The central fovea of ​​the macula. Age-related macular degeneration

3-10-2014, 15:15

Description

Changes macular spot can develop in isolation, but more often they are a consequence of a general disease of the retina.

Although the fovea centralis of the macula is functionally the most important part of the retina, based on ophthalmoscopic changes in this area it is not always possible to draw a correct conclusion about the degree of impairment central vision, since in some cases, insignificant macular changes cause a sharp decrease in visual acuity, and in other cases, with significant damage to the macula area, central vision remains normal.

It is also necessary to always take into account the fact that even severe changes macular spots may undergo reverse development. As for central vision, it can be restored not only with an ophthalmoscopically noticeable reverse development of changes in the macula, but also with such macular lesions, the picture of which remains unchanged.

Macular diseases are often accompanied by changes in both the choroid and optic nerve.

The macular lesions described below often have quite pronounced ophthalmoscopic signs, but sometimes they are so small that they can only be detected during examination direct form and with a dilated pupil.

Visual acuity in these lesions is in most cases significantly reduced, and on the part of the papilla, a slight blanching of its temporal half is sometimes noted, associated with atrophy of the nerve fibers of the papillomacular bundle, which develops as a result of the death of ganglion cells of the macula, which, therefore, should direct the attention of the ophthalmoscopist to especially thorough examination of the lacular area.

1. Central serous retinitis (retinitis centralis serosa).
This disease, described by some authors under the name "retinitis angiospastica", is associated with impaired permeability the smallest vessels and capillaries, its etiology has not yet been sufficiently elucidated.

The ophthalmoscopic picture is distinguished by the presence of sharply defined retinal edema in the area of ​​the macula, which, in connection with this, even protrudes slightly forward. In the edematous zone, the size of which reaches 6-4 diameters of the optic nerve papilla, small yellowish or grayish-white lesions are noted. After a few weeks, the protrusion of the macula decreases, but the number of lesions may increase, but after 3-4 months, as a rule, all signs of damage to the macula completely disappear.

The disease has a tendency to relapse, which occurs in approximately 30% of cases and can be repeated several times.
The prognosis for both treated and untreated cases is generally good.

2. Macular degeneration in familial amaurotic idiocy. In amaurotic idiocy, there are two forms degenerative changes yellow spots, characteristic of childhood and adolescence.
a) Macular degeneration with familial amaurotic idiocy childhood. Amauritic idiocy of childhood is quite rare disease, affecting children under 2 years of age. Children are usually born healthy, and then, during the first months of life, develop muscle weakness and blindness sets in. Such children also experience rapidly progressing dementia and paralysis.

Ophthalmoscopically, in the area of ​​the macula, a grayish-white turbidity is detected in the form of a horizontal oval, measuring about 1/2-2 the diameter of the papilla. In the center of the turbidity there is a cherry-red spot, as in an embolism central artery. The papilla shows signs of primary atrophy: it is pale and has sharply defined contours. The retinal vessels are not changed.

The disease usually ends in death.

b) Macular degeneration Priamaurotic idiocy of adolescence. This type of macular degeneration occurs in children aged 6-12 years and older, general disease which is characterized by progressive decline mental abilities, paralysis and epileptiform seizures; they usually die at the age of 15-20 years. The disease often affects several family members.

Vision is sometimes impaired even before the appearance of ophthalmoscopic signs, which are as follows: at the very beginning of the disease, there is uneven pigmentation in the area of ​​the macula, later gray spots appear, which gradually acquire a yellowish or orange color.

Eventually, the lesions merge together and occupy a space measuring about 2 papilla diameters, and sometimes more. Pigment spots of varying sizes are often found in the affected area. In the later stages of the disease, individual yellow choroidal vessels are sometimes visible within the fused lesions. On the side of the papilla, pallor of its temporal part is noted, which is associated, as mentioned above, with the death of ganglion cells in the area of ​​the macula.

With amaurotic idiocy of adolescence, another form of retinal damage is observed, which occurs as pigmentary degeneration of the retina.

3. Carpal macular degeneration. Racemose degeneration of the macula is observed with vascular damage, retinal detachment, glaucoma, uveitis and other diseases, after traumatic eye injuries and burns with radiant energy, as well as in old age.

An ophthalmic examination reveals a grayish color change in the central part of the macula, reminiscent of a honeycomb (accumulation of cystic formations).

Subsequently, a holey rupture of the degenerated retina occurs in this place; it has a round or oval shape and differs from the surrounding retina by its dark red color.

The boundaries of the perforated tear are clearly defined; along its edge one can see the remains of the degenerated retina, which have grey colour and the structure of the honeycomb.

In the area of ​​the retinal defect, fine, granular pigmentation is noted (Table 4, Fig. 3). IN initial stages cystic retinal degeneration can be detected only with ophthalmoscopy in excellent light (Table 4, Fig. 4).

Central vision with this lesion of the macula is significantly impaired.

4. Senile macular degeneration (dcgeneratio maniau luteae senilis). Age-related macular degeneration is almost always a bilateral process, which appears to be associated with arteriosclerotic changes in blood vessels in the macular region, leading to disruption of the nutrition of the outer layers of the retina.
There are two types of this disease.

The first type of deheperation is characterized by the fact that the area of ​​the macula, due to mild impairment, acquires a dark brown tint, and dark red and yellowish small patches appear in the center. Sometimes in the macular area, instead these changes, only an accumulation of small lumps of pigment is noticeable.

Over time, the affected area increases very slowly, but, in general, its size rarely exceeds the size of the optic nerve papilla.

IN late stage The disease often develops blanching of the temporal part of the papilla, due to the degeneration of the nerve fibers of the papillomacular bundle, which occurs after the ganglion cells of the macula.

Vision is impaired already at the very beginning of the disease: visual acuity is reduced, a central scotoma appears, but it never reaches complete blindness.

The second type of senile macular degeneration is distinguished by the fact that in the macular area, due to atrophy of the pigment epithelium, a light, wavy-lined focus appears, 1-2 millimeters in size, with a papilla. Changes in both eyes usually have a similar pattern.

IN initial period In this type of macular degeneration, central vision is less impaired than in the first type, and the central scotoma is often not noted for color.

As an exception, there is a form of macular degeneration when changes in the form of a cluster of small gray spots and pigment spots spread beyond the macula and the affected area reaches a size of 1-3 times the diameter of the optic nerve papilla.

5. Perforation of the macula. The hole in the area looks like a sharply defined, round or oval, dark red spot on a grayish-muddy background. In the area of ​​the hole, it is sometimes possible to see exposed pigment epithelium, which is recognized by its characteristic shagreen pattern; occasionally small white or shiny dots are found here.

In general, the ophthalmoscopic picture bears some resemblance to neutral artery embolism, when a cherry-red spot is noted in the area of ​​the macula lutea on a cloudy, gray background. In more late period disease, retinal swelling around the hole usually goes away and the contrast between the color of the spot and the surrounding pink background becomes significantly less (Table 26, Fig. 2).

Important diagnostic sign is that between the edge of the hole and its bottom there is often a parallactic displacement, and there is also a difference in refraction of approximately one diopter.

Over time, the appearance of the hole in the macula usually does not change. On the part of the optic nerve papilla, as with other lesions of the macula, pallor of its temporal part often subsequently develops.

Macular perforation may be due to various diseases: retinal degeneration, chorioretinitis, high myopia, retinal detachment, traumatic injuries eyes.

6. Congenital absence pigment epithelium in the area of ​​the macula - a malformation of the retina, often combined with a defect in the internal (adjacent to the retina) layer of the choroid. Ophthalmoscopically, in the area of ​​the macula and around it, there is an accumulation of irregularly shaped yellowish-red spots that can merge together.

The spots have irregular outlines and myopia: they are bordered by an uneven accumulation of pigment. If there is also a defect in the inner layers of the choroid, yellowish-white areas are visible among the yellowish-red spots, within which the ribbon-like vessels of the chorioid pass (Table 24, Fig. 5).

Congenital absence of pigment in the area of ​​the yellow five is often observed in both eyes.

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Article from the book: ..

The inner far part of the eye is covered with a special fabric. It's called the retina. This tissue sends and receives visual signals. The macula is part of the retina. It is responsible for the stability of central vision. When one or another appears ophthalmological pathologies Vision may be impaired until it is gradually lost. One such disease is macular degeneration. Next, we will consider what this pathology is, how it manifests itself and why it is dangerous.

General information

Age-related macular degeneration - what is it? In general, the pathology is characterized by a deterioration in the condition of the cells that make up this area. Macular degeneration (both eyes or one) usually occurs in older people. It is extremely rare that pathology is diagnosed in young people. For this reason, the disease is often referred to as age-related macular degeneration. Let's look at the disease in more detail.

Classification

Macular degeneration can be of two types:

• Neovascular (wet). In this case, degeneration is caused by growing blood vessels in the retina. Quite often they leak fluid and blood. These processes can lead to irreversible damage in the macular area. The neovascular form is diagnosed in only 10% of patients suffering from the disease. However, this type of pathology accounts for greatest number cases of complete loss of vision.
• Atrophic (dry). In this case, experts point to the gradual death of light-sensitive cells as the cause. This also causes vision loss. On atrophic form Macular degeneration accounts for the majority of cases overall (about 90%).

Causes

Why does macular degeneration occur? Experts have not yet established exact reasons development of this pathology. There are quite a lot of different versions. Some of them are confirmed by research and observations, some remain at the level of theories. Thus, a number of experts argue that with a deficiency of certain mineral compounds and vitamins, a person becomes more susceptible to developing the disease. For example, a number of studies have found that the likelihood of macular degeneration increases several times in the absence of vitamins E and C and antioxidants. Great importance has a deficiency of zinc (it is present in the body, but is concentrated in the area of ​​the visual organs), as well as zeaxanthin and lutein carotenoids. The latter are pigments directly from the macula itself.

Experts name human cytomegalovirus as one of the provoking factors. Some researchers argue that the development of pathology is greatly facilitated by a diet in which the level of saturated fats is very high. In this case, monounsaturated compounds are considered potentially protective. In accordance with some observations, it has been established that it is possible to reduce the likelihood of pathology by taking ω-3 fatty acids. The results of more than ten studies have revealed a statistically significant relationship between macular degeneration and smoking. In this case, the probability of the occurrence of pathology increases by 2-3 times in nicotine abusers (compared to persons who have never smoked). However, five studies found no association.

Risk factors

The likelihood of pathology occurring increases under certain conditions. The most common risk factors include:

• age;
• presence of relatives who suffered or have the disease;
• belonging to the white race;
• smoking;
• being female;
• disturbances in activity of cardio-vascular system(for example, these include increased cholesterol levels and high blood pressure).

Macular Degeneration: Symptoms

The manifestation of pathology is different in all patients. For example, in some patients, macular degeneration may develop quite slowly. In other patients, on the contrary, the course of the disease is rapid, which leads to significant deterioration of vision. Soreness does not accompany either the wet or dry form of the pathology. Some of the main symptoms of macular degeneration include:

• blurred vision;
• distortion of straight lines (for example, the contours of a doorway may appear curved);
• difficulties in the process of considering details (when reading, for example);
• the presence of a small black dot in the center that increases in size over time.

Diagnostic measures

A specialist may suspect the occurrence of degeneration when examining an elderly patient complaining of decreased vision. Special drops are used to dilate the pupils. Thanks to this manipulation, it becomes available for inspection rear end eyes. The Amsler test is also used in the diagnostic process - a sheet with a grid and a black dot in the middle. If, when examining the central mark, the cell lines appear curved (distorted), this may indicate pathology.

Macular degeneration: treatment

As practice shows, in most cases any therapeutic measures are not carried out. Some patients, however, with a dry form of the pathology are prescribed low-intensity, or threshold, laser exposure. Its essence is to remove drusen (specific yellowish deposits) using moderate doses of radiation. Until recently, when wet form pathology method was used photodynamic therapy using Visudin. The drug is administered to the patient intravenously. From systemic blood flow the medication is selectively absorbed exclusively by newly formed regional vessels. Thus, Visudin has virtually no effect on the pigment epithelium in the retina. Along with the use of the drug, a laser therapy session is performed. The procedure is carried out under computer control. Low-intensity radiation is directed to the area of ​​the neovascular membrane (a fiber optic device is used for this). Pathologically dangerous vessels become empty and begin to stick together. As a result, hemorrhages stop. As practice shows, the therapeutic effect lasts for 1-1.5 years.

Modern therapeutic methods

During the research, the drug Ranibizumab was created. The product is intended for administration into the eye cavity. The drug inhibits the activity and development of newly formed vessels and neovascular subretinal membranes. As a result, vision is not only stabilized, but in some cases significantly improved. As a rule, five injections per year are sufficient. The therapeutic course lasts two years. Already after the first injection, most patients experience improvement in vision. The use of the drug "Ranibizumab" is allowed for both dry and wet forms of pathology. Indications also include: The product can be used in combination with photodynamic therapy.

Preventive actions

A person cannot stop the aging process and return age. But it’s quite possible to eliminate a number of things. For example, quit smoking. The environment is of great importance in the prevention of pathology. Experts do not recommend going outside in the middle of a hot day. If such a need arises, the eyes should be protected from direct exposure ultraviolet radiation. Diet is also important. When eating foods rich in cholesterol, the risk of spot degeneration increases significantly. However, eating fish and nuts reduces the risk. Spinach is recommended as a preventative measure.

The eye consists of eyeball with a diameter of 22-24 mm, covered with an opaque shell, sclera, and the front is transparent cornea(or cornea). The sclera and cornea protect the eye and serve as anchorage for the oculomotor muscles.

Iris- a thin vascular plate that limits the passing beam of rays. Light enters the eye through pupil. Depending on the lighting, the diameter of the pupil can vary from 1 to 8 mm.

Lens is an elastic lens that is attached to the muscles ciliary body. The ciliary body changes the shape of the lens. The lens divides inner surface eyes at the anterior chamber, filled with aqueous humor, and the posterior chamber, filled vitreous body.

The inner surface of the rear camera is covered with a photosensitive layer - retina. From the retina, the light signal is transmitted to the brain by optic nerve. Between the retina and sclera is choroid, consisting of a network blood vessels, nourishing the eye.

The retina has yellow spot- the area of ​​the clearest vision. The line passing through the center of the macula and the center of the lens is called visual axis. It is tilted upward from the optical axis of the eye by an angle of about 5 degrees. The diameter of the macula is about 1 mm, and the corresponding field of vision of the eye is 6-8 degrees.

The retina is covered with light-sensitive elements: with chopsticks And cones. Rods are more sensitive to light, but do not distinguish colors and are used for twilight vision. Cones are sensitive to colors but less sensitive to light and therefore serve for daytime vision. In the macula area, cones predominate and rods are few; To the periphery of the retina, on the contrary, the number of cones quickly decreases, and only rods remain.

In the middle of the macula is central fossa. The bottom of the pit is lined only with cones. The diameter of the fovea is 0.4 mm, the field of view is 1 degree.

In the macula, individual fibers of the optic nerve approach most of the cones. Outside the macula, one optic nerve fiber serves a group of cones or rods. Therefore, in the area of ​​the fovea and the macula, the eye can distinguish fine details, and the image falling on the rest of the retina becomes less clear. The peripheral part of the retina serves mainly for orientation in space.

The sticks contain pigment rhodopsin, gathering in them in the dark and fading in the light. The perception of light by rods is due to chemical reactions under the influence of light on rhodopsin. Cones react to light through a reaction iodopsin.

In addition to rhodopsin and iodopsin, there is a black pigment on the posterior surface of the retina. When exposed to light, this pigment penetrates the layers of the retina and, absorbing a significant part of the light energy, protects the rods and cones from strong light exposure.

At the site of the optic nerve trunk is located blind spot. This part of the retina is not sensitive to light. The diameter of the blind spot is 1.88 mm, which corresponds to a field of view of 6 degrees. This means that a person from a distance of 1 m may not see an object with a diameter of 10 cm if its image is projected onto a blind spot.

The optical system of the eye consists of the cornea, aqueous humor, lens and vitreous. Refraction of light in the eye occurs mainly at the cornea and lens surfaces.

Light from the observed object passes through the optical system of the eye and is focused on the retina, forming a reverse and reduced image on it (the brain “inverts” the reverse image, and it is perceived as direct).

The refractive index of the vitreous body is greater than unity, therefore focal lengths the eyes in the outer space (front focal length) and inside the eye (back focal length) are not the same.

The optical power of the eye (in diopters) is calculated as the inverse of the back focal length of the eye, expressed in meters. The optical power of the eye depends on whether it is at rest (58 diopters for a normal eye) or in the state of greatest accommodation (70 diopters).

Accommodation is the ability of the eye to clearly distinguish objects located at different distances. Accommodation occurs due to changes in the curvature of the lens when the muscles of the ciliary body are tense or relaxed. When the ciliary body is taut, the lens stretches and its radii of curvature increase. As muscle tension decreases, the curvature of the lens increases under the influence of elastic forces.

In a free, relaxed state of a normal eye, clear images of infinitely distant objects are obtained on the retina, and with the greatest accommodation, the closest objects are visible.

The position of an object at which a sharp image is created on the retina for an unstrained eye is called farthest point of the eye.

The position of the object at which a sharp image is created on the retina with the greatest possible eye strain is called the closest point of the eye.

When the eye accommodates to infinity, the back focus coincides with the retina. At the highest voltage on the retina, an image of an object located at a distance of about 9 cm is obtained.

The difference between the reciprocals of the distances between the near and far points is called range of accommodation of the eye(measured in diopters).

With age, the eye's ability to accommodate decreases. At the age of 20, for the average eye, the nearest point is at a distance of about 10 cm (accommodation range of 10 diopters), at 50 years of age, the nearest point is already at a distance of about 40 cm (accommodation range of 2.5 diopters), and by the age of 60 it goes to infinity , that is, accommodation stops. This phenomenon is called age-related farsightedness or presbyopia.

Best vision distance is the distance at which a normal eye experiences lowest voltage when examining the details of an object. With normal vision, it averages 25-30 cm.

Adaptation of the eye to changing lighting conditions is called adaptation. Adaptation occurs due to changes in the diameter of the pupil opening, movement of black pigment in the layers of the retina and different reactions to light of rods and cones. The pupil contracts in 5 seconds, and its full dilation occurs in 5 minutes.

Dark adaptation occurs during the transition from high to low brightnesses. In bright light, the cones work, but the rods are “blinded”, the rhodopsin has faded, the black pigment has penetrated into the retina, shielding the cones from the light. At sharp decline brightness, the opening of the pupil opens, allowing more light to pass through. Then the black pigment leaves the retina, rhodopsin is restored, and when there is enough of it, the rods begin to function. Since cones are not sensitive to low brightness, at first the eye does not distinguish anything. The sensitivity of the eye reaches its maximum value after 50-60 minutes of being in the dark.

Light adaptation- this is the process of adaptation of the eye during the transition from low brightness to high brightness. At first, the rods are greatly irritated, “blinded” due to the rapid decomposition of rhodopsin. The cones, not yet protected by grains of black pigment, are also too irritated. After 8-10 minutes the feeling of blindness stops and the eye sees again.

line of sight the eyes are quite wide (125 degrees vertically and 150 degrees horizontally), but only a small part of it is used for clear discrimination. The field of the most perfect vision (corresponding to the fovea) is about 1-1.5°, satisfactory (in the area of ​​the entire macula) is about 8° horizontally and 6° vertically. The rest of the visual field serves for rough orientation in space. To view the surrounding space, the eye has to make a continuous rotational movement in its orbit within 45-50°. This rotation brings images of various objects to the fovea and makes it possible to examine them in detail. Eye movements occur without the participation of consciousness and, as a rule, are not noticed by a person.

Angular limit of eye resolution- this is the minimum angle at which the eye observes two luminous points separately. The angular resolution limit of the eye is about 1 minute and depends on the contrast of objects, illumination, pupil diameter and wavelength of light. In addition, the resolution limit increases as the image moves away from the fovea and in the presence of visual defects.

Visual defects and their correction

With normal vision, the far point of the eye is infinitely distant. This means that the focal length of the relaxed eye is equal to the length of the eye axis, and the image falls exactly on the retina in the area of ​​the fovea.

Such an eye can distinguish objects well in the distance, and with sufficient accommodation, also near objects.

Myopia

With myopia, rays from an infinitely distant object are focused in front of the retina, so a blurry image is formed on the retina.

Most often this occurs due to elongation (deformation) of the eyeball. Less commonly, myopia occurs when normal length eyes (approx. 24 mm) due to too much optical power optical system eyes (more than 60 diopters).

In both cases, the image from distant objects is inside the eye and not on the retina. The retina receives only the focus from objects close to the eye, that is, the far point of the eye is at a finite distance in front of it.

Far point of the eye

Myopia is corrected using negative lenses, which construct an image of an infinitely distant point at the farthest point of the eye.

Far point of the eye

Myopia most often appears in childhood and adolescence, and as the eyeball grows in length, myopia increases. True myopia is usually preceded by the so-called false myopia- a consequence of a spasm of accommodation. In this case, normal vision can be restored with the help of means that dilate the pupil and relieve tension in the ciliary muscle.

Farsightedness

With farsightedness, rays from an infinitely distant object are focused behind the retina.

Farsightedness is caused by poor optical power eyes for a given length of the eyeball: either a short eye with normal optical power, or low optical power eyes at normal length.

To focus the image on the retina, you have to constantly strain the muscles of the ciliary body. The closer objects are to the eye, the further their image goes beyond the retina and the more effort the eye muscles require.

The far point of the farsighted eye is behind the retina, i.e., in a relaxed state, it can clearly see only an object that is behind it.

Far point of the eye

Of course, you cannot place an object behind the eye, but you can project its image there using positive lenses.

Far point of the eye

With slight farsightedness, distance and near vision is good, but there may be complaints about fatigue and headache at work. At medium degree farsightedness, distance vision remains good, but near vision is difficult. With high farsightedness, both distance and near vision becomes poor, since all the eye’s ability to focus images on the retina even of distant objects has been exhausted.

The newborn's eye is slightly compressed horizontal direction Therefore, the eye has a slight farsightedness, which disappears as the eyeball grows.

Ametropia

Ametropia (nearsightedness or farsightedness) of the eye is expressed in diopters as the reciprocal of the distance from the surface of the eye to the far point, expressed in meters.

The optical power of the lens required to correct myopia or hyperopia depends on the distance from the glasses to the eye. Contact lenses are placed close to the eye, so their optical power is equal to ametropia.

For example, if, with myopia, the far point is located in front of the eye at a distance of 50 cm, then to correct it you need contact lenses with an optical power of −2 diopters.

A weak degree of ametropia is considered to be up to 3 dioptres, a medium degree is considered to be from 3 to 6 dioptres, and a high degree is considered to be above 6 dioptres.

Astigmatism

With astigmatism, the focal lengths of the eye are different in different sections passing through its optical axis. With astigmatism in one eye, the effects of myopia, farsightedness and farsightedness are combined. normal vision. For example, an eye may be nearsighted in a horizontal section and farsighted in a vertical section. Then at infinity he will not be able to clearly see horizontal lines, but he will clearly distinguish vertical ones. At a close distance, on the contrary, such an eye sees vertical lines well, but horizontal ones will be blurry.

The cause of astigmatism is either irregular shape cornea, or in the deviation of the lens from the optical axis of the eye. Astigmatism is most often congenital, but can result from surgery or eye injury. In addition to visual perception defects, astigmatism is usually accompanied by eye fatigue and headaches. Astigmatism is corrected using cylindrical (converging or diverging) lenses in combination with spherical lenses.

Rays of light falling on the retina do not excite all of its areas. The entry point of the optic nerve is a blind spot, insensitive to light, so the rays hitting it are lost and the image disappears.

The most sensitive place of the retina, as we already know, is the yellow spot and the depression that is in its center, - central hole

Being richly supplied with cones, the fovea is the place of best vision. Therefore, when examining an object, a person tries to position this object so that the rays from it fall on the central fossa. It is quite clear that in this way a person establishes an object unconsciously.

Rice.5. Ocular fundus. 1 - yellow spot; 2 - central fossa; 3 - blind spot; 4 - retinal arteries; 5 - veins

The role of rods and cones in daytime and twilight vision

Cones are cells that carry out daytime and color vision. When exposed to sunlight or bright electric light, the cones are excited. Rods provide twilight, night vision.

Under the influence of light, physical and chemical processes. The rods contain a special substance called visual purple, or rhodopsin. Under the influence of light, visual purple undergoes changes. In the light it disintegrates, and in the dark it is restored.

It is assumed that when visual purple disintegrates, substances are formed that, acting on the endings of the optic nerve, cause excitation in it.

The chemical structure of visual purple is based on vitamin A, the supply of which is mandatory for the synthesis of visual purple and, therefore, normal night vision.

IN Lately a special light-sensitive substance was also found in cones. The formation of this substance, similar to visual purple, occurs in the dark, and destruction occurs under the influence of light. It differs from visual purple in that its decomposition proceeds 4 times slower than the decomposition of visual purple.

Night blindness

Disruption of the normal activity of the rod layer in the retina causes a disease known as night blindness.

The disease lies in the fact that, although the patient sees perfectly during the day and in bright light does not show any signs of visual impairment, in the evening, as soon as twilight sets in, vision is impaired and the patient almost ceases to see; When darkness falls, he completely loses his sight.

Night blindness is often caused by a lack of vitamin A in food. This circumstance suggests that the basis of night blindness is a violation of the formation of visual purpura. This is confirmed by the fact that night blindness easy to cure when provided with food for the patient sufficient quantity vitamin A.

Feeling the colors

All the objects he sees human eye, have one color or another. Light is perceived by our eyes when the light wave oscillates within 400-800 millimicrons (a millimicron is one millionth of a millimeter).

If you miss the beam white light through a prism and thereby decompose it, it is divided into several colors, which are arranged in a certain order. The resulting arrangement of different colors with their divisions into adjacent colors is called the light spectrum.

At one end of the spectrum is red, which has a wavelength of 800 millimicrons, and at the other end is violet, which has a wavelength of 400 millimicrons. Between them are other colors. If you count from the end where it is located purple, then the spectrum will be in the following order: violet, blue, cyan, bluish-green, green, yellow, orange, red. Rays with a wavelength longer than 800 millimicrons (infrared) and shorter than 400 millimicrons (ultraviolet) are not perceived by our eyes. Between the 8 colors of the spectrum there is a very a large number of passing colors. Our eye distinguishes about 200 such transitional colors.

The colors of objects are perceived by us depending on the ability of the object to absorb or reflect light waves of different lengths. If an object absorbs some light waves and reflects others, it will have the color of those waves that are reflected by its surface.

So, for example, if an object reflects light with a wavelength of 580 millimicrons, it will appear green; in the case of reflection of waves with a length of 500 millimicrons, its color will be blue. The reflection of all waves of the spectrum causes a feeling white, and when an object absorbs all colors, it will appear black. Between white and black lies gray with various shades. If you pass a white ray of sunlight through a prism, it will decompose into the colors of the spectrum. A similar phenomenon can be observed after rain, when a rainbow forms in the sky, which represents decomposition sunbeam into individual components.

The cellular elements of the retina that perceive color are cones. Rods do not perceive the color of an object. Therefore, at night, when we see only with the help of a rod apparatus, all objects appear equally gray.

Colors are best perceived by those parts of the retina that are rich in cones, i.e. the most color-sensitive are the macula and the fovea.

Color blindness

Exists certain type vision disorders when a person partially or completely loses color perception. This disease is called color blindness. Quite rare is complete color blindness. A person suffering from this disorder does not perceive any colors. Everything around him has only one gray color various shades. One type of violation color vision is color blindness (named after the English chemist Dalton, in whom color blindness was first discovered). Colorblind people usually cannot distinguish between red and green colors. Different shades of these colors are perceived as different shades of gray. Color blindness is a disease that is widespread. Men suffer from it more often than women. About 4-5% of all men suffer from color blindness, while the number of affected women does not exceed 0.5%.

To detect color blindness, special tables are used. Not everyone suffering from color blindness knows about their disease. Sometimes years pass before this color vision disorder is discovered.

More rare than people who are color blind to red and green are people who are yellow and violet color blind.

Eye adaptation

Adaptation of the eye to vision varying degrees illumination is called adaptation.

Everyone knows perfectly well that if you enter a dark room from a brightly lit room or from a sunlit street, then at first a person sees nothing. Then the eye gradually begins to get used to it and the person can already distinguish the contours of objects, and after a while even all the details. All this occurs due to changes in the sensitivity of the eye. The sensitivity of the retina in a dark room increases and a person gradually begins to see. Adaptation of the eye to vision in a dark room is called dark adaptation.

The sensitivity of the eye during dark adaptation increases approximately 200 thousand times. This enormous increase in sensitivity occurs after being in the dark for 60-80 minutes. Especially sharp increase sensitivity is observed in the first minutes.

An increase in retinal excitability is simultaneously accompanied by a certain chemical process.

When staying in a brightly lit room, visual purple completely disintegrates. Therefore, the sticks, which are photosensitive element, with the help of which we see in the dark, are not excited. In the dark, visual purple is restored.

A slightly different phenomenon is observed when moving from a dark room to a brightly lit room. At first, a person does not see anything, he is blinded. There is pain in his eyes, tears flow, and he is forced to close his eyes. Then the eyes begin to gradually get used to it and normal vision is soon restored.

Adaptation of the eye to seeing objects in bright light is called light adaptation.

With light adaptation, the sensitivity of the eye decreases sharply. Light adaptation, unlike dark adaptation, occurs within 1-2 minutes.

Visual acuity

The eye makes it possible to see an object, distinguish its shape, color, size, distance at which it is located, and also determine the direction in which it is moving. In order to clearly distinguish a form, a person must clearly see the boundaries and details of the object. The ability to distinguish small details of an object under consideration is the basis of the so-called visual acuity. Visual acuity is determined by the smallest distance that must be between two points for the eye to perceive them separately. The smaller this distance is when perceiving two points, the sharper vision. The macula and fovea have the greatest visual acuity. The further to the periphery from the macula, the lower the visual acuity. Thus, the value of visual acuity in to a large extent associated with the activity of cones. At night, visual acuity decreases sharply.

To measure a person’s visual acuity, they use special tables that have letters or some other designation.

The largest letters are on the top line, then the letters gradually become smaller and become the smallest on the bottom line.

When determining visual acuity, a person must be at a distance of 5 m from the table hanging on the wall. First, the visual acuity of one eye is determined, and then the other. During the determination, the test subject covers the other eye with a piece of paper or hand. After the eye is covered, the subject is asked to read the letters. The test starts with larger letters. The indicator of visual acuity is considered to be the line with the smallest letters on which the subject can distinguish several letters.

There is a line in the table that corresponds to full visual acuity and is indicated by an indicator of 1.0. If the subject can only read those letters that are above the line taken as 1.0, then visual acuity is considered below normal. Visual acuity decreases by 0.1 for each unread line above normal. For example, if a subject can read the letters of a line that is directly above a line that has an index of 1.0, visual acuity is considered to be 0.9, if this is the second line - 0.8, etc.

) eyes of vertebrates and humans; It has an oval shape, located opposite the pupil, slightly above the entry point of the optic nerve into the eye. The cells of the belly contain a yellow pigment (hence the name). Blood capillaries are present only in the lower part of the housing; in its middle part, the retina becomes very thin, forming a central fovea (fovea), containing only photoreceptors. Most animals and humans have only cone cells in the fovea; some deep-sea fish with telescopic eyes have only rod cells in the central fovea. Birds with good vision may have up to three central foveae. In humans, the diameter of the spot is about 5 mm, in the fovea the cones are rod-like (the longest receptors in the retina). The diameter of the area free of rod cells is 500-550 µm; There are about 30 thousand cone cells here.

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

See what the “Yellow Spot” is in other dictionaries:

- (lat. macula lutea) the place of greatest visual acuity in the retina of vertebrates, including humans. It has an oval shape, located opposite the pupil, slightly above the entry point of the optic nerve into the eye. In the cells of the macula... ... Wikipedia

The place of greatest visual acuity in the retina (maximum concentration of photoreceptors). The cells of the macula contain yellow pigment (hence the name). * * * YELLOW SPOT YELLOW SPOT, the place of greatest visual acuity in the retina... ... encyclopedic Dictionary

- (macula lutea), the area of ​​maximum photoreceptor concentration and highest visual acuity in the vertebrate retina. Contains yellow pigments carotenoids (hence the name). Located in the pentrum of the fundus along the line of passage of the optic. axis or shifted to... ... Biological encyclopedic dictionary

yellow spot- geltonoji dėmė statusas T sritis fizika atitikmenys: engl. yellow strain vok. gelber Fleck, m rus. yellow spot, n pranc. tache jaune, f … Fizikos terminų žodynas

- (macula lutea, BNA, JNA) see Spot... Medical encyclopedia

Place max. visual acuity in the retina (maximum concentration of photoreceptors). The cells of the belly contain a yellow pigment (hence the name) ... Natural science. encyclopedic Dictionary

Noun, s., used. often Morphology: (no) what? stains, why? spot, (see) what? stain of what? stain, what about? about the stain; pl. What? spots, (no) what? stains, why? spots, (see) what? stains, what? spots, about what? about stains 1. A stain is a stained... ... Dictionary Dmitrieva

spot- A/; pl. pya/tna, gen. ten, dat. tnam; Wed see also spot 1) Stained with something. place on what l. surfaces. Dirty, greasy stain/. Coffee, oil, oil stain/. Sauce stain... Dictionary of many expressions

A; pl. spots, gen. ten, dat. tnam; Wed 1. Stained with something. place on what l. surfaces. Dirty, greasy p. Coffee, oil, oil p. from sauce. P. blood. Removing stains. Place the item on the dress. The whole skirt is stained. 2. About the fact that... ... encyclopedic Dictionary