The organ of vision is one of the the most important organs senses accessible to a person, because a person perceives about 70% of information about the outside world through visual analyzers. Organ of vision or visual analyzer - it's not just the eye. The eye itself- This is the peripheral part of the organ of vision.
Information received through the apparatus of the eyeball is transmitted along the visual pathways (optic nerve, optic chiasm, optic tract) first to the subcortical centers of vision (external geniculate bodies), then along the optic radiation and the optic fascicle of Graziole to the higher visual center in the occipital lobes brain.
The peripheral part of the organ of vision is:
Eyeball,
Protective apparatus of the eyeball (upper and lower eyelids, orbit),
The adnexal apparatus of the eye (the lacrimal gland, its ducts, as well as the oculomotor apparatus, consisting of muscles).
The eyeball occupies the main place in the orbit or orbit, which is the bony seat of the eye and also serves to protect it. Between the orbit and the eyeball there is fatty tissue, which performs shock-absorbing functions and contains blood vessels, nerves and muscles. The eyeball weighs about 7 grams.
Eyeball
is a sphere with a diameter of about 25 mm, consisting of three shells. The outer, fibrous membrane consists of an opaque sclera about 1 mm thick, which passes into the cornea in front.On the outside, the sclera is covered with a thin transparent mucous membrane - conjunctiva. The middle shell is called vascular. From its name it is clear that it contains a lot of vessels that nourish eyeball. It forms, in particular, the ciliary body and the iris. The inner layer of the eye is the retina.
The eye also has adnexal apparatus, in particular, the eyelids and lacrimal organs. Eye movements are controlled six muscles- four straight and two oblique. In terms of their structure and functions, the eyes can be compared to the optical system of, for example, a camera. The image on the retina (analogue of photographic film) is formed as a result of the refraction of light rays in the system of lenses located in the eye (cornea and lens) (analogue of a lens). Let's look at how this happens in more detail.
Light entering the eye first passes through the cornea - a transparent lens that has a dome shape (radius of curvature approximately 7.5 mm, thickness in the central part approximately 0.5 mm). It lacks blood vessels and has many nerve endings Therefore, when the cornea is damaged or inflamed, the so-called corneal syndrome develops (lacrimation, photophobia and inability to open the eye).
The anterior surface of the cornea is covered with epithelium, which has the ability to regenerate (repair) when damaged. Deeper is the stroma, consisting of collagen fibers, and from the inside the cornea is covered with one layer of cells - the endothelium, which, if damaged, is not restored, which leads to the development of corneal dystrophy, that is, to a violation of its transparency.
Cornea- this is a lens that accounts for 40 diopters out of all 60 diopters of the total refractive power of the eye. That is, the cornea is the strongest lens in the optical system of the eye. This is a consequence of the difference in the refractive index of the air in front of the cornea and the refractive index of its substance.
After leaving the cornea, light enters the fluid-filled so-called anterior chamber of the eye - the space between the inner surface of the cornea and the iris.
Iris is a diaphragm with a hole in the center - the pupil, the diameter of which can vary depending on the lighting, regulating the flow of light entering the eye.
Corneal periphery along the entire circumference it practically connects with the iris, forming the so-called anterior chamber angle, through the anatomical elements of which (Schlemm’s canal, trabecula and other formations, which have a common name - drainage paths of the eye), there is an outflow of fluid constantly circulating in the eye into venous system. Behind the iris is the lens, another lens that refracts light. The optical power of this lens is less than that of the cornea - it is approximately 18-20 diopters. The entire circumference of the lens has thread-like ligaments (the so-called zonular ligaments), which connect to the ciliary muscles located in the wall of the eye. These muscles can contract and relax. Depending on this, the ligaments of Zinn can also relax or tighten, as a result of which the radius of curvature of the lens changes - so a person can see clearly both near and far.
This ability, called accommodation, is lost with age (after 40 years) due to the compaction of the lens substance - near vision deteriorates.
Lens in its structure it is similar to a berry with one seed - it has a shell - a capsular bag, a denser substance - the core (resembling a seed), and a less dense substance (resembling the pulp of the berry) - lens masses. In youth, the core of the lens is soft, however, by the age of 40-50 it becomes denser. The anterior capsule of the lens faces the iris, the posterior capsule faces the vitreous body, and the border between them is the ligaments of Zinn. Around the equator of the lens, along its entire circumference, is the ciliary body, which is part choroid. It has processes that produce intraocular fluid. This fluid enters the anterior chamber of the eye through the pupil and is removed through the angle of the anterior chamber into the venous system of the eye. The balance between the production and outflow of this fluid is very important, since its disturbance leads to the development of glaucoma.
Located behind the lens vitreous. The main functions of the vitreous body are maintaining the shape and tone of the eyeball, conducting light, and participating in intraocular metabolism. As a refractive medium it is weak. When examined in transmitted light, the normal vitreous body appears completely transparent.
It has a jelly-like structure in most cases, but sometimes it can become liquefied. On the other hand, compacted areas in the form of threads or lumps may appear in it, the presence of which the patient feels in the form of “flies” and floating dots. In some places, the vitreous body is closely fused with the retina, so when seals form in it, the vitreous body can pull on the retina, sometimes causing its detachment.
After passing through all of the above structures, the light hits the retina, which plays the role of photographic film in the eye. Consisting of ten layers, the retina is designed to convert light energy into nerve impulse energy. The transformation of light energy in the retina is carried out thanks to a complex photochemical process, accompanied by the breakdown of photoreagents with subsequent restoration and with the participation of vitamin A and other substances.
Millions of small retinal cells called photoreceptors(rods and cones) convert light energy into the energy of nerve impulses and send it to the brain. The total number of cones in the retina of the human eye is 7 million, rods - 130 million. Rods have very high light sensitivity and provide twilight and peripheral vision. Cones perform a subtle function: central shaped vision and color perception. The central part of the retina, called the macula lutea, has the highest visual functions. This name comes from the yellow color of the pit. macular spot(fovea).
The central recess (foveola), the diameter of which is 0.2-0.4 mm, is the thinnest place of the retina, no more than 0.18 mm thick. The retina here consists almost exclusively of visual cells.
Nerve impulses are collected from the retina by the optic nerve, which consists of approximately 1 million nerve fibers. Thus, the information is transmitted to the occipital lobe of the brain, where the visual image is analyzed.
Damage, trauma or compression optic nerve at any level lead to almost irreversible loss of vision even with normal functioning other anatomical structures of the eye and the transparency of the ocular media.
Based on the above, we can say that The organ of vision is a delicate system, all links of which function in close interaction with each other, and disruption of the functioning of at least one of them leads to decreased vision.
Able to see under illumination of several photons and in direct light sunlight. It is capable of focusing in just a third of a second. Due to this and due to its structural features (which will be discussed later), the eye is considered one of the most complex organs of the body. What is this? The result of evolution or an incredible coincidence? Let's try to figure this out.
Some scientists considered the idea of the evolution of the organ of vision extremely absurd. But is this really so? Charles Darwin offered his explanation of the mechanism of evolution. He believed that if the organ of vision continuously changes, then these changes are inherited. This means that the most complex organ of vision could have been created in the form in which we now observe it, by natural selection. He analyzed the structure of the organ of vision of many creatures, and also showed changes in the structure of the eye - from the simplest to the most complex organisms.
The evolution of the human eye began more than 500,000,000 years ago. It was then that the development of a light-sensitive spot began, consisting of several cells in a simple organism. The spot helped distinguish light from darkness. And although it could not determine distance or image, it was from it that the development of the eye began. Evolution is supported by the fact that for the spot to develop and eventually become the spot of a planarian (flatworm) or the ordinary eye of a fish, the development of many components and systems of the body would be required.
Each component requires the presence of proteins (proteins) that would perform special functions. These functions must be ingrained in the creature's DNA. The existence of such substances means that a system of other proteins or genes with their own function is involved in the interaction and process of evolution. Without them, vision is impossible.
The human eye does not pretend to be perfect, if only because it is not ideal. This means that the eye is the result of evolution. On the other hand, what we think is a design flaw may actually be quite beneficial. What defects in the design of the human eye do we know?
Biologist Richard Dawkins in his book “The Blind Watchmaker” rightly argued that from the point of view of photo engineering, photographic elements should be directed towards the light, and the wires connecting the elements with the organ of reproduction and analysis should be directed towards the brain (in our case). If the elements are connected “back to front”, and the wires are located on the side close to the light, the light overcomes their mass, is weakened and distorted. From Dawkins' point of view, this is aesthetically incorrect. However, this assumption does not explain why such a system has been successfully used by vertebrates throughout history. for long years. But the same Dawkins adds that the difference is insignificant, because most photons are directed directly and in any case will be caught by the eye.
The most developed non-inverted retinas of the eye belong to cephalopods - squid and octopus. The octopus's retina contains 20,000,000 photoreceptor cells. But this is not the limit. Humans have 126 million of them, and birds have 10 times more.
The human eye contains a "central fovea". This is the "center of the center" - a place in the "spot" - the center of the human retina. This is where photoreceptors and cones are most abundant. All vessels are located towards it in such a way that an area of high visual sharpness is created with a gradual decrease in visual sharpness towards the periphery of the retina. And the spot itself is 100 times more sensitive than the retina. This allows the human eye to focus on a specific area without being distracted by peripheral vision.
The situation is different with the eyes of birds. Their retina does not have a central fovea or macula. The octopus retina also does not have a fovea, but the octopus does have a linear centralis. This organ forms the range of sharpness along the retina. The octopus's eye has another feature. Using the statocyst (the organ of balance), the eye always maintains one position relative to the Earth's gravitational field.
The energy consumption for maintaining such a complex organ is very high. Thus, oxygen consumption by the retina (per gram of tissue) is 50% more than in the liver, and 600% more than in the heart muscle (myocardium). The proximity of the photoreceptors to the capillaries and the absence of nerves in their path ensures the rapid supply of nutrients and the removal of waste.
Vision first appeared about 540,000,000 years ago. The evolutionary process was complex. First, the single-celled green Euglena developed a light-sensitive spot - an “eye.” The ability to distinguish light was vital for euglena. As life became more complex and new species appeared, the eye also evolved.
Thus, a grouping of light-sensitive cells in the form of a “spot” occurred. With its help, the body could evaluate the movements of the predator. With the advent eyespots in jellyfish (about 500 million years ago), these organisms could navigate in space.
Ciliated worms already have two spots, and each of them contains thousands of photosensitive cells. These spots are only half immersed in a cup of pigment - the prototype modern eye. Gradually a groove is formed, the so-called “glass of the eye”. For example, this can be seen in river snails. Visibility with this eye is like through frosted glass.
Visual acuity increases as the external opening of the eye narrows. In the mollusk nautilus, the 1-centimeter eye contains millions of cells, but still captures little light.
At a certain stage of evolution, two organs of vision appeared. One allowed us to see the world in bright colors. The other made it possible to distinguish the outlines of objects. It is from the second that the human organ of vision originates. A little later, a transparent film is formed, which protects the pupil from contamination and changes its ability to refract light. This is how the first lens appears. The larger it is, the sharper the gaze.
The eye turns out to be such a perfect organ that nature needed to invent it twice, separately for invertebrates and for vertebrates. The development process was also different. In the case of mollusks, the eye originated from the epithelium, and in the case of humans, from epithelium (cornea and lens) and nervous tissue (vitreous body and retina). There is also a third, compound eye. It is more complex and consists of many ommatidia (individual ocelli). Trilobites, insects, crustaceans and some invertebrates have this eye.
Our body interacts with the environment using sense organs, or analyzers. With their help, a person is not only able to “feel” the external world, based on these sensations he has special forms reflection - self-awareness, creativity, the ability to foresee events, etc.
According to I.P. Pavlov, each analyzer (and even the organ of vision) is nothing more than a complex “mechanism”. It is capable of not only perceiving environmental signals and converting their energy into impulse, but also performing higher analysis and synthesis.
The organ of vision, like any other analyzer, consists of 3 integral parts:
The peripheral part, which is responsible for perceiving the energy of external irritation and processing it into nerve impulse;
Pathways through which the nerve impulse passes directly to the nerve center;
The cortical end of the analyzer (or sensory center), located directly in the brain.
The rods consist of inner and outer segments. The latter is formed with the help of double membrane disks, which are folds of the plasma membrane. Cones differ in size (they are larger) and the nature of the disks.
There are three types of cones and only one type of rods. The number of rods can reach 70 million, or even more, while the number of cones is only 5-7 million.
As already mentioned, there are three types of cones. Each of them perceives different colour: blue, red or yellow.
Rods are needed to perceive information about the shape of an object and the illumination of the room.
From each of the photoreceptor cells there is a thin process that forms a synapse (the place where two neurons contact) with another process of bipolar neurons (neuron II). The latter transmit excitation to larger ganglion cells (neuron III). The axons (processes) of these cells form the optic nerve.
This is a biconvex crystal clear lens with a diameter of 7-10 mm. It has neither nerves nor blood vessels. Under the influence of the ciliary muscle, the lens is able to change its shape. It is these changes in the shape of the lens that are called accommodation of the eye. When set to distance vision, the lens flattens, and when set to near vision, it enlarges.
Together with the lens, it forms the light-refracting medium of the eye.
It fills all the free space between the retina and the lens. It has a jelly-like transparent structure.
The structure of the organ of vision is similar to the principle of the camera. The pupil acts as a diaphragm, narrowing or expanding depending on the lighting. The lens is the vitreous body and the lens. Light rays hit the retina, but the image comes out upside down.
Thanks to the light-refracting media (the lens and the vitreous body), the light beam hits the yellow spot on the retina, which is the best vision zone. Light waves reach the cones and rods only after they have passed through the entire thickness of the retina.
The motor apparatus of the eye consists of 4 striated rectus muscles (inferior, superior, lateral and medial) and 2 oblique muscles (inferior and superior). The rectus muscles are responsible for turning the eyeball in the appropriate direction, and the oblique muscles are responsible for turning around the sagittal axis. The movements of both eyeballs are synchronous only thanks to the muscles.
Skin folds, the purpose of which is to limit the palpebral fissure and close it when closed, provide protection to the eyeball from the front. There are about 75 eyelashes on each eyelid, the purpose of which is to protect the eyeball from foreign objects.
A person blinks approximately once every 5-10 seconds.
Consists of lacrimal glands and system tear ducts. Tears neutralize microorganisms and can moisturize the conjunctiva. Without tears, the conjunctiva of the eye and the cornea would simply dry out, and the person would go blind.
The lacrimal glands produce about one hundred milliliters of tears every day. Interesting fact: Women cry more often than men, because the secretion of tear fluid is promoted by the hormone prolactin (of which girls have much more).
Basically, tears consist of water containing approximately 0.5% albumin, 1.5% sodium chloride, some mucus and lysozyme, which has a bactericidal effect. Has a slightly alkaline reaction.
Let's take a closer look at the anatomy of the organ of vision with the help of drawings.
The figure above schematically shows parts of the organ of vision in a horizontal section. Here:
1 - tendon of the middle rectus muscle;
2 - rear camera;
3 - cornea of the eye;
4 - pupil;
5 - lens;
6 - anterior chamber;
7 - iris;
8 - conjunctiva;
9 - tendon of the rectus lateral muscle;
10 - vitreous body;
11 - sclera;
12 - choroid;
13 - retina;
14 - yellow spot;
15 - optic nerve;
16 - blood vessels of the retina.
This figure shows the schematic structure of the retina. The arrow shows the direction of the light beam. The numbers indicate:
1 - sclera;
2 - choroid;
3 - retinal pigment cells;
4 - sticks;
5 - cones;
6 - horizontal cells;
7 - bipolar cells;
8 - amacrine cells;
9 - ganglion cells;
10 - optic nerve fibers.
The figure shows a diagram of the optical axis of the eye:
1 - object;
2 - cornea of the eye;
3 - pupil;
4 - iris;
5 - lens;
6 - central point;
7 - image.
As already mentioned, human vision transmits almost 90% of the information about the world around us. Without him, the world would be the same and uninteresting.
The organ of vision is a rather complex and not fully studied analyzer. Even in our time, scientists sometimes have questions about the structure and purpose of this organ.
The main functions of the organ of vision are the perception of light, forms of the surrounding world, the position of objects in space, etc.
Light is capable of causing complex changes in and is thus an adequate stimulus for the visual organs. It is believed that rhodopsin is the first to perceive irritation.
The highest quality visual perception will be provided that the image of the object falls on the area of the retinal spot, preferably on its central fovea. The further from the center the projection of the image of an object, the less distinct it is. This is the physiology of the organ of vision.
Let's look at some of the most common eye diseases.
First of all, you need to remember that your eyes also need to rest, and excessive stress will not lead to anything good.
Use only high-quality lighting with a lamp power of 60 to 100 W.
Do eye exercises more often and be examined by an ophthalmologist at least once a year.
Remember that eye diseases are a fairly serious threat to your quality of life.
People have always thought about complex structure human body. This is how the wise Greek Herophilus, back in ancient times, described the retina of the eye: “A fisherman’s net, thrown to the bottom of the eye glass, which catches the sun’s rays.” This poetic comparison turned out to be surprisingly accurate. Today we can confidently say that the retina of the eye is precisely a “grid” capable of “catching” even individual quanta of light.
The retina can be defined as a multi-element photoreceiver of images, which, in a simplified structure, is represented as a branch of the optic nerve with additional image processing functions.
The retina of the eye occupies an area with a diameter of about 22 mm, and due to this, it almost completely (about 72% of the inner surface of the eyeball) covers the fundus of the eye with photoreceptors from the ciliary body to the blind spot - the zone where the optic nerve exits the fundus. With ophthalmoscopy, it looks like a light disk due to the higher light reflection coefficient (than in other areas of the retina).
Blind spot and central retina
In the area where the optic nerve exits, the retina does not have photosensitive receptors. Therefore, a person does not see the image of objects that fall into this place (hence the name “blind spot”). It measures approximately 1.8 - 2 mm in diameter, located in the horizontal plane at a distance of 4 mm from the posterior pole of the eyeball towards the nose below the pole of the eyeball.
The central zone of the retina, called the macula, macula, or macular zone, appears as the darkest area of the fundus. U different people its color can vary from dark yellow to dark brown. The central zone has a somewhat elongated oval shape in the horizontal plane. The size of the macula is not precisely determined, but it is generally accepted that in the horizontal plane it ranges from 1.5 to 3 mm.
The macula, like the blind spot, is not located at the pole of the eyeball. Its center is shifted in the horizontal plane in the direction opposite to the blind spot: at a distance of about 1 mm from the axis of symmetry of the optical system of the eye.
The retina of the eye has different thickness. In the blind spot area it is thickest (0.4 - 0.5 mm). It has the smallest thickness in the central zone of the macula (0.07 - 0.1 mm), where the so-called central fossa is formed. At the edges of the retina (the dentate line), its thickness is approximately 0.14 mm.
Although the retina looks like a thin film, it still has a complex microstructure. In the direction of the rays that enter the retina through the transparent media of the eye and the membrane separating the vitreous body from the retina, the first layer of the retina is transparent nerve fibers. They are “conductors” through which photoelectric signals are transmitted to the brain, carrying information about the visual picture of objects of observation: images that are focused by the optical system of the eye on the fundus.
Light, the distribution density of which on the surface of the retina is proportional to the brightness of the field of objects, penetrates through all layers of the retina and falls on the photosensitive layer, composed of cones and rods. This layer actively absorbs light.
Cones have a length of 0.035 mm and a diameter from 2 µm in the central zone of the macula to 6 µm in the peripheral zone of the retina. The sensitivity threshold of cones is approximately 30 quanta of light, and the threshold energy is 1.2 10 -17 J. Cones are photoreceptors for day “color” vision.
The most accepted is the three-component theory of G. Helmholtz, according to which the perception of color by the eye is ensured by three types of cones with different color sensitivities. Each cone contains three types of pigment, a light-sensitive substance, in varying concentrations:
— the first type of pigment (blue-blue) absorbs light in the wavelength range 435-450 nm;
- second type (green) - in the range of 525-540 nm;
- third type (red) - in the range of 565-570 nm.
Rods are receptors for night, “black and white” vision. Their length is 0.06 mm and their diameter is about 2 microns. They have a threshold sensitivity of 12 quanta of light at a wavelength of 419 nm or a threshold energy of 4.8 0 -18 J. Therefore, they are much more sensitive to luminous flux.However, due to the weak spectral sensitivity of the rods, objects observed at night are perceived by humans as gray or black and white.
The density of cones and rods across the retina is not the same. Highest density observed in the macula zone. As you approach the periphery of the retina, the density decreases.
In the center of the fovea (foveola) there are only cones. Their diameter in this place is the smallest; they are tightly hexagonally enclosed. In the foveal zone, the density of cones is 147,000-238,000 per 1 mm. This area of the retina has the greatest spatial resolution, and therefore is intended for observing the most important fragments of space on which a person fixes his gaze.
Further from the center, the density decreases to 95,000 per 1 mm, and in the parafovea - to 10,000 per 1 mm. The density of rods is highest in the parafoveoli - 150,000-160,000 per 1 mm. Further from the center, their density also decreases, and at the periphery of the retina it is only 60,000 per 1 mm. Average density rods on the retina is 80,000-100,000 per 1 mm.
Retinal functions
There is a discrepancy between the number of individual photoreceptors (7,000,000 cones and 12,000,000 rods) and the 1.2 million fibers of the optic nerve. It manifests itself in the fact that the number of “photodetectors” is more than 10 times greater than the number of “conductors” that connect the retina with the corresponding centers of the brain.
This makes clear the function of the layers of the retina: it is to carry out commutation between individual photoreceptors and areas of the visual center of the brain. On the one hand, they do not overload the brain with “small”, secondary information, and on the other hand, they do not allow the loss of an important component of visual information about the environment that the eye observes. Therefore, each cone from the foveal zone has its own personal channel for the passage of nerve impulses to the brain.
However, as we move away from the foveola, such channels are formed for groups of photoreceptors. This is served by the horizontal, bipolar amacrine and, as well as its external and internal layers. If each ganglion cell has only its own personal fiber (axon) for transmitting signals to the brain, this means that, thanks to the switching action of bipolar and horizontal cells, it must have synaptic contact with either one (in the foveal zone) or several (in the peripheral zone) photoreceptors.
It is clear that for this it is necessary to carry out appropriate horizontal switching of photoreceptors and bipolar cells at a lower level, as well as bipolar and ganglion cells at a higher level. This switching is provided through the processes of horizontal and amacrine cells.
Synaptic contacts are electrochemical contacts (synapses) between cells, which are carried out due to electrochemical processes involving specific substances (neurotransmitters). They ensure “transfer of matter” along “conductor nerves”. Therefore, connections between different dendrites of the retina depend not only on nerve impulses, but also on processes throughout the body. These processes can deliver neurotransmitters to synaptic areas in the retina and into the brain, both with the participation of nerve impulses and with the flow of blood and other fluids.
Dendrites are processes of nerve cells that receive signals from other neurons, receptor cells, and conduct nerve impulses through synaptic contacts to the body of neurons. The collection of dendrites forms a dendritic branch. The set of dendritic branches is called a dendritic tree.
Amacrine cells exert “lateral inhibition” between neighboring ganglion cells. This feedback switching of bipolar and ganglion cells is ensured. This not only solves the problem of connecting a limited number of nerve fibers to the brain of a large number of photoreceptors, but also carries out preliminary processing of information coming from the retina to the brain, that is, spatial and temporal filtering of visual signals.
These are the functions of the retina. As you can see, she is very fragile and important. Take care of her!
Rice. 144. Structure of the eye (diagram):
1 - sclera; 2 - choroid; 3 - retina; 4 - central fossa; 5 - blind spot; 6 - optic nerve; 7— conjunctiva; 8—ciliary ligament; 9—cornea; 10—pupil; 11, 18—optical axis; 12 - anterior chamber; 13 - lens; 14 - iris; 15 — rear camera; 16 - ciliary muscle; 17—vitreous body
The eye (oculus) consists of the eyeball and the optic nerve with its membranes. The eyeball has a round shape, anterior and posterior poles. The first corresponds to the most protruding part of the outer fibrous membrane(cornea), and the second is the most protruding part, which is located lateral to the exit of the optic nerve from the eyeball. The line connecting these points is called the external axis of the eyeball, and the line connecting a point on the inner surface of the cornea with a point on the retina is called the internal axis of the eyeball. Changes in the ratios of these lines cause disturbances in the focusing of images of objects on the retina, the appearance of myopia (myopia) or farsightedness (hyperopia).
The eyeball consists of fibrous and choroidal membranes, the retina and the nucleus of the eye (aqueous humor of the anterior and posterior chambers, lens, vitreous body).
The fibrous membrane is an outer dense membrane that performs protective and light-transmitting functions. Its front part is called the cornea, the back part is called the sclera. The cornea is the transparent part of the shell that has no blood vessels and is shaped like a watch glass. The diameter of the cornea is 12 mm, thickness is about 1 mm.
The sclera consists of dense fibrous connective tissue, about 1 mm thick. At the border with the cornea in the thickness of the sclera there is a narrow canal - the venous sinus of the sclera. The extraocular muscles are attached to the sclera.
The choroid contains a large number of blood vessels and pigment. It consists of three parts: the choroid, the ciliary body and the iris. The choroid proper forms a large part of the choroid and lines the posterior part of the sclera, fused loosely with the outer membrane; between them there is a perivascular space in the form of a narrow gap.
The ciliary body resembles a moderately thickened section of the choroid, which lies between the choroid proper and the iris. The basis of the ciliary body is loose connective tissue, rich in blood vessels and smooth muscle cells. The anterior section has about 70 radially located ciliary processes that make up the ciliary crown. The radially located fibers of the ciliary girdle are attached to the latter, which then go to the anterior and posterior surfaces of the lens capsule. The posterior part of the ciliary body - the ciliary circle - resembles thickened circular stripes that pass into the choroid. The ciliary muscle consists of complexly intertwined bundles of smooth muscle cells. When they contract, a change in the curvature of the lens occurs and adaptation to a clear vision of the object (accommodation).
The iris is the most anterior part of the choroid, shaped like a disk with a hole (pupil) in the center. It consists of connective tissue with blood vessels, pigment cells that determine eye color, and muscle fibers located radially and circularly.
The iris is distinguished by the anterior surface, which forms the posterior wall of the anterior chamber of the eye, and the pupillary edge, which limits the opening of the pupil. The posterior surface of the iris constitutes the anterior surface of the posterior chamber of the eye; the ciliary margin is connected to the ciliary body and sclera by means of the pectineal ligament. Muscle fibers the irises, contracting or relaxing, reduce or increase the diameter of the pupils.
The inner (sensitive) membrane of the eyeball - the retina - fits tightly to the choroid. The retina has a large posterior visual part and a smaller anterior “blind” part, which combines the ciliary and iris parts of the retina. The visual part consists of internal pigment and internal nerve parts. The latter has up to 10 layers of nerve cells. In inner part The retina includes cells with processes in the form of cones and rods, which are the light-sensitive elements of the eyeball. Cones perceive light rays in bright (daylight) light and are also color receptors, while rods function in dim light and play the role of twilight light receptors. The remaining nerve cells play a connecting role; the axons of these cells, united into a bundle, form a nerve that exits the retina.
On the posterior part of the retina there is this exit of the optic nerve - the optic disc, and lateral to it is a yellowish spot. Here it is greatest number cones; this something is the essence of the greatest vision.
The nucleus of the eye includes the anterior and posterior chambers filled with aqueous humor, the lens and the vitreous body. The anterior chamber of the eye is the space between the cornea at the front and the anterior surface of the iris at the back. This circumference, where the edge of the cornea and iris is located, is limited by the pectineal ligament. Between the bundles of this ligament is the space of the iridocorneal ganglion (fountain spaces). Through these spaces, aqueous humor from the anterior chamber flows into the venous sinus of the sclera (Schlemm's canal), and then enters the anterior ciliary veins. Through the opening of the pupil, the anterior chamber connects to the posterior chamber of the eyeball. The posterior chamber in turn connects to the spaces between the lens fibers and the ciliary body. Along the periphery of the lens lies a space in the form of a belt (Petite canal), filled with aqueous humor.
The lens is a biconvex lens that is located behind the chambers of the eye and has light refractive ability. It distinguishes between the front and back surfaces and the equator. The substance of the lens is colorless, transparent, dense, and has no vessels or nerves. Its inner part - the core - is much denser than the peripheral part. On the outside, the lens is covered with a thin transparent elastic capsule, to which the ciliary band (ligament of Zinn) is attached. When the ciliary muscle contracts, the size of the lens and its refractive power change.
The vitreous body is a jelly-like transparent mass that has no blood vessels or nerves and is covered with a membrane. It is located in the vitreous chamber of the eyeball, behind the lens and fits tightly to the retina. On the side of the lens in the vitreous body there is a depression called the vitreous fossa. The refractive power of the vitreous body is close to that of the aqueous humor that fills the chambers of the eye. In addition, the vitreous body performs supporting and protective functions.
Accessory organs of the eye. The auxiliary organs of the eye include the muscles of the eyeball (Fig. 145), fascia of the orbit, eyelids, eyebrows, lacrimal apparatus, fatty body, conjunctiva, vagina of the eyeball. 
Rice. 145. Muscles of the eyeball:
A — view from the lateral side: 1 — superior rectus muscle; 2 - levator muscle upper eyelid; 3 - inferior oblique muscle; 4 - inferior rectus muscle; 5 - lateral rectus muscle; B — top view: 1 — block; 2 - sheath of the tendon of the superior oblique muscle; 3 - superior oblique muscle; 4—medial rectus muscle; 5 - inferior rectus muscle; 6 - superior rectus muscle; 7 - lateral rectus muscle; 8 - muscle that lifts the upper eyelid
The motor system of the eye is represented by six muscles. The muscles start from the tendon ring around the optic nerve in the depths of the orbit and are attached to the eyeball. There are four rectus muscles of the eyeball (superior, inferior, lateral and medial) and two oblique muscles (superior and inferior). The muscles act in such a way that both eyes rotate in concert and are directed to the same point. The muscle that lifts the upper eyelid also begins from the tendon ring. The muscles of the eye are striated muscles and contract voluntarily.
The orbit, in which the eyeball is located, consists of the periosteum of the orbit, which in the area of the optic canal and the superior orbital fissure fuses with the dura mater of the brain. The eyeball is covered by a membrane (or Tenon's capsule), which is loosely connected to the sclera and forms the episcleral space. Between the vagina and the periosteum of the orbit is the fatty body of the orbit, which acts as an elastic cushion for the eyeball.
The eyelids (upper and lower) are formations that lie in front of the eyeball and cover it from above and below, and when closed, they completely cover it. The eyelids have anterior and back surface and free edges. The latter, connected by commissures, form the medial and lateral corners of the eye. In the medial angle there are the lacrimal lake and the lacrimal caruncle. On the free edge of the upper and lower eyelids near the medial angle, a small elevation is visible - the lacrimal papilla with an opening at the apex, which is the beginning of the lacrimal canaliculus.
The space between the edges of the eyelids is called the palpebral fissure. The eyelashes are located along the front edge of the eyelids. The basis of the eyelid is cartilage, which is covered with skin on top, and with inside- the conjunctiva of the eyelid, which then passes into the conjunctiva of the eyeball. The depression that forms when the conjunctiva of the eyelids passes to the eyeball is called the conjunctival sac. Eyelids, in addition to their protective function, reduce or block access to light flux.
At the border of the forehead and the upper eyelid there is an eyebrow, which is a ridge covered with hair and performs a protective function.
The lacrimal apparatus consists of the lacrimal gland with excretory ducts and lacrimal ducts. The lacrimal gland is located in the fossa of the same name in the lateral corner, at the upper wall of the orbit and is covered with a thin connective tissue capsule. The excretory ducts (there are about 15 of them) of the lacrimal gland open into conjunctival sac. The tear washes the eyeball and constantly moisturizes the cornea. The movement of tears is facilitated by the blinking movements of the eyelids. Then the tear flows through the capillary gap near the edge of the eyelids into the lacrimal lake. This is where the lacrimal canaliculi originate, which open into the lacrimal sac. The latter is located in the fossa of the same name in the inferomedial corner of the orbit. Downwards it passes into a rather wide nasolacrimal canal, through which tear fluid enters the nasal cavity.
Conducting pathways of the visual analyzer (Fig. 146). The light that hits the retina first passes through the transparent light-refracting apparatus of the eye: the cornea, aqueous humor anterior and posterior chambers, lens and vitreous body. The beam of light along its path is regulated by the pupil. The refractive apparatus directs a beam of light to the more sensitive part of the retina - this is the best vision - the spot with its central fovea. Having passed through all layers of the retina, light causes complex photochemical transformations of visual pigments there. As a result of this, a nerve impulse arises in light-sensitive cells (rods and cones), which is then transmitted to the next neurons of the retina - bipolar cells (neurocytes), and after them - to the neurocytes of the ganglion layer, ganglion neurocytes. The processes of the latter go towards the disc and form the optic nerve. Having passed into the skull through the optic nerve canal along the lower surface of the brain, the optic nerve forms an incomplete optic chiasm. From the optic chiasm begins the optic tract, which consists of nerve fibers of ganglion cells of the retina of the eyeball. Then the fibers along the optic tract go to the subcortical visual centers: the lateral geniculate body and the superior colliculus of the midbrain roof. In the lateral geniculate body, the fibers of the third neuron (ganglionic neurocytes) of the optic pathway end and come into contact with the cells of the next neuron. The axons of these neurocytes pass through the internal capsule and reach the cells occipital lobe near the calcarine groove, where they end (cortical end of the visual analyzer). Some of the axons of ganglion cells pass through the geniculate body and enter the superior colliculus as part of the handle. Next, from the gray layer of the superior colliculus, impulses go to the nucleus oculomotor nerve and into the accessory nucleus, where innervation occurs oculomotor muscles, muscles that constrict the pupils, and the ciliary muscle. These fibers carry an impulse in response to light stimulation and the pupils constrict ( pupillary reflex), the eyeballs also turn in the required direction.
Rice. 146. Scheme of the structure of the visual analyzer:
1 - retina; 2—uncrossed fibers of the optic nerve; 3 - crossed fibers of the optic nerve; 4— optic tract; 5—cortical analyzer
The mechanism of photoreception is based on the gradual transformation of the visual pigment rhodopsin under the influence of light quanta. The latter are absorbed by a group of atoms (chromophores) of specialized molecules - chromolipoproteins. Vitamin A alcohol aldehydes, or retinal, act as a chromophore, which determines the degree of light absorption in visual pigments. The latter are always in the form of 11-cisretinal and normally bind to the colorless opsin protein, thereby forming visual pigment rhodopsin, which, through a series of intermediate stages, again undergoes cleavage into retinal and opsin. In this case, the molecule loses color and this process is called fading. The transformation scheme of the rhodopsin molecule is presented as follows.
The process of visual excitation occurs in the period between the formation of lumi- and metarhodopsin II. After the cessation of exposure to light, rhodopsin is immediately resynthesized. First, with the participation of the enzyme retinal isomerase, trans-retinal is converted into 11-cisretinal, and then the latter combines with opsin, again forming rhodopsin. This process is continuous and underlies dark adaptation. In complete darkness, it takes about 30 minutes for all the rods to adapt and the eyes to acquire maximum sensitivity. The formation of an image in the eye occurs with the participation of optical systems (cornea and lens), which produce an inverted and reduced image of an object on the surface of the retina. The adaptation of the eye to clearly seeing distant objects at a distance is called accommodation. The accommodation mechanism of the eye is associated with contraction of the ciliary muscles, which change the curvature of the lens.
When viewing objects at close range, convergence also acts simultaneously with accommodation, i.e., the axes of both eyes are brought together. The closer the object being examined is, the closer the visual lines converge.
The refractive power of the optical system of the eye is expressed in diopters (“D” - diopter). The power of a lens whose focal length is 1 m is taken as 1 D. The refractive power of the human eye is 59 diopters when viewing distant objects and 70.5 diopters when viewing close objects.
There are three main anomalies of refraction of rays in the eye (refraction): myopia, or myopia; farsightedness, or hypermetropia; senile farsightedness, or presbyopia (Fig. 147). The main reason for all eye defects is that the refractive power and the length of the eyeball are not consistent with each other, as in a normal eye. With myopia (myopia), the rays converge in front of the retina in the vitreous body, and a circle of light scattering appears on the retina at another point, and the eyeball is longer than normal. For vision correction, concave lenses with negative diopters are used.
Rice. 147. Path of light rays in a normal eye (A), with myopia
(B1 and B2), with farsightedness (B1 and B2) and with astigmatism (G1 and G2):
B2, B2 - biconcave and biconvex lenses for correcting myopia and hyperopia; G2 - cylindrical lens for astigmatism correction; 1 - zone of clear vision; 2 - blurred image area; 3 - corrective lenses
With farsightedness (hyperopia), the eyeball is short, and therefore parallel rays coming from distant objects are collected behind the retina, and it produces an unclear, blurry image of the object. This disadvantage can be compensated for by using the refractive power of convex lenses with positive diopters.
Senile farsightedness (presbyopia) is associated with weak elasticity of the lens and weakening of the tension of the zonular ligaments during normal length eyeball.
This refractive error can be corrected using biconvex lenses. Vision with one eye gives us an idea of an object in only one plane. Only when seeing with both eyes simultaneously is depth perception and a correct idea of relative position items. The ability to merge individual images received by each eye into a single whole provides binocular vision.
Visual acuity characterizes the spatial resolution of the eye and is determined by the smallest angle at which a person is able to distinguish two points separately. The smaller the angle, the better vision. Normally, this angle is 1 minute, or 1 unit.
To determine visual acuity, special tables are used that depict letters or figures of various sizes.
The visual field is the space that is perceived by one eye when it is stationary. Changes in the field of view may be early sign some diseases of the eyes and brain.
Color perception is the ability of the eye to distinguish colors. Thanks to this visual function, a person is able to perceive about 180 shades of color. Color vision is of great practical importance in a number of professions, especially in art. Like visual acuity, color perception is a function of the cone apparatus of the retina. Violations color vision can be congenital and inherited and acquired.
Violation of color perception is called color blindness and is determined using pseudo-isochromatic tables, which present a set of colored dots that form a sign. A person with normal vision can easily distinguish the contours of a sign, but a colorblind person cannot.
Is it possible to register an employee for the position of financial director - chief accountant? The chief accountant claims that yes, but...
The head of a small business can easily manage the budget independently. CHECKED! If you're budgeting...
The creation of new projects involves a preliminary economic justification for their feasibility, subsequent...
Reporting is generated by the RM, is agreed upon (approved) by the Risk Committee under the Management Board and transmitted to...
At the edge of a large, very large meadow, on a long emerald blade of grass lived a tiny Ladybug. Little Ladybug...
Nowadays, it is quite common for people to turn to the stars. With the help of a horoscope a person can learn a lot about...
Business or friendly. If you were pursuing a stranger, it means your level of trust in the female sex...
according to Freud's dream book If you dreamed about how you were fishing, it means that in real life you can hardly switch off...
The New Year's whirlwind will spin us around so quickly and rapidly that it's time to thoroughly think through the New Year's...
The article offers you some of the most delicious recipes for making vinaigrette. Vinaigrette is a delicious salad...
July 4th, 2015 , 08:33 pm Lately our Nastena has been completely giving up sweets (and thank God), but...
Fragrant, very tasty chocolate cake, it’s impossible to stop eating! The Negro’s Kiss cake is very easy to prepare,...
Dream interpretation is an ancient art that allows you to at least assess the psychological status...
As soon as the first mirrors were born, people immediately endowed them with all sorts of mystical abilities....
The head of a small business can easily manage the budget independently. CHECKED! If you manage...
The creation of new projects involves a preliminary economic justification for their feasibility, subsequent...
