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The structure of the human eye. How is it arranged? Expert opinion. General structure of the organ of vision Visual defects and their correction

The pigment layer from the inside is adjacent to the structure of the eye, referred to as Bruch's membrane. The thickness of this membrane is from 2 to 4 microns, it is also called a vitreous plate due to its complete transparency. The functions of the Bruch's membrane are to create antagonism of the ciliary muscle at the time of accommodation. Bruch's membrane also delivers nutrients and fluids to the pigment layer of the retina and to the choroid.

As the body ages, the membrane thickens and its protein composition changes. These changes lead to a slowdown in metabolic reactions, and the pigment epithelium in the form of a layer also develops in the boundary membrane. The ongoing changes indicate age-related diseases of the retina.

The size of the retina of an adult eye reaches 22 mm and covers approximately 72% of the entire area of ​​the internal surfaces of the eyeball. The pigment epithelium of the retina, that is, its outermost layer, is associated with the choroid of the human eye more closely than with other structures of the retina.

In the center of the retina, in the part that is closer to the nose, on the back side of the surface there is an optic disc. There are no photoreceptors in the disc, and therefore it is designated in ophthalmology by the term "blind spot". In the photo taken during microscopic examination of the eye, the "blind spot" looks like an oval shape of a pale shade, slightly rising above the surface and having a diameter of about 3 mm. It is in this place that the primary structure of the optic nerve begins from the axons of ganglionic neurocytes. The central part of the human retinal disc has a depression through which the vessels pass. Their function is to supply blood to the retina.

On the side of the optic disc, at a distance of about 3 mm, there is a spot. In the central part of this spot is located the central fossa - a recess, which is the most sensitive area of ​​​​the human retina to the light flux.

The fovea fovea is the so-called "yellow spot", which is responsible for clear and sharp central vision. In the "yellow spot" of the human retina, there are only cones.

Humans (as well as other primates) have their own peculiarities in the structure of the retina. Humans have a central fovea, while some species of birds, as well as cats and dogs, have a "optic streak" instead of this fovea.

The retina in its central part is represented only by the fovea and the area surrounding it, which is located within a radius of 6 mm. Then comes the peripheral part, where the number of cones and rods gradually decreases towards the edges. All the inner layers of the retina end with a jagged edge, the structure of which does not imply the presence of photoreceptors.

The thickness of the retina throughout its length is not the same. In the thickest part near the edge of the optic disc, the thickness reaches 0.5 mm. The smallest thickness was found in the region of the corpus luteum, or rather its fossa.

Microscopic structure of the retina

The anatomy of the retina at the microscopic level is represented by several layers of neurons. There are two layers of synapses and three layers of nerve cells located radically.
In the deepest part of the human retina, there are ganglion neurons, rods and cones, while they are farthest from the center. In other words, this structure makes the retina an inverted organ. That is why light, before reaching the photoreceptors, must penetrate all the inner layers of the retina. However, the light flux does not penetrate the pigment epithelium and choroid, as they are opaque.

There are capillaries in front of the photoreceptors, which is why leukocytes, when looking at a blue light source, are often perceived as tiny moving dots that have a light color. Such features of vision in ophthalmology are referred to as the Shearer phenomenon or the entopic phenomenon of the blue field.

In addition to ganglion neurons and photoreceptors, there are also bipolar nerve cells in the retina, their functions are to transfer contacts between the first two layers. Horizontal connections in the retina are carried out by amacrine and horizontal cells.

On a highly enlarged photo of the retina, between the layer of photoreceptors and the layer of ganglion cells, you can see two layers consisting of plexuses of nerve fibers and having many synaptic contacts. These two layers have their own names - the outer plexiform layer and the inner plexiform layer. The functions of the first are to make continuous contacts between cones and rods and also between vertical bipolar cells. The inner plexiform layer switches the signal from bipolar cells to ganglion neurons and to amacrine cells located in the horizontal and vertical direction.

From this we can conclude that the nuclear layer, located outside, contains photosensory cells. The inner nuclear layer includes the bodies of bipolar amacrine and horizontal cells. The ganglionic layer directly includes the ganglionic cells themselves and also a small number of amacrine cells. All layers of the retina are permeated with Muller cells.

The structure of the outer limiting membrane is represented by synaptic complexes, which are located between the outer layer of ganglion cells and between photoreceptors. The layer of nerve fibers is formed by the axons of ganglion cells. The basement membranes of Müller cells and the endings of their processes take part in the formation of the inner limiting membrane. Axons of ganglion cells that do not have Schwann membranes, having reached the inner border of the retina, turn at a right angle and go to the place where the optic nerve is formed.
The retina of any person contains from 110 to 125 million rods and from 6 to 7 million cones. These photosensitive elements are located unevenly. In the central part there is the maximum number of cones, in the peripheral part there are more rods.

Retinal diseases

Many acquired and hereditary eye diseases have been identified, in which the retina can also be involved in the pathological process. This list includes the following:

  • pigmentary degeneration of the retina (it is hereditary, with its development the retina is affected and peripheral vision is lost);
  • macular degeneration (a group of diseases, the main symptom of which is the loss of central vision);
  • macular degeneration of the retina (also hereditary, associated with a symmetrical bilateral lesion of the macular zone, loss of central vision);
  • rod-cone dystrophy (occurs when the photoreceptors of the retina are damaged);
  • retinal detachment (separation from the back of the eyeball, which can occur under the influence of inflammation, degenerative changes, as a result of injuries);
  • retinopathy (caused by diabetes mellitus and arterial hypertension);
  • retinoblastoma (malignant tumor);
  • macular degeneration (pathology of blood vessels and malnutrition of the central region of the retina).

The eyeball consists of three shells: outer, middle and inner. The outer, or fibrous, membrane is formed from dense connective tissue - the cornea (in front) and the opaque sclera, or tunica (back). The middle (vascular) membrane contains blood vessels and consists of three sections:

1) anterior section (iris, or iris). The iris contains smooth muscle fibers that make up two muscles: a circular, constricting pupil, located almost in the center of the iris, and a radial, dilating the pupil. Closer to the anterior surface of the iris is a pigment that determines the color of the eye and the opacity of this shell. The iris adjoins with its back surface to the lens;

2) middle section (ciliary body). The ciliary body is located at the junction of the sclera with the cornea and has up to 70 ciliary radial processes. Inside the ciliary body is the ciliary, or ciliary, muscle, which consists of smooth muscle fibers. The ciliary muscle is attached by ciliary ligaments to the tendon ring and the lens bag;

3) the posterior section (the choroid itself).

The most complex structure has an inner shell (retina). The main receptors in the retina are rods and cones. The human retina contains about 130 million rods and about 7 million cones. Each rod and cone has two segments - outer and inner, the cone has a shorter outer segment. The outer segments of the rods contain visual purple, or rhodopsin (purple substance), in the outer segments of the cones - iodopsin (violet). The inner segments of the rods and cones are connected to neurons that have two processes (bipolar cells) that are in contact with ganglion neurons that are part of the optic nerve with their fibers. Each optic nerve contains about 1 million nerve fibers.

The distribution of rods and cones in the retina has the following order: in the middle of the retina there is a central fovea (yellow spot) with a diameter of 1 mm, it contains only cones, closer to the central fovea are cones and rods, and on the periphery of the retina - only rods. In the fovea, each cone is connected to one neuron through a bipolar cell, and to the side of it, several cones are also connected to one neuron. Rods, unlike cones, are connected to one bipolar cell in several pieces (about 200). Due to this structure, the greatest visual acuity is provided in the fovea. At a distance of approximately 4 mm medially from the central fossa is the papilla of the optic nerve (blind spot), in the center of the nipple are the central artery and the central vein of the retina.

Between the posterior surface of the cornea and the anterior surface of the iris and part of the lens is the anterior chamber of the eye. Between the posterior surface of the iris, the anterior surface of the ciliary ligament and the anterior surface of the lens is the posterior chamber of the eye. Both chambers are filled with transparent aqueous humor. The entire space between the lens and the retina is occupied by a transparent vitreous body.

Light refraction in the eye. The refractive media of the eye include: the cornea, the aqueous humor of the anterior chamber of the eye, the lens and the vitreous body. In many ways, the clarity of vision depends on the transparency of these media, but the refractive power of the eye depends almost entirely on the refraction in the cornea and lens. Refraction is measured in diopters. The diopter is the reciprocal of the focal length. The refractive power of the cornea is constant and equal to 43 diopters. The refractive power of the lens is unstable and varies over a wide range: when looking at the near distance - 33 diopters, at a distance - 19 diopters. The refractive power of the entire optical system of the eye: when looking into the distance - 58 diopters, at a short distance - 70 diopters.

Parallel light rays, after refraction in the cornea and lens, converge to one point in the fovea. The line passing through the centers of the cornea and lens to the center of the macula is called the visual axis.

Accommodation. The ability of the eye to clearly distinguish objects at different distances is called accommodation. The phenomenon of accommodation is based on the reflex contraction or relaxation of the ciliary, or ciliary, muscle, innervated by the parasympathetic fibers of the oculomotor nerve. Contraction and relaxation of the ciliary muscle changes the curvature of the lens:

a) when the muscle contracts, the ciliary ligament relaxes, which causes an increase in light refraction, because the lens becomes more convex. Such a contraction of the ciliary muscle, or visual tension, occurs when an object approaches the eye, that is, when viewing an object that is as close as possible;

b) when the muscle relaxes, the ciliary ligaments stretch, the lens bag squeezes it, the curvature of the lens decreases and its refraction decreases. This occurs when the object is removed from the eye, i.e., when looking into the distance.

The contraction of the ciliary muscle begins when an object approaches a distance of about 65 m, then its contractions increase and become distinct when an object approaches a distance of 10 m. Further, as the object approaches, the contractions of the muscles increase more and more and finally reach the limit at which clear vision becomes impossible. The minimum distance from an object to the eye at which it is clearly visible is called the nearest point of clear vision. In a normal eye, the far point of clear vision is at infinity.

Farsightedness and myopia. A healthy eye, when looking into the distance, refracts a beam of parallel rays so that they are focused in the fovea. With myopia, parallel rays are focused in front of the fovea, diverging rays fall into it and therefore the image of the object is blurred. The causes of myopia may be the tension of the ciliary muscle during accommodation at a close distance or too long the longitudinal axis of the eye.

In farsightedness (due to the short longitudinal axis), parallel rays are focused behind the retina, and converging rays enter the fovea, which also causes blurred images.

Both vision defects can be corrected. Myopia is corrected by biconcave lenses, which reduce refraction and shift the focus to the retina; farsightedness - biconvex lenses that increase the refraction and therefore move the focus to the retina.

The organ of vision is the most important of all human senses, because about 90% of information about the outside world a person receives through a visual analyzer or visual system.

The organ of vision is the most important of all human senses, because about 90% of information about the outside world a person receives through a visual analyzer or visual system. The main functions of the organ of vision are central, peripheral, color and binocular vision, as well as light perception.

A person sees not with his eyes, but through his eyes, from where information is transmitted through the optic nerve to certain areas of the occipital lobes of the cerebral cortex, where the picture of the outside world that we see is formed.

The structure of the visual system

The visual system consists of:

* Eyeball;

* Protective and auxiliary apparatus of the eyeball (eyelids, conjunctiva, lacrimal apparatus, oculomotor muscles and orbital fascia);

* Life support systems of the organ of vision (blood supply, production of intraocular fluid, regulation of hydro and hemodynamics);

* Conducting pathways - optic nerve, optic chiasm and optic tract;

* Occipital lobes of the cerebral cortex.

Eyeball

The eye has the shape of a sphere, so the allegory of an apple began to be applied to it. The eyeball is a very delicate structure, therefore it is located in the bony recess of the skull - the eye socket, where it is partially sheltered from possible damage.

The human eye is not quite the correct spherical shape. In newborns, its dimensions are (on average) along the sagittal axis 1.7 cm, in adults 2.5 cm. The mass of the eyeball of a newborn is up to 3 g, an adult - up to 7-8 g.

Features of the structure of the eyes in children

In newborns, the eyeball is relatively large, but short. By 7-8 years, the final size of the eyes is established. The newborn has a relatively larger and flatter cornea than adults. At birth, the shape of the lens is spherical; throughout life, it grows and becomes flatter. In newborns, there is little or no pigment in the stroma of the iris. The bluish color of the eyes is due to the translucent posterior pigment epithelium. When the pigment begins to appear in the iris, it takes on its own color.

The structure of the eyeball

The eye is located in the orbit and is surrounded by soft tissues (fatty tissue, muscles, nerves, etc.). In front, it is covered with conjunctiva and covered with eyelids.

Eyeball consists of three membranes (outer, middle and inner) and contents (vitreous body, lens, and aqueous humor of the anterior and posterior chambers of the eye).

Outer, or fibrous, shell of the eye represented by dense connective tissue. It consists of a transparent cornea in the anterior part of the eye and a white opaque sclera. With elastic properties, these two shells form the characteristic shape of the eye.

The function of the fibrous membrane is to conduct and refract light rays, as well as protect the contents of the eyeball from adverse external influences.

Cornea- transparent part (1/5) of the fibrous membrane. The transparency of the cornea is due to the uniqueness of its structure, in it all the cells are located in a strict optical order and there are no blood vessels in it.

The cornea is rich in nerve endings, so it is very sensitive. The impact of unfavorable external factors on the cornea causes a reflex contraction of the eyelids, providing protection for the eyeball. The cornea not only transmits, but also refracts light rays, it has a large refractive power.

Sclera- the opaque part of the fibrous membrane, which has a white color. Its thickness reaches 1 mm, and the thinnest part of the sclera is located at the exit of the optic nerve. The sclera consists mainly of dense fibers that give it strength. Six oculomotor muscles are attached to the sclera.

Functions of the sclera- protective and shaping. Numerous nerves and vessels pass through the sclera.

choroid, the middle layer, contains the blood vessels that carry blood to feed the eye. Just below the cornea, the choroid passes into the iris, which determines the color of the eyes. At its center is pupil. The function of this shell is to limit the entry of light into the eye at high brightness. This is achieved by constricting the pupil in high light and dilating in low light.

Behind the iris is located lens, similar to a biconvex lens that catches light as it passes through the pupil and focuses it on the retina. Around the lens, the choroid forms a ciliary body, in which the ciliary (ciliary) muscle is embedded, which regulates the curvature of the lens, which provides a clear and distinct vision of objects at different distances.

When this muscle is relaxed, the ciliary band attached to the ciliary body is stretched and the lens is flattened. Its curvature, and hence the refractive power, is minimal. In this state, the eye sees distant objects well.

In order to see near objects, the ciliary muscle contracts and the tension of the ciliary girdle weakens, so that the lens becomes more convex, hence more refractive.

This property of the lens to change its refractive power of the beam is called accommodation.

Inner shell eyes presented retina– highly differentiated nervous tissue. The retina of the eye is the front edge of the brain, an extremely complex formation both in structure and in function.

Interestingly, during embryonic development, the retina is formed from the same group of cells as the brain and spinal cord, so it is true to say that the surface of the retina is an extension of the brain.

In the retina, light is converted into nerve impulses, which are transmitted along the nerve fibers to the brain. There they are analyzed, and the person perceives the image.

The main layer of the retina is a thin layer of light-sensitive cells - photoreceptors. They are of two types: responding to weak light (rods) and strong (cones).

Sticks there are about 130 million, and they are located throughout the retina, except for the very center. Thanks to them, a person sees objects on the periphery of the field of view, including in low light.

There are about 7 million cones. They are located mainly in the central zone of the retina, in the so-called yellow spot. The retina here is maximally thinned, all layers are missing, except for the layer of cones. A person sees best with a yellow spot: all light information that falls on this area of ​​\u200b\u200bthe retina is transmitted most fully and without distortion. Only day and color vision is possible in this region.

Under the influence of light rays in photoreceptors, a photochemical reaction occurs (disintegration of visual pigments), as a result of which energy (electric potential) is released that carries visual information. This energy in the form of nervous excitation is transmitted to other layers of the retina - to bipolar cells, and then to ganglion cells. At the same time, due to the complex connections of these cells, random “noise” in the image is removed, weak contrasts are enhanced, moving objects are perceived more sharply.

Ultimately, all visual information in an encoded form is transmitted in the form of impulses along the fibers of the optic nerve to the brain, its highest instance - the posterior cortex, where the visual image is formed.

Interestingly, the rays of light, passing through the lens, are refracted and turned over, due to which an inverted reduced image of the object appears on the retina. Also, the picture from the retina of each eye enters the brain not entirely, but as if cut in half. However, we see the world normally.

Therefore, it is not so much in the eyes as in the brain. In essence, the eye is simply a perceiving and transmitting instrument. The brain cells, having received an inverted image, turn it over again, creating a true picture of the surrounding world.

Contents of the eyeball

The contents of the eyeball are the vitreous body, the lens, and the aqueous humor of the anterior and posterior chambers of the eye.

The vitreous body by weight and volume is approximately 2/3 of the eyeball and more than 99% consists of water, in which a small amount of protein, hyaluronic acid and electrolytes are dissolved. This is a transparent, avascular gelatinous formation that fills the space inside the eye.

The vitreous body is quite firmly connected with the ciliary body, the lens capsule, as well as with the retina near the dentate line and in the region of the optic nerve head. With age, the connection with the lens capsule weakens.

Auxiliary apparatus of the eye

The auxiliary apparatus of the eye includes the oculomotor muscles, lacrimal organs, as well as the eyelids and conjunctiva.

oculomotor muscles

The oculomotor muscles provide the mobility of the eyeball. There are six of them: four straight and two oblique.

The rectus muscles (superior, inferior, external, and internal) originate from a ring of tendons located at the apex of the orbit around the optic nerve and insert into the sclera.

The superior oblique muscle starts from the periosteum of the orbit above and medially from the visual opening, and, going somewhat backwards and downwards, is attached to the sclera.

The inferior oblique muscle originates from the medial wall of the orbit behind the inferior orbital fissure and inserts on the sclera.

The blood supply to the oculomotor muscles is carried out by the muscular branches of the ophthalmic artery.

The presence of two eyes allows us to make our vision stereoscopic (that is, to form a three-dimensional image).

Precise and well-coordinated work of the eye muscles allows us to see the world around us with two eyes, i.e. binocularly. In case of dysfunction of the muscles (for example, with paresis or paralysis of one of them), double vision occurs or the visual function of one of the eyes is suppressed.

It is also believed that the oculomotor muscles are involved in the process of adjusting the eye to the process of vision (accommodation). They compress or stretch the eyeball so that the rays coming from the observed objects, whether far or near, can hit the retina exactly. In this case, the lens provides finer adjustment.

Blood supply to the eye

The brain tissue that conducts nerve impulses from the retina to the visual cortex, as well as the visual cortex, normally almost everywhere has a good supply of arterial blood. Several large arteries that are part of the carotid and vertebrobasilar vascular systems participate in the blood supply of these brain structures.

Arterial blood supply to the brain and visual analyzer is carried out from three main sources - the right and left internal and external carotid arteries and the unpaired basilar artery. The latter is formed as a result of the fusion of the right and left vertebral arteries located in the transverse processes of the cervical vertebrae.

Almost the entire visual cortex and partly the cortex of the parietal and temporal lobes adjacent to it, as well as the occipital, midbrain and pontine oculomotor centers are supplied with blood by the vertebrobasilar basin (vertebra - translated from Latin - vertebra).

In this regard, circulatory disorders in the vertebrobasilar system can cause dysfunction of both the visual and oculomotor systems.

Vertebrobasilar insufficiency, or vertebral artery syndrome, is a condition in which blood flow in the vertebral and basilar arteries is reduced. The cause of these disorders may be compression, increased tone of the vertebral artery, incl. as a result of compression by bone tissue (osteophytes, herniated disc, subluxation of the cervical vertebrae, etc.).

As you can see, our eyes are an exceptionally complex and amazing gift of nature. When all departments of the visual analyzer work harmoniously and without interference, we see the world around us clearly.

Treat your eyes carefully and carefully!

Located in the eye socket (orbit). The walls of the orbit are formed by the facial and cranial bones. The visual apparatus consists of the eyeball, optic nerve and a number of auxiliary organs (muscles, lacrimal apparatus, eyelids). Muscles allow the eyeball to move. These are a pair of oblique muscles (upper and lower muscles) and four rectus muscles (upper, lower, internal and external).

The eye as an organ

The human organ of vision is a complex structure that includes:

  • Peripheral organ of vision (eyeball with appendages);
  • Pathways (optic nerve, optic tract);
  • Subcortical centers and higher visual centers.

The peripheral organ of vision (eye) is a paired organ, the device of which allows you to perceive light radiation.

Eyelashes and eyelids perform a protective function. Accessory organs include the lacrimal glands. Tear fluid is needed to warm, moisturize and clean the surface of the eyes.

Basic structures

The eyeball is an organ of complex structure. The internal environment of the eye is surrounded by three shells: outer (fibrous), middle (vascular) and inner (reticulate). The outer shell for the most part consists of protein opaque tissue (sclera). In its anterior part, the sclera passes into the cornea: the transparent part of the outer shell of the eye. Light enters the eyeball through the cornea. The cornea is also necessary for the refraction of light rays.

The cornea and sclera are strong enough. This allows them to maintain intraocular pressure and maintain the shape of the eye.

The middle layer of the eye is:

  • Iris;
  • Vascular membrane;
  • Ciliary (ciliary) body.

The iris consists of loose connective tissue and a network of blood vessels. In its center is the pupil - a hole with a diaphragm device. In this way, it can regulate the amount of light entering the eye. The edge of the iris passes into the ciliary body, covered with sclera. The annular ciliary body consists of the ciliary muscle, vessels, connective tissue and processes of the ciliary body. The lens is attached to the processes. The functions of the ciliary body are the process of accommodation and production. This fluid nourishes some parts of the eye and maintains a constant intraocular pressure.

It also forms the substances necessary to ensure the process of vision. In the next layer of the retina are processes called rods and cones. Through the processes, the nervous excitation that provides visual perception is transmitted to the optic nerve. The active part of the retina is called the fundus, which contains blood vessels, and the macula, where most of the cone processes responsible for color vision are located.

Shape of rods and cones

Inside the eyeball are:

  • intraocular fluid;
  • vitreous body.

The posterior surface of the eyelids and the anterior part of the eyeball over the sclera (to the cornea) are covered by the conjunctiva. This is the mucous membrane of the eye, which looks like a thin transparent film.

The structure of the anterior part of the eyeball and lacrimal apparatus

Optical system

Depending on the functions performed by different parts of the organs of vision, it is possible to distinguish between the light-transmitting and light-perceiving parts of the eye. The light-perceiving part is the retina. The image of objects perceived by the eye is reproduced on the retina using the optical system of the eye (the light-conducting section), which consists of the transparent medium of the eye: the vitreous body, the moisture of the anterior chamber and the lens. But mainly the refraction of light occurs on the outer surface of the eye: the cornea and in the lens.

Optical system of the eye

Rays of light pass through these refractive surfaces. Each of them deflects a light beam. In the focus of the optical system of the eye, the image appears as its inverted copy.

The process of refraction of light in the optical system of the eye is denoted by the term "refraction". The optical axis of the eye is a straight line that passes through the center of all refractive surfaces. Light rays emanating from infinitely distant objects are parallel to this straight line. Refraction in the optical system of the eye collects them in the main focus of the system. That is, the main focus is the place where objects at infinity are projected. From objects that are at a finite distance, the rays, refracting, are collected in additional foci. Additional tricks are further than the main one.

In studies of the functioning of the eye, the following parameters are usually taken into account:

  • Refractive, or refraction;
  • Corneal curvature radius;
  • Refractive index of the vitreous.

It is also the radius of curvature of the retinal surface.

Age development of the eye and its optical power

After the birth of a person, his organs of vision continue to form. In the first six months of life, the area of ​​the macula and the central area of ​​the retina are formed. The functional mobility of the visual pathways also increases. During the first four months, the morphological and functional development of the cranial nerves occurs. Until the age of two years, the improvement of the cortical visual centers, as well as the visual cellular elements of the cortex, continues. In the first years of a child's life, the connections between the visual analyzer and other analyzers are formed and strengthened. The development of human organs of vision is completed by the age of three.

Light sensitivity in a child appears immediately after birth, but a visual image cannot yet appear. Quite quickly (within three weeks), the baby develops conditioned reflex connections, which lead to the improvement of the functions of spatial, objective and.

Central vision develops in humans only in the third month of life. Subsequently, it is improved.

The visual acuity of the newborn is very low. By the second year of life, it rises to 0.2-0.3. By the age of seven, it develops to 0.8–1.0.

The ability to perceive color appears at the age of two to six months. At the age of five, color vision in children is fully developed, although it continues to improve. Also gradually (approximately by school age) they reach the normal level of the border of the field of view. Binocular vision develops much later than other functions of the eye.

Adaptation

Adaptation is the process of adapting the organs of vision to a changing level of illumination of the surrounding space and objects in it. Distinguish between the process of dark adaptation (changes in sensitivity when moving from bright light to complete darkness) and light adaptation (when moving from darkness to light).

The "adaptation" of the eye, which perceived bright light, to vision in the dark develops unevenly. At first, the sensitivity increases quite quickly, and then slows down. Complete completion of the dark adaptation process can take several hours.

Light adaptation takes a much shorter period of time - about one to three minutes.

Accommodation

Accommodation is the process of "adaptation" of the eye to a clear distinction between those objects that are located in space at different distances from the perceiver. The mechanism of accommodation is associated with the possibility of changing the curvature of the surfaces of the lens, that is, changing the focal length of the eye. This occurs when the ciliary body is stretched or relaxed.

With age, the ability of the organs of vision to accommodate gradually decreases. Develops (age farsightedness).

Visual acuity

The concept of "visual acuity" refers to the ability to see separately points that are located in space at a certain distance from each other. In order to measure visual acuity, the concept of "visual angle" is used. The smaller the angle of view, the higher the visual acuity. Visual acuity is considered one of the most important functions of the eye.

Determining visual acuity is one of the key work of the eye.

Hygiene is a part of medicine that develops rules that are important for preventing diseases and promoting the health of various organs and body systems. The main rule aimed at maintaining the health of vision is to prevent eye fatigue. It is important to learn how to relieve stress, use vision correction methods if necessary.

Also, hygiene of vision provides for measures that protect the eyes from pollution, injuries, burns.

Hygiene

Workplace equipment is part of the activities that allow the eyes to function normally. The organs of vision "work" best in conditions closest to natural. Unnatural lighting, low eye mobility, dry indoor air can lead to visual impairment.

Eye health is greatly influenced by the quality of nutrition.

Exercises

There are quite a few exercises that help maintain good vision. The choice depends on the state of vision of a person, his capabilities, lifestyle. It is best to get expert advice when choosing certain types of gymnastics.

A simple set of exercises designed to relax and train:

  1. Blink intensively for one minute;
  2. "Blink" with closed eyes;
  3. Direct your gaze to a certain point located far from the person. Look into the distance for a minute;
  4. Look at the tip of the nose, look at it for ten seconds. Then again look into the distance, close your eyes;
  5. Lightly patting with your fingertips, massage the eyebrows, temples and infraorbital region. After that, you need to cover your eyes with your palm for one minute.

Exercise should be done once or twice a day. It is also important to use the complex to relax from intense visual stress.

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conclusions

The eye is a sensory organ that provides the function of vision. Most of the information about the world around us (about 90%) comes to a person through vision. The unique optical system of the eye allows you to get a clear image, distinguish colors, distances in space, and adapt to changing lighting conditions.

The eyes are a complex and sensitive organ. Its pretty, but also creating unnatural operating conditions. In order to maintain eye health, hygiene recommendations must be followed. In the event of problems with vision or the occurrence of eye diseases, it is necessary to seek the advice of a specialist. This will help a person maintain visual functions.


The visual organs of fish are basically the same as those of other vertebrates. The mechanism of perception of visual sensations is similar to other vertebrates: light passes into the eye through the transparent cornea, then the pupil - a hole in the iris - passes it to the lens, and the lens transmits and focuses the light on the inner wall of the eye to the retina, where it is directly perceived. . The retina consists of light-sensitive (photoreceptor), nerve, as well as supporting cells.

Light-sensitive cells are located on the side of the pigment membrane. In their processes, shaped like rods and cones, there is a photosensitive pigment. The number of these photoreceptor cells is very large - there are 50 thousand of them per 1 mm 2 of the retina in carp (in squid - 162 thousand, spider - 16 thousand, human - 400 thousand, owl - 680 thousand). Through a complex system of contacts between the terminal branches of sensory cells and dendrites of nerve cells, light stimuli enter the optic nerve.

Cones in bright light perceive the details of objects and color. Rods perceive weak light, but they cannot create a detailed image.

The position and interaction of the cells of the pigment membrane, rods and cones change depending on the illumination. In the light, the pigment cells expand and cover the rods located near them; cones are drawn to the nuclei of cells and thus move towards the light. In the dark, sticks are drawn to the nuclei (and are closer to the surface); the cones approach the pigment layer, and the pigment cells reduced in the dark cover them.

The number of receptors of various kinds depends on the way of life of fish. In diurnal fish, cones prevail in the retina, in twilight and nocturnal fish, rods: burbot has 14 times more rods than pike. Deep-sea fish living in the darkness of the depths do not have cones, and the rods become larger and their number increases sharply - up to 25 million / mm 2 of the retina; the probability of capturing even weak light increases. Most fish distinguish colors, which is confirmed by the possibility of developing conditioned reflexes in them for a certain color - blue, green, red, yellow, blue.

Some deviations from the general scheme of the structure of the eye of a fish are associated with the characteristics of life in the water. The eye of the fish is elliptical. Among others, it has a silvery shell (between the vascular and protein), rich in guanine crystals, which gives the eye a greenish-golden sheen.

The cornea is almost flat (rather than convex), the lens is spherical (rather than biconvex) - this expands the field of view. The hole in the iris - the pupil - can change the diameter only within small limits. As a rule, fish do not have eyelids. Only sharks have a nictitating membrane that covers the eye like a curtain, and some herring and mullet have a fatty eyelid - a transparent film covering part of the eye.

The location of the eyes on the sides of the head (in most species) is the reason why fish have mostly monocular vision, and the ability for binocular vision is very limited. The spherical shape of the lens and moving it forward to the cornea provides a wide field of view: light enters the eye from all sides. The vertical angle of view is 150°, horizontally 168–170°. But at the same time, the sphericity of the lens causes myopia in fish. The range of their vision is limited and fluctuates due to the turbidity of the water from a few centimeters to several tens of meters.

Vision over long distances becomes possible due to the fact that the lens can be pulled back by a special muscle, a sickle-shaped process extending from the choroid of the bottom of the eyecup.

With the help of vision, fish are also guided by objects on the ground. Improved vision in the dark is achieved by the presence of a reflective layer (tapetum) - guanine crystals, underlain by pigment. This layer does not transmit light to the tissues lying behind the retina, but reflects it and returns it back to the retina. This increases the ability of the receptors to use the light that has entered the eye.

Due to habitat conditions, the eyes of fish can change greatly. In cave or abyssal (deep water) forms, the eyes can be reduced and even disappear. Some deep-sea fish, on the contrary, have huge eyes that allow them to capture very faint traces of light, or telescopic eyes, the collecting lenses of which the fish can put in parallel and acquire binocular vision. The eyes of some eels and larvae of a number of tropical fish are carried forward on long outgrowths (stalked eyes).

An unusual modification of the eyes of a four-eyed bird from Central and South America. Her eyes are placed on the top of her head, each of them is divided by a partition into two independent parts: the upper fish sees in the air, the lower one in the water. In the air, the eyes of fish crawling ashore or trees can function.

The role of vision as a source of information from the outside world for most fish is very large: when orienting during movement, when searching for and capturing food, while maintaining a flock, during the spawning period (the perception of defensive and aggressive postures and movements by rival males, and between individuals of different sexes - wedding attire and spawning "ceremonial"), in the relationship of the victim-predator, etc.

The ability of fish to perceive light has long been used in fishing (fishing by the light of a torch, fire, etc.).

It is known that fish of different species react differently to light of different intensities and different wavelengths, i.e., different colors. Thus, bright artificial light attracts some fish (Caspian sprat, saury, horse mackerel, mackerel, etc.) and scares away others (mullet, lamprey, eel, etc.). Different species are also selectively related to different colors and different light sources - surface and underwater. All this is the basis for the organization of industrial fishing for electric light (this is how sprat, saury and other fish are caught).