

Choose one, the most correct option. The energy of light rays entering the eye causes nervous excitement
1) in the lens
2) in the vitreous body
3) in visual receptors
4) in the optic nerve

Answer

Choose one, the most correct option. Behind the pupil in the human organ of vision is located
1) choroid
2) vitreous body
3) lens
4) retina

Answer

1. Establish the path of the light beam in the eyeball
1) pupil
2) vitreous body
3) retina
4) lens

Answer

2. Establish the sequence of passage of the light signal to the visual receptors. Write down the corresponding sequence of numbers.
1) pupil
2) lens
3) vitreous body
4) retina
5) cornea

Answer

3. Establish the sequence of arrangement of the structures of the eyeball, starting with the cornea. Write down the corresponding sequence of numbers.
1) retinal neurons
2) vitreous body
3) pupil in the pigment membrane
4) light-sensitive rod and cone cells
5) convex transparent part of the tunica albuginea

Answer

4. Establish the sequence of signals passing through the sensory visual system. Write down the corresponding sequence of numbers.
1) optic nerve
2) retina
3) vitreous body
4) lens
5) cornea
6) visual cortex

Answer

5. Establish the sequence of processes of passage of a ray of light through the organ of vision and a nerve impulse in the visual analyzer. Write down the corresponding sequence of numbers.
1) conversion of a ray of light into a nerve impulse in the retina
2) information analysis
3) refraction and focusing of a light beam by the lens
4) transmission of nerve impulses along the optic nerve
5) passage of light rays through the cornea

Answer

Choose one, the most correct option. The light-sensitive receptors of the eye - rods and cones - are located in the membrane
1) rainbow
2) protein
3) vascular
4) mesh

Answer

1. Choose the three correct options: light-refracting structures of the eye include:
1) cornea
2) pupil
3) lens
4) vitreous body
5) retina
6) yellow spot

Answer

2. Choose three correct answers out of six and write down the numbers under which they are indicated. The optical system of the eye consists of
1) lens
2) vitreous body
3) optic nerve
4) macula of the retina
5) cornea
6) tunica albuginea

Answer

1. Select three correctly labeled captions for the drawing “Structure of the eye.” Write down the numbers under which they are indicated.
1) cornea
2) vitreous body
3) iris
4) optic nerve
5) lens
6) retina

Answer

2. Select three correctly labeled captions for the drawing “Structure of the eye.” Write down the numbers under which they are indicated.
1) iris
2) cornea
3) vitreous body
4) lens
5) retina
6) optic nerve

Answer

3. Select three correctly labeled captions for the picture that depicts the internal structure of the organ of vision. Write down the numbers under which they are indicated.
1) pupil
2) retina
3) photoreceptors
4) lens
5) sclera
6) yellow spot

Answer

4. Select three correctly labeled captions for the picture that depicts the structure of the human eye. Write down the numbers under which they are indicated.
1) retina
2) blind spot
3) vitreous body
4) sclera
5) pupil
6) cornea

Answer

Establish a correspondence between visual receptors and their characteristics: 1) cones, 2) rods. Write numbers 1 and 2 in the correct order.
A) perceive colors
B) active in good lighting
B) visual pigment rhodopsin
D) exercise black-and-white vision
D) contain the pigment iodopsin
E) distributed evenly across the retina

Answer

Choose three correct answers out of six and write down the numbers under which they are indicated. The differences between human daytime vision and twilight vision are that
1) cones work
2) color discrimination is not carried out
3) visual acuity is low
4) sticks work
5) color discrimination is carried out
6) visual acuity is high

Answer

Choose one, the most correct option. When viewing an object, a person’s eyes continuously move, providing
1) prevention of eye blindness
2) transmission of impulses along the optic nerve
3) the direction of light rays to the macula of the retina
4) perception of visual stimuli

Answer

Choose one, the most correct option. Human vision depends on the condition of the retina, since it contains light-sensitive cells in which
1) vitamin A is formed
2) visual images arise
3) black pigment absorbs light rays
4) nerve impulses are formed

Answer

Establish a correspondence between the characteristics and membranes of the eyeball: 1) albuginea, 2) vascular, 3) retina. Write numbers 1-3 in the order corresponding to the letters.
A) contains several layers of neurons
B) contains pigment in cells
B) contains the cornea
D) contains the iris
D) protects the eyeball from external influences
E) contains a blind spot

Answer

, lens and vitreous body. Their combination is called a diopter apparatus. Under normal conditions, light rays are refracted (bended) 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.