Eye structure
The eye is a complex optical system. Light rays from surrounding objects enter the eye through the cornea. The cornea in the optical sense is a strong converging lens that focuses light rays diverging in different directions. Moreover, the optical power of the cornea normally does not change and always gives a constant degree of refraction. The sclera is the opaque outer shell of the eye; therefore, it does not participate in conducting light into the eye.
Having been refracted on the front and back surfaces of the cornea, the light rays pass unhindered through the transparent liquid that fills the anterior chamber, up to the iris. The pupil, the circular opening in the iris, allows the centrally located rays to continue their journey into the eye. The more peripheral rays are retained by the pigment layer of the iris. Thus, the pupil not only regulates the amount of light flux to the retina, which is important for adapting to different levels of illumination, but also eliminates lateral, random, distorting rays. Then the light is refracted by the lens. The lens is also a lens, like the cornea. Its fundamental difference is that in people under 40, the lens is able to change its optical power - a phenomenon called accommodation. Thus, the lens produces more accurate pre-focusing. Behind the lens is the vitreous, which extends down to the retina and fills a large volume of the eyeball.
The rays of light, focused by the optical system of the eye, eventually fall on the retina. The retina serves as a kind of spherical screen onto which the surrounding world is projected. From the school physics course, we know that a collecting lens gives an inverted image of an object. The cornea and the lens are two collecting lenses, and the image projected onto the retina is also inverted. In other words, the sky is projected onto the lower half of the retina, the sea is projected onto the upper half, and the ship we are looking at is displayed on the macula. The macula, the central part of the retina, is responsible for high visual acuity. Other parts of the retina will not allow us to read or enjoy working on a computer. Only in the macula are all conditions created for the perception of small details of objects.
In the retina, optical information is perceived by light-sensitive nerve cells, encoded into a sequence of electrical impulses and transmitted through the optic nerve to the brain for final processing and conscious perception.
Cornea
The transparent convex window in front of the eye is the cornea. The cornea is a strong refractive surface, providing two-thirds of the eye's optical power. Resembling a peephole in shape, it allows you to see the world around us well.
Since the cornea has no blood vessels, it is perfectly transparent. The absence of blood vessels in the cornea determines the characteristics of its blood supply. The posterior surface of the cornea is nourished by moisture from the anterior chamber, which is produced by the ciliary body. The front part of the cornea receives oxygen for the cells from the surrounding air, that is, it essentially does without the help of the lungs and the circulatory system. Therefore, at night, when the eyelids are closed, and when wearing contact lenses, the supply of oxygen to the cornea is significantly reduced. The vascular network of the limbus plays an important role in providing the cornea with nutrients.
The cornea normally has a shiny and mirror-like surface. This is largely due to the work of the tear film, constantly wetting the corneal surface. Constant wetting of the surface is achieved by blinking movements of the eyelids, which are carried out unconsciously. There is the so-called blinking reflex, which is activated when microscopic zones of the dry surface of the cornea appear with a prolonged absence of blinking movements. This opportunity is felt by nerve endings ending between the cells of the surface epithelium of the cornea. Information about this enters the brain through the nerve trunks and is transmitted as a command to the muscles of the eyelids to contract. The whole process takes place without the participation of consciousness, than the latter, naturally, is significantly freed for the implementation of other benefits. Although, if desired, consciousness can suppress this reflex for a rather long time. This skill is especially useful during the children's game "who will look over whom".
The thickness of the cornea in a healthy adult eye is on average slightly more than half a millimeter. It is in the very center of it. The closer to the edge of the cornea, the thicker it becomes, reaching one millimeter. Despite such diminutiveness, the cornea consists of various layers, each of which has its own specific function. There are five such layers (in order of location from outside to inside) - epithelium, Bowman's sheath, stroma, Descemet's sheath, endothelium. The structural base of the cornea, its most powerful layer, is the stroma. The stroma consists of the thinnest plates formed by strictly oriented collagen protein fibers. Collagen is one of the strongest proteins in the body and provides strength to bones, joints and ligaments. Its transparency in the cornea is associated with the strict periodicity of the collagen fibers in the stroma.
Conjunctiva
The conjunctiva is a thin, transparent tissue that covers the outside of the eye. It starts at the limbus, the outer edge of the cornea, and covers the visible part of the sclera as well as the inner surface of the eyelids. In the thickness of the conjunctiva, there are vessels that feed it. These vessels can be viewed with the naked eye. With inflammation of the conjunctiva, conjunctivitis, the vessels dilate and give a picture of a red irritated eye, which most had the opportunity to contemplate in their mirror.
The main function of the conjunctiva is to secrete the mucous and liquid portion of the tear fluid, which moisturizes and lubricates the eye.
Limbo
The dividing strip between the cornea and the sclera, 1.0-1.5 millimeters wide, is called the limbus. Like much in the eye, the small size of its individual part does not exclude the critical importance for the normal functioning of the entire organ as a whole. The limbus contains many vessels that take part in the nutrition of the cornea. The limbus is an important growth zone for the corneal epithelium. There is a whole group of eye diseases, which are caused by damage to the growth or stem cells of the limbus. An insufficient number of stem cells often occurs with an eye burn, most of all with a chemical burn. The inability to form cells in the required amount for the corneal epithelium leads to the ingrowth of vessels and scar tissue onto the cornea, which inevitably leads to a decrease in its transparency. The result is a sharp deterioration in vision.
Choroid
The choroid consists of three parts: in front - the iris, then - the ciliary body, behind - the most extensive part - the choroid itself. The choroid itself, hereinafter called the choroid, is located between the retina and the sclera. It consists of blood vessels that feed the posterior segment of the eye, primarily the retina, where active processes of light perception, transmission and primary processing of visual information take place. The choroid is connected to the ciliary body in front and attaches to the edges of the optic nerve in the back.
Iris
The part of the eye that is used to judge the color of the eyes is called the iris. Eye color depends on the amount of melanin pigment in the back layers of the iris. The iris controls the penetration of light rays into the eye in various lighting conditions, similar to the diaphragm in a camera. The round hole in the center of the iris is called the pupil. The structure of the iris includes microscopic muscles that constrict and dilate the pupil.
The muscle that constricts the pupil is located at the very edge of the pupil. In bright light, this muscle contracts, causing the pupil to constrict. The fibers of the muscle that dilate the pupil are oriented in the thickness of the iris in the radial direction, therefore, their contraction in a dark room or when frightened leads to the dilation of the pupil.
Approximately, the iris is a plane that conventionally divides the anterior section of the eyeball into the anterior and posterior chambers.
Pupil
The pupil is the opening in the center of the iris that allows rays of light to enter the eye to be perceived by the retina. By changing the size of the pupil by contracting special muscle fibers in the iris, the eye controls the amount of light in the retina. This is an important adaptive mechanism, because the variation in illumination in physical quantities between a cloudy autumn night in a forest and a bright sunny afternoon in a snow-covered field is measured millions of times. Both in the first and in the second case, and with all other levels of illumination between them, the healthy eye does not lose the ability to see and receives the maximum possible information about the surrounding situation.
Ciliary body
The ciliary body is located directly behind the iris. Thin fibers are attached to it, on which the lens is suspended. The fibers on which the lens is suspended are called zonular. The ciliary body continues posteriorly into the choroid itself.
The main function of the ciliary body is to produce the aqueous humor of the eye, a clear fluid that fills and nourishes the anterior regions of the eyeball. That is why the ciliary body is extremely rich in vessels. The work of special cellular mechanisms achieves filtration of the liquid part of the blood in the form of aqueous humor, which normally contains practically no blood cells and has a strictly regulated chemical composition.
In addition to the abundant vascular network, muscle tissue is well developed in the ciliary body. The ciliary muscle, through its contraction and relaxation and the associated change in the tension of the fibers on which the lens is suspended, changes the shape of the latter. The contraction of the ciliary body leads to relaxation of the zonular fibers and to a greater thickness of the lens, which increases its optical power. This process is called accommodation, and it turns on when there is a need to consider closely spaced objects. When looking into the distance, the ciliary muscle relaxes and stretches the zonular fibers. The lens becomes thinner, its strength as a lens decreases, and the eye is focused on distance vision.
With age, the eye's ability to optimally adjust to near and far distances is lost. Optimum focusing is at a certain distance from the eyes. More often than not, in people who had good vision in their youth, the eye remains "tuned" to a long distance. This condition is called presbyopia and is manifested primarily by reading difficulties.
Retina
The retina is the thinnest inner lining of the eye that is sensitive to light. This photosensitivity is provided by so-called photoreceptors - millions of nerve cells that convert a light signal into an electrical one. Further, other nerve cells of the retina initially process the information received and transmit it in the form of electrical impulses along their fibers to the brain, where the final analysis and synthesis of visual information and the perception of the latter at the level of consciousness take place. The bundle of nerve fibers from the eye to the brain is called the optic nerve.
There are two types of photoreceptors - cones and rods. There are fewer cones - there are only about 6 million of them in each eye. Cones are practically only found in the macula, the part of the retina responsible for central vision. Their maximum density is reached in the central part of the macula, known as the dimple. The cones work in good light and make it possible to distinguish color. They are responsible for daytime vision.
The retina also contains up to 125 million cones. They are scattered around the periphery of the retina and provide lateral vision, albeit indistinct, but possible at dusk.
Retinal vessels
The cells of the retina have a high demand for oxygen and nutrients. The retina has a dual blood supply system. The leading role is played by the choroid, which covers the retina from the outside. Photoreceptors and other retinal nerve cells receive everything they need from the capillaries of the choroid.
Those vessels, which are indicated in the figure, form a second blood supply system responsible for nourishing the inner layers of the retina. These vessels originate from the central retinal artery, which enters the eyeball in the thickness of the optic nerve and appears in the fundus of the optic nerve head. Further, the central retinal artery is divided into superior and inferior branches, which, in turn, branch into the temporal and nasal artery. Thus, the arterial system, visible in the fundus, consists of four main trunks. Veins follow the course of arteries and serve as a conduit for blood in the opposite direction.
Sclera
The sclera is the strong outer skeleton of the eyeball. Its anterior part is visible through the transparent conjunctiva as the "white of the eye". Six muscles are attached to the sclera, which control the direction of the gaze and simultaneously turn both eyes to either side.
The strength of the sclera depends on age. The thinnest sclera in children. Visually, this is manifested by a bluish tint of the sclera of children's eyes, which is explained by the transmission of the dark pigment of the fundus through the thin sclera. The sclera becomes thicker and stronger with age. Thinning of the sclera is most common with myopia.
Macula
The macula is the central part of the retina, which is located towards the temple from the optic nerve head. The vast majority of those who have ever attended school have heard that there are rods and cones in the retina. So, there are only cones in the macula, which are responsible for detailed color vision. Without the macula, it is impossible to read, distinguish between small details of objects. All conditions have been created in the macula for the maximum possible detailed registration of light rays. The retina in the macular area becomes thinner, which allows light rays to directly hit the light-sensitive cones. There are no retinal vessels in the macula that would interfere with clear vision. Macular cells are nourished from the deeper choroid.
Lens
The lens is located directly behind the iris and, due to its transparency, is no longer visible to the naked eye. The main function of the lens is to dynamically focus the image on the retina. The lens is the second (after the cornea) optical power lens of the eye, which changes its refractive power depending on the distance of the object under consideration from the eye. At a close distance to the object, the lens enhances its strength, at a distance, it weakens.
The lens is suspended on the finest fibers woven into its shell - a capsule. At the other end, these fibers are attached to the processes of the ciliary body. The innermost part of the lens, the densest, is called the nucleus. The outer layers of the lens material are called the cortex. The lens cells are constantly multiplying. Since the lens is limited from the outside by the capsule, and the volume available to it in the eye is limited, the density of the lens increases with age. This is especially true for the nucleus of the lens. As a result, with age, people develop a condition called presbyopia, i.e. the inability of the lens to change its optical power leads to difficulty in seeing the details of objects close to the eye.
Vitreous
The vast space between the lens and the retina is filled with a gel-like, gelatinous transparent substance called the vitreous body. It occupies about 2/3 of the volume of the eyeball and gives it shape, turgor and incompressibility. 99 percent of the vitreous body consists of water, especially associated with special molecules, which are long chains of repeating links - sugar molecules. These chains, like the branches of a tree, are connected at one end to the trunk, represented by a protein molecule.
The vitreous body has many useful functions, the most important of which is maintaining the retina in its normal position. In newborns, the vitreous body is a homogeneous gel. With age, for not fully known reasons, the vitreous degenerates, leading to the adhesion of individual molecular chains into large clusters. Homogeneous in infancy, the vitreous body divides into two components with age - an aqueous solution and an accumulation of chain molecules. In the vitreous body, water cavities and floating, visible to the person himself in the form of "flies", accumulations of molecular chains are formed. Ultimately, this process leads to the fact that the posterior surface of the vitreous body exfoliates from the retina. This can lead to a sharp increase in the number of floating opacities - flies. By itself, such a detachment of the vitreous body is not dangerous, but in rare cases it can lead to a detachment of the retina.
Optic nerve
The optic nerve transmits information received in light rays and perceived by the retina in the form of electrical impulses to the brain. The optic nerve serves as a link between the eye and the central nervous system. It comes out of the eye near the macula. When the doctor examines the fundus with a special device, he sees the exit site of the optic nerve in the form of a rounded, pale pink formation called the optic nerve head.
There are no light-sensing cells on the surface of the optic nerve head. Therefore, a so-called blind spot is formed - an area of \u200b\u200bspace where a person does not see anything. Normally, a person usually does not notice such a phenomenon, since he uses two eyes, the fields of view of which overlap, and also due to the ability of the brain to ignore the blind spot and complete the image.
Lacrimal meat
This rather large part of the eye surface is clearly visible in the inner (close to the nose) corner of the eye in the form of a pink convex formation. The lacrimal meatus is covered with conjunctiva. In some people, it may be covered with fine hairs. The conjunctiva of the inner corner of the eye is generally very sensitive to touch, especially the lacrimal caruncle.
The lacrimal meat does not carry any specific functions in the eye and is in essence a rudiment, that is, a residual organ inherited from our common ancestors with snakes and other amphibians. Snakes have a third eyelid that attaches to the inner corner of the eye and, being transparent, allows these creatures to see well without risking damage to the delicate eye structures. The lacrimal meat in the human eye is the third eyelid of amphibians and reptiles, atrophied as unnecessary.
Anatomy and physiology of the lacrimal apparatus
The lacrimal organs include the lacrimal organs (lacrimal glands, accessory lacrimal glands in the conjunctiva) and the lacrimal ducts (lacrimal openings, tubules, lacrimal sac and nasolacrimal duct).
The lacrimal points, located at the inner corner of the palpebral fissure, are the beginning of the lacrimal ducts and lead to the lacrimal canals, which flow, joining into one, or each individually, into the upper part of the lacrimal sac.
The lacrimal sac is located under the medial ligament in the lacrimal fossa and below it passes into the nasolacrimal duct, located in the bony nasolacrimal canal and opening under the inferior turbinate into the lower nasal passage. There are folds and ridges along the duct, the most pronounced of them at the outlet of the nasolacrimal duct is called the Gasner valve. The folds provide a "locking" mechanism that prevents the contents of the nasal cavity from penetrating into the conjunctival cavity. There are massive venous plexuses in the walls of the nasolacrimal duct.
A tear consists mainly of water (over 98 percent), it contains mineral salts, mainly sodium chloride, a little protein and, in addition, a weakly bactericidal substance - lysozyme. The tear produced by the lacrimal glands, under its own weight and with the help of the blinking movements of the eyelids, flows into the "lacrimal lake" at the inner corner of the palpebral fissure, from where it moves through the lacrimal openings into the lacrimal tubules due to their sucking action when blinking. The compression and expansion of the lacrimal sac and the suction effect of nasal breathing also contribute to the movement of the tear further.
Tears moisten the surface of the eyeball, as if washing away small foreign particles from it, helping to ensure that the cornea of \u200b\u200bthe eye is transparent, and protect it from drying out. Tears also detoxify microbes in the conjunctival sac. The lacrimal fluid that enters the nasal cavity is evaporated along with the exhaled air.
Spasm of accommodation
To understand the mechanism of accommodation spasm, it is necessary to find out what accommodation is. The human eye has a natural property to change its refractive power to different distances by changing the shape of the lens. The body of the eye contains a muscle that is connected to the lens and regulates its curvature. As a result of its contraction, the lens changes its shape and, accordingly, more or less refracts the rays of light entering the eye.
To obtain clear images on the retina located close to objects, such an eye must increase the refractive power due to the tension of accommodation, i.e., by increasing the curvature of the lens. The closer the object is, the more convex the lens becomes in order to transfer the focal image to the retina. When examining distant objects, the lens should be flattened as much as possible. This requires relaxation of the accommodative muscle.
Intense visual work at close range (reading, working on a computer) leads to a spasm of accommodation and is characterized by features of a serious illness. The visual working area shifts closer to the eye and is sharply limited when the patient tries to overcome the difficulties that arise during visual work. People who suffer from accommodation spasm for a long time become irritable, get tired quickly, and often complain of a headache. According to some reports, every sixth student suffers from spasms. Some children develop persistent school myopia, after the formation of which the eye is fully adapted to work at close range. However, in this case, the high visual acuity in the distance is lost, which, of course, is undesirable, but with the indicated restructuring, it is inevitable. Preservation of good eyesight requires prevention activities in schools.
With age, there is a natural change in accommodation. The reason for this is the condensation of the lens. It becomes less and less plastic and loses its ability to change shape. This usually happens after 40 years. But true spasm in adulthood is a rare phenomenon that occurs in severe disorders of the central nervous system. Spasm of accommodation is also noted in hysteria, functional neuroses, general contusions, closed skull injuries, metabolic disorders, menopause. The strength of the spasm can reach from 1 to 3 diopters.
The duration of this disease ranges from several months to several years, depending on the general condition of the patient, his mode of life, and the nature of his work. A spasm of accommodation is revealed by an ophthalmologist when choosing corrective glasses or with typical patient complaints.
Choroid of the eyeball (tunica vasculosa bulbi). Embryogenetically, it corresponds to the pia mater and contains a dense plexus of vessels. It is divided into three sections: the iris ( iris), ciliary or ciliary body ( corpus ciliare) and the choroid itself ( chorioidea). Each of these three sections of the vascular tract has specific functions.
Iris is the front clearly visible section of the vascular tract.
The physiological significance of the iris is that it is a kind of diaphragm that regulates the flow of light into the eye, depending on the conditions. Optimal conditions for high visual acuity are provided with a pupil width of 3 mm. In addition, the iris takes part in ultrafiltration and outflow of intraocular fluid, and also ensures the constancy of the moisture temperature of the anterior chamber and the tissue itself by changing the width of the vessels. The iris is a pigmented circular plate located between the cornea and the lens. In its center there is a round hole, the pupil ( pupilla), the edges of which are covered with a pigmented fringe. The iris has an exceptionally peculiar pattern due to the radially located rather densely intertwined vessels and connective tissue beams (lacunae and trabeculae). Due to the looseness of the iris tissue, many lymphatic spaces are formed in it, which open on the anterior surface with pits or lacunae, crypts of various sizes.
In the anterior part of the iris, there are many process pigment cells - chromatophores containing golden xanthophores and silvery guanophores. The posterior region of the iris is black due to the large number of fuscin-filled pigment cells.
In the anterior mesodermal layer of the iris of the newborn, the pigment is almost absent and the posterior pigment plate shines through the stroma, causing the bluish color of the iris. The iris acquires a permanent color by the age of 10-12 years of a child's life. In places of accumulation of pigment "freckles" of the iris are formed.
In old age, depigmentation of the iris is observed in connection with sclerotic and dystrophic processes in the aging body, and it again acquires a lighter color.
There are two muscles in the iris. The circular muscle, constricting the pupil (m. Sphincter pupillae), consists of circular smooth fibers located concentrically to the pupillary edge to a width of 1.5 mm - the pupillary girdle; innervated by parasympathetic nerve fibers. The muscle that dilates the pupil (m. Dilatator pupillae) consists of pigmented smooth fibers lying radially in the posterior layers of the iris and having sympathetic innervation. In young children, the iris muscles are poorly expressed, the dilator is almost non-functional; the sphincter predominates and the pupil is always narrower than in older children.
The peripheral part of the iris is the ciliary (ciliary) belt up to 4 mm wide. At the border of the pupillary and ciliary zones, by 3-5 years, a collar (mesentery) is formed, in which the small arterial circle of the iris is located, formed by the anastomosing branches of the great circle and providing blood supply to the pupillary girdle.
The large arterial circle of the iris is formed on the border with the ciliary body due to the branches of the posterior long and anterior ciliary arteries, anastomosed with each other and giving return branches to the choroid itself.
The iris is innervated by sensitive (ciliary), motor (oculomotor) and sympathetic nerve branches. Constriction and dilation of the pupil is carried out mainly through the parasympathetic (oculomotor) and sympathetic nerves. In the case of damage to the parasympathetic pathways, while maintaining sympathetic pathways, there is absolutely no reaction of the pupil to light, convergence and accommodation. The elasticity of the iris, which depends on the person's age, also affects the size of the pupil. In children under 1 year old, the pupil is narrow (up to 2 mm) and reacts poorly to light, weakly expands, in adolescence and young age it is wider than average (up to 4 mm), reacts vividly to light and other influences; to old age, when the elasticity of the iris decreases sharply, the pupils, on the contrary, narrow and their reactions are weakened. None of the parts of the eyeball contains as many indicators for understanding the physiological and especially the pathological state of the human central nervous system as the pupil. This unusually sensitive apparatus easily reacts to various psycho-emotional shifts (fear, joy), diseases of the nervous system (tumors, congenital syphilis), diseases of internal organs, intoxication (botulism), childhood infections (diphtheria), etc.
Ciliary body - this is, figuratively speaking, the gland of the internal secretion of the eye. The main functions of the ciliary body are the production (ultrafiltration) of intraocular fluid and accommodation, that is, the creation of conditions for clear vision near and far. In addition, the ciliary body takes part in the blood supply to the underlying tissues, as well as in maintaining normal ophthalmotonus due to both the production and the outflow of intraocular fluid.
The ciliary body is like a continuation of the iris. Its structure can be found only with tonnage and cycloscopy. The ciliary body is a closed ring about 0.5 mm thick and almost 6 mm wide, located under the sclera and separated from it by the supraciliary space. On the meridional section, the ciliary body has a triangular shape with a base towards the iris, one apex towards the choroid, the other towards the lens and contains the ciliary (accommodative muscle - m. ciliaris), consisting of smooth muscle fibers. On the tuberous front inner surface of the ciliary muscle, there are more than 70 ciliary processes ( processus ciliares). Each ciliary process consists of a stroma with a rich network of vessels and nerves (sensory, motor, trophic), covered with two sheets (pigment and non-pigment) epithelium. The anterior segment of the ciliary body, which has pronounced processes, is called the ciliary crown ( corona ciliaris), and the posterior processless part is the ciliary circle ( orbiculus ciliaris) or flat section ( pars plana). The stroma of the ciliary body, like the iris, contains a large number of pigment cells - chromatophores. However, the ciliary processes do not contain these cells.
The stroma is covered with an elastic vitreous plate. Further inwards, the surface of the ciliary body is covered with ciliary epithelium, pigment epithelium and, finally, the inner vitreous membrane, which are a continuation of similar formations of the retina. Zonal fibers are attached to the vitreous membrane of the ciliary body ( fibrae zonulares), on which the lens is fixed. The posterior border of the ciliary body is the dentate line (ora serrata), where the vascular itself begins and the optically active part of the retina ends ( pars optica retinae).
The blood supply to the ciliary body is carried out through the posterior long ciliary arteries and anastomoses with the vasculature of the iris and choroid. Thanks to the rich network of nerve endings, the ciliary body is very sensitive to any irritation.
In newborns, the ciliary body is underdeveloped. The ciliary muscle is very thin. However, by the second year of life, it significantly increases and due to the appearance of combined contractions of all eye muscles, it acquires the ability to accommodate. With the growth of the ciliary body, its innervation is formed and differentiated. In the first years of life, the sensitive innervation is less perfect than the motor and trophic, and this is manifested in the painlessness of the ciliary body in children during inflammatory and traumatic processes. In seven-year-old children, all relationships and sizes of the morphological structures of the ciliary body are the same as in adults.
Choroid itself (chorioidea) is the posterior part of the vascular tract, visible only with biomicro- and ophthalmoscopy. It is located under the sclera. The choroid accounts for 2/3 of the entire vascular tract. The choroid takes part in feeding the avascular structures of the eye, photoenergetic layers of the retina, in ultrafiltration and outflow of intraocular fluid, in maintaining normal ophthalmotonus. The choroid is formed by the posterior short ciliary arteries. In the anterior section, the vessels of the choroid are anastomosed with the vessels of the great arterial circle of the iris. In the posterior section, around the optic nerve head, there are anastomoses of the vessels of the choriocapillary layer with the capillary network of the optic nerve from the central retinal artery. The choroid thickness is up to 0.2 mm at the posterior pole and up to 0.1 mm in the front. Between the choroid and the sclera there is a perichoroidal space (spatium perichorioidale), filled with outflowing intraocular fluid. In early childhood, the perichoroidal space is almost completely absent; it develops only by the second half of the child's life, opening in the first months first in the area of \u200b\u200bthe ciliary body.
Choroid is a multi-layered formation. The outer layer is formed by large vessels (vascular plate, lamina vasculosa). Between the vessels of this layer there is a loose connective tissue with cells - chromatophores, the color of the choroid depends on their number and color. As a rule, the number of chromatophores in the choroid corresponds to the general pigmentation of the human body and is relatively small in children. Thanks to the pigment, the choroid forms a kind of dark camera obscura, preventing the reflection of the rays coming through the pupil into the eye and providing a clear image on the retina. If there is little or no pigment in the choroid (more often in fair-haired persons), then there is an albino picture of the fundus. In such cases, the function of the eye is significantly reduced. In this shell, in the layer of large vessels, there are also 4-6 vorticose, or vortex, veins ( v. vorticosae), through which venous outflow occurs mainly from the posterior part of the eyeball.
Next comes the layer of middle vessels. There are fewer connective tissue and chromatophores, and veins predominate over arteries. Behind the middle vascular layer is a layer of small vessels, from which branches branch off into the innermost - choriocapillary layer ( lamina choriocapillaris). The choriocapillary layer has an unusual structure and passes through its lumen (lacunae) not one uniform blood element, as usual, but several in one row. In terms of diameter and number of capillaries per unit area, this layer is the most powerful in comparison with others. The upper wall of the capillaries, that is, the inner shell of the choroid, is the vitreous plate, which serves as the border with the retinal pigment epithelium, which, however, is intimately connected with the choroid. It should be noted that the vasculature is most dense in the posterior choroid. It is very intense in the central (macular) region and poor in the area of \u200b\u200bthe optic nerve exit and near the dentate line.
The choroid contains, as a rule, the same amount of blood (up to 4 drops). An increase in the choroidal volume by one drop can cause an increase in pressure inside the eye by more than 30 mm Hg. Art. A relatively large amount of blood continuously passing through the choroid provides constant nutrition for the retinal pigment epithelium associated with the choroid, where active photochemical processes take place. The innervation of the choroid is mainly trophic. Due to the absence of sensitive nerve fibers in it, its inflammation, injuries and tumors are painless.
The choroid is the middle membrane of the eyeball, and is located between the outer membrane (sclera) and the inner membrane (retina). The choroid is also called the vascular tract (or “uvea” in Latin).
During embryonic development, the vascular tract has the same origin as the pia mater. There are three main parts in the choroid:
The choroid is a layer of a special connective tissue that contains many small and large vessels. Also, the choroid consists of a large number of pigment cells and smooth muscle cells. The vascular system of the choroid is formed by long and short posterior ciliary arteries (branches of the orbital artery). The outflow of venous blood occurs through the vorticose veins (4-5 in each eye). The vorticose veins are usually located posterior to the equator of the eyeball. The vorticose veins have no valves; from the choroid they pass through the sclera, after which they flow into the veins of the orbit. From the ciliary muscle, blood also flows through the anterior ciliary veins.
The choroid is adjacent to the sclera almost throughout. However, there is a perichoroidal space between the sclera and the choroid. This space is filled with intraocular fluid. The periochoroidal space is of great clinical importance, since it is an additional pathway for the outflow of aqueous humor (the so-called uveoscleral pathway. Also in the periochoroidal space, detachment of the anterior part of the choroid usually begins in the postoperative period (after operations on the eyeball). the choroid cause the development of various diseases in it.
Diseases of the choroid are classified as follows:
1. Congenital diseases (or abnormalities) of the choroid.
2. Acquired diseases of the choroid
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The following research methods are used to examine the choroid and diagnose various diseases: biomicroscopy, gonioscopy, cycloscopy, ophthalmoscopy, fluorescent angiography. Additionally, methods for studying the hemodynamics of the eye are used: rheoophthalmography, ophthalmodynamography, ophthalmoplethysmography. An ultrasound scan of the eye is also indicative for detecting choroidal detachment or tumor formations.
The choroid of the eyeball (tunica fasculisa bulbi) is the middle shell of the eyeball. It contains a plexus of blood vessels and pigment cells. This membrane is divided into 3 parts: the iris, the ciliary body, and the choroid itself. The median location of the choroid between the fibrous and reticular membranes contributes to the retention of excess rays falling on the retina by the pigment layer and the distribution of vessels in all layers of the eyeball.
Iris(iris) - the anterior part of the choroid of the eyeball, has the form of a circular, upright plate with a round hole - the pupil (pupilla). The pupil does not lie exactly in the middle of it, but is slightly displaced towards the nose. The iris plays the role of a diaphragm, which regulates the amount of light entering the eyes, due to which the pupil narrows in strong light, and dilates in weak light.
With its outer edge, the iris is connected to the ciliary body and the sclera, its inner edge surrounding the pupil is free. In the iris, the anterior surface facing the cornea and the posterior surface adjacent to the lens are distinguished. The anterior surface seen through the transparent cornea has a different coloration in different people and determines the color of the eyes. The color depends on the amount of pigment in the surface layers of the iris. If there is a lot of pigment, then the eyes are brown (brown) up to black, if the pigment layer is poorly developed or even absent, then mixed greenish-gray and blue tones are obtained. The latter mainly arise from the translucency of the black retinal pigment on the back of the iris.
The iris, performing the function of the diaphragm, has amazing mobility, which is ensured by the subtle adaptability and correlation of its constituent components. The base of the iris (stroma iridis) consists of connective tissue, which has a lattice architecture, into which vessels are inserted that extend radially from the periphery to the pupil. These vessels, which are the only carriers of elastic elements, together with the connective tissue form the elastic skeleton of the iris, which allows it to easily change in size.
The movements of the iris are carried out by the muscular system, which lies in the thickness of the stroma. This system consists of smooth muscle fibers, which are partly arranged in an annular manner around the pupil, forming a muscle that narrows the pupil (m. Sphincter pupillae), and partly diverge radially from the pupillary opening and form a muscle that dilates the pupil (m. Dilatator pupillae). Both muscles are mutually connected: the sphincter stretches the dilator, and the dilator stretches the sphincter. The impermeability of the diaphragm to light is achieved by the presence of a bilayer pigment epithelium on its posterior surface. On the anterior surface, washed by fluid, it is covered with the anterior chamber endothelium.
Ciliary body (corpus ciliare) is located on the inner surface at the junction of the sclera into the cornea. In cross section, it has the shape of a triangle, and when viewed from the side of the posterior pole, it is in the shape of a circular ridge, on the inner surface of which there are radially oriented processes (processus ciliares) numbering about 70.
The ciliary body and the iris are attached to the sclera by spongy ligaments. These cavities are filled with fluid coming from the anterior chamber and then into the circular venous sinus (helmet canal). Annular ligaments depart from the ciliary processes, which are woven into the lens capsule.
Process accommodation, i.e. adaptation of the eye to near or distant vision, is possible due to the weakening or tension of the annular ligaments. They are under the control of the muscles of the ciliary body, consisting of meridian and circular fibers. With the contraction of the circular muscles, the ciliary processes approach the center of the ciliary circle and the annular ligaments weaken. Due to internal elasticity, the lens straightens and its curvature increases, thereby reducing the focal length.
Simultaneously with the contraction of the circular muscle fibers, the meridional muscle fibers also contract, which tighten the posterior part of the choroid and the ciliary body as much as the focal length of the light beam decreases. When relaxing due to elasticity, the ciliary body takes its original position and, stretching the annular ligaments, strains the lens capsule, flattening it. In this case, the posterior pole of the eye also takes up its original position.
In old age, part of the muscle fibers of the ciliary body is replaced by connective tissue. The elasticity and firmness of the lens also decreases, resulting in impaired vision.
Choroid itself (chorioidea) - the back of the choroid, covering 2/3 of the eyeball. The shell consists of elastic fibers, blood and lymph vessels, and pigment cells that create a dark brown background. It is loosely adhered to the inner surface of the tunica albuginea and is easily displaced during accommodation. In animals, calcium salts accumulate in this part of the choroid, which form an eye mirror that reflects light rays, which creates conditions for the glow of the eyes in the dark.
Retina
The retina is the innermost shell of the eyeball, extending to the serrated edge (area serrata), which lies at the junction of the ciliary body into the choroid itself. Along this line, the retina is divided into anterior and posterior parts. The mesh shell has 11 layers that can be combined into 2 sheets: pigmentary - outdoor and cerebral - internal. Light-sensitive cells are located in the medulla - rods and cones; their outer light-sensitive segments are directed towards the pigment layer, i.e. outward. The next layer is bipolar cellsthat form contacts with rods, cones and ganglion cells, the axons of which form the optic nerve. In addition, there are horizontal cellslocated between rods and bipolar cells and amacrine cells to combine the function of ganglion cells.
The human retina contains about 125 million rods and 6.5 million cones. The macula has only cones, and the rods are located on the periphery of the retina. Retinal pigment cells isolate each light-sensitive cell from the other and from side rays, creating the conditions for imaginative vision. In bright light, rods and cones are immersed in the pigment layer. In the corpse, the retina is dull white, without characteristic anatomical features. When viewed with an ophthalmoscope, the retina (fundus) in a living person has a bright red background due to the translucence of the blood in the choroid. Against this background, bright red blood vessels of the fiber are visible.
Cones are photoreceptors of the vertebrate retina that provide day (photopic) and color vision. The thickened outer receptor process, directed towards the pigment layer of the retina, gives the cell the shape of a bulb (hence the name). Unlike rods, each fovea cone is usually connected via a bipolar neuron to a separate ganglion cell. As a result, the cones perform a detailed analysis of the image, have a high response rate, but low light sensitivity (more sensitive to the action of long waves). In the cones, as in rods, the outer and inner segments, the connective fiber, the nucleus-containing part of the cell, and the inner fiber that carry out synaptic communication with bipolar and horizontal neurons. The outer segment of the cone (a derivative of the cilium), consisting of numerous membrane discs, contains visual pigments - rhodopsins, which react to light of different spectral composition. The cones of the human retina contain 3 types of pigments, and each of them contains a pigment of the same type, which provides selective perception of one color or another: blue, green, red. The inner segment includes an accumulation of numerous mitochondria (ellipsoid), the contractile element is an accumulation of contractible fibrils (myoid) and glycogen granules (paraboloid). In most vertebrates, an oil drop is located between the outer and inner segments, selectively absorbing light before it reaches the visual pigment.
Sticks- photoreceptors of the retina, providing twilight (scotopic) vision. The outer receptor process gives the cell the shape of a rod (hence the name). Several rods are connected by synaptic communication with one bipolar cell, and several bipolar cells, in turn, with one ganglion cell, the axon of which enters the optic nerve. The outer segment of the rod, which consists of numerous membrane discs, contains the visual pigment, rhodopsin. In most diurnal animals and humans, rods prevail over cones at the retinal periphery.
At the posterior pole of the eye is located oval spot - a disc of the optic nerve (discus n. optici) measuring 1.6 - 1.8 mm with a depression in the center (excavatio disci). Branches of the optic nerve, devoid of the myelin sheath, and veins converge radially to this spot; arteries diverge into the visual part of the retina. These vessels supply blood only to the retina. By the vascular pattern of the retina, one can judge the state of the blood vessels of the whole organism and some of its diseases (iridology).
Laterally by 4 mm at the level of the optic nerve head lies spot (macula) with fovea (fovea centralis), colored red-yellow-brown. The focus of light rays is concentrated in the spot; it is the place of the best perception of light rays. The spot contains light-sensitive cells - cones. Rods and cones lie near the pigment layer. The light rays thus penetrate all layers of the transparent retina. Under the influence of light, rhodopsin of rods and cones breaks down into retinene and protein (scotopsin). As a result of the decay, energy is generated, which is captured by the bipolar cells of the retina. Rhodopsin is constantly resynthesized from scotopsin and vitamin A.
Visual pigment - structural and functional unit of the photosensitive membrane of the photoreceptors of the retina - rods and cones. The visual pigment molecule consists of a chromophore that absorbs light and opsin, a complex of protein and phospholipids. The chromophore is represented by the aldehyde of vitamin A1 (retinal) or A2 (dehydroretinal).
Opsins (rod and cone) and retinal, connecting in pairs, form visual pigments that differ in the absorption spectrum: rhodopsin (rod pigment), iodopsin(cone pigment, absorption maximum 562 nm), porphyropsin (rod pigment, absorption maximum 522 nm). Differences in the maxima of pigment absorption in animals of different species are also associated with differences in the structure of opsins, which interact differently with the chromophore. In general, these differences are of an adaptive nature, for example, species in which the absorption maximum is shifted to the blue part of the spectrum, live at great depths of the ocean, where light with a wavelength of 470 to 480 nm penetrates much better.
Rhodopsin,visual purple - pigment of rods of the retina of animals and humans; a complex protein, which includes the chromophore group of the retinal carotenoid (aldehyde of vitamin A1) and opsin - a complex of glycoprotein and lipids. The maximum of the absorption spectrum is about 500 nm. In the visual act, under the influence of light, rhodopsin undergoes cis-trans isomerization, accompanied by a change in the chromophore and its separation from the protein, a change in ion transport in the photoreceptor, and the appearance of an electrical signal, which is then transmitted to the neural structures of the retina. The synthesis of retinal is carried out with the participation of enzymes through vitamin A. Visual pigments close to rhodopsin (iodopsin, porphyropsin, cyanopsin) differ from it either in chromophore or opsin and have slightly different absorption spectra.
Eye cameras
Chambers of the eye - the space between the front surface of the iris and the back of the cornea is called anterior chamber eyeball (camera anterior bulbi). The anterior and posterior walls of the chamber converge together along its circumference at the angle formed by the junction of the cornea into the sclera, on the one hand, and the ciliary edge of the iris, on the other. Angle(angulus iridocornealis) is rounded off by a network of crossbars, which together make up baby ligament... Between the crossbars, the ligaments are slit spaces (fountain spaces). The angle is of great physiological importance for the circulation of fluid in the chamber, which, through the fountain spaces, is emptied into the adjacent scleral schlemm Canal.
Behind the iris is a narrower back camera eye (camera posterior bulbi), which is limited in front by the posterior surface of the iris, behind - lens, along the periphery - by the ciliary body. Through the pupillary opening, the back chamber communicates with the anterior chamber. The liquid serves as a nutrient for the lens and cornea, and also participates in the formation of the lenses of the eye.
Lens
Lens (lens) - light refracting medium of the eyeball. It is completely transparent and has the appearance of a lentil or biconvex glass. The central points of the anterior and posterior surfaces are called the lens poles, and the peripheral edge, where both surfaces merge into each other, is called the equator. The axis of the lens connecting both poles is 3.7 mm when looking into the distance and 4.4 mm when accommodated, when the lens is convex. The equatorial diameter is 9 mm. The lens with the plane of its equator stands at right angles to the optical axis, adjoining its front surface to the iris, and back to the vitreous body.
The lens is enclosed in a thin, also completely transparent structureless bag (capsula lentis) and is held in its position by a special ligament (zonula ciliaris), which is composed of many fibers going from the bag of the lens to the ciliary body. Between the fibers are fluid-filled spaces that communicate with the chambers of the eye.
Vitreous
The vitreous body (corpus vitreum) is a transparent jelly-like mass located in the cavity between the retina and the posterior surface of the lens. The vitreous body is formed by a transparent colloidal substance, consisting of thin rare connective tissue fibers, proteins and hyaluronic acid. Due to the depression from the side of the lens, a fossa (fossa hyaloidea) is formed on the anterior surface of the vitreous body, the edges of which are connected to the lens bag by means of a special ligament.
Eyelids
Eyelids (palpebrae) - connective tissue formations, covered with a thin layer of skin, limiting their front and rear edges (limbus palpebralis anteriores et posteriores) the palpebral fissure (rima palpebrum). The mobility of the upper eyelid (palpebra superior) is greater than that of the lower (palpebra inferior). The lowering of the upper eyelid is carried out due to the part of the muscle surrounding the orbit (m. Orbicularis oculi). As a result of the contraction of this muscle, the curvature of the arch of the upper eyelid decreases, as a result of which it shifts downward. The eyelid is lifted by a special muscle (m. Levator palpebrae superioris).
The inner surface of the eyelid is lined with a connecting sheath - conjunctiva... In the medial and lateral corners of the palpebral fissure there are ligaments of the eyelids. The medial corner is rounded, it contains lacrimal pool (lacus lacrimalis), in which there is an elevation - lacrimal meat (caruncula lacrimalis). At the edge of the connective tissue base of the eyelid, there are fatty glands (gll.tarsales), called the meibomian glands, the secret of which lubricates the edges of the eyelids and eyelashes.
Eyelashes(cilia) - short hard hairs that grow from the edge of the eyelid, serving as a lattice to protect the eye from getting small particles into it. The conjunctiva (tunica conjunctiva) starts from the edge of the eyelids, covers their inner surface, and then wraps around the eyeball, forming a conjunctival sac that opens from the front into the palpebral fissure. It is firmly adhered to the cartilage of the eyelids and loosely connected to the eyeball. In the places of transition of the connective tissue membrane from the eyelids to the eyeball, folds are formed, as well as the upper and lower fornices, which do not interfere with the movement of the eyeball and eyelids. Morphologically, the fold is a rudiment of the third century (nictitating membrane).
8.4.10. Lacrimal apparatus
The lacrimal apparatus (apparatus lacrimalis) is a system of organs designed to release tears and drain along the lacrimal ducts. The lacrimal apparatus includes lacrimal gland, lacrimal canal, lacrimal sac and nasolacrimal duct.
Lacrimal gland (gl. lacrimalis) secretes a clear liquid containing water, the enzyme lysozyme and a small amount of protein substances. The upper most part of the gland is located in the fossa of the lateral angle of the orbit, the lower part is under the upper part. Both lobes of the gland have an alveolar-tubular structure and 10 - 12 common ducts (ductuli excretorii), which open into the lateral part of the conjunctival sac. The lacrimal fluid along the capillary gap formed by the conjunctiva of the eyelid, the conjunctiva and the cornea of \u200b\u200bthe eyeball, washes it and merges along the edges of the upper and lower eyelids to the medial corner of the eye, penetrating into the lacrimal canals.
Lacrimal tubule (canaliculus lacrimalis) is represented by the upper and lower tubes with a diameter of 500 microns. They are located vertically in their initial part (3 mm), and then take a horizontal position (5 mm) and with a common trunk (22 mm) are poured into the lacrimal sac. The tubule is lined with squamous epithelium. The lumen of the tubules is not the same: the narrow spaces are located in the corner at the place of the transition of the vertical part to the horizontal one and at the place where it flows into the lacrimal sac.
Lacrimal sac (saccus lacrimalis) is located in the fossa of the medial wall of the orbit. The medial ligament of the eyelid passes in front of the sac. From its wall, bundles of muscle surrounding the orbit begin. The upper part of the sac begins blindly and forms a vault (fornix sacci lacrimalis), the lower part passes into the nasolacrimal duct. The nasolacrimal duct (ductus nasolacrimalis) is an extension of the lacrimal sac. It is a straight flattened tube 2 mm in diameter, 5 mm long with a bag, which opens into the anterior part of the nasal passage. The sac and duct are composed of fibrous tissue; their lumen is lined with flat epithelium.
Choroid of the eye (tunica vasculosa bulbi) is located between the outer capsule of the eye and the retina, therefore it is called the middle shell, vascular or uveal tract of the eye. It consists of three parts: the iris, the ciliary body and the choroid itself (choroid).
All complex functions of the eye are carried out with the participation of the vascular tract. At the same time, the vascular tract of the eye plays the role of an intermediary between metabolic processes taking place throughout the body and in the eye. A ramified network of wide thin-walled vessels with rich innervation carries out the transmission of general neurohumoral effects. The anterior and posterior parts of the vascular tract have different sources of blood supply. This explains the possibility of their separate involvement in the pathological process.
14.1. Anterior choroid - iris and ciliary body
14.1.1. The structure and function of the iris
Iris (iris) - the anterior part of the vascular tract. It determines the color of the eye, is the light and dividing diaphragm (Fig. 14.1).
Unlike other parts of the vascular tract, the iris does not touch the outer lining of the eye. The iris extends from the sclera slightly behind the limbus and is located freely in the frontal plane in the anterior segment of the eye. The space between the cornea and the iris is called the anterior chamber of the eye. Its depth in the center is 3-3.5 mm.
Behind the iris, between it and the lens, the posterior chamber of the eye is located in the form of a narrow slit. Both chambers are filled with intraocular fluid and communicate through the pupil.
The iris is visible through the cornea. The diameter of the iris is about 12 mm, its vertical and horizontal dimensions can differ by 0.5 - 0.7 mm. The peripheral part of the iris, called the root, can only be seen with a special technique called gonioscopy. In the center, the iris has a round hole - pupil (pupilla).
The iris consists of two leaves. The anterior leaflet of the iris is of mesodermal origin. Its outer boundary layer is covered with epithelium, which is a continuation of the posterior corneal epithelium. The basis of this leaf is the iris stroma, represented by blood vessels. With biomicroscopy, on the surface of the iris, one can see a lace pattern of the interlacing of the vessels, forming a kind of relief, individual for each person (Fig. 14.2). All vessels have a connective tissue cover. The towering details of the lace pattern of the iris are called trabeculae, and the recesses between them are called gaps (or crypts). The color of the iris is also individual: from blue, gray, yellowish-green in blondes to dark brown and almost black in brunettes. The differences in color are due to the different numbers of poly-processous pigment cells of melanoblasts in the iris stroma. In dark-skinned people, the number of these cells is so large that the surface of the iris does not look like lace, but like a thick-woven carpet. Such an iris is characteristic of the inhabitants of the southern and extreme northern latitudes as a factor of protection against blinding light flux.
Concentric to the pupil on the surface of the iris there is a jagged line formed by the interlacing of the vessels. It divides the iris into pupillary and ciliary (ciliary) edges. In the ciliary girdle, elevations are distinguished in the form of uneven circular contraction grooves, along which the iris folds when the pupil expands. The iris is the thinnest on the extreme periphery at the beginning of the root, so it is here that "the iris can be detached in case of contusion injury (Fig. 14.3).
The posterior layer of the iris has a todermal origin, it is a pigment-muscle formation. Embryologically, it is a continuation of the undifferentiated part of the retina. A dense pigment layer protects the eye from excessive light flux. At the edge of the pupil, the pigment leaf is inverted anteriorly and forms a pigment border. Two muscles of multidirectional action carry out constriction and expansion of the pupil, providing a dosed supply of light into the eye cavity. The sphincter, which constricts the pupil, is located in a circle at the very edge of the pupil. The dilator is located between the sphincter and the root of the iris. Dilator smooth muscle cells are arranged radially in one layer.
The rich innervation of the iris is carried out by the autonomic nervous system. The dilator is innervated by the sympathetic nerve, and the sphincter is innervated by the parasympathetic fibers of the ciliary node - by the oculomotor nerve. The trigeminal nerve provides sensory innervation to the iris.
The blood supply to the iris is carried out from the anterior and two posterior long ciliary arteries, which form a large arterial circle at the periphery. Arterial branches are directed towards the pupil, forming arcuate anastomoses. This is how a convoluted network of vessels of the iris ciliary girdle is formed. Radial branches extend from it, forming a capillary network along the pupillary edge. The veins of the iris collect blood from the capillary bed and are directed from the center to the root of the iris. The structure of the circulatory network is such that even with the maximum dilation of the pupil, the vessels do not bend at an acute angle and there is no disturbance of blood circulation.
Studies have shown that the iris can be a source of information about the state of internal organs, each of which has its own zone of representation in the iris. According to the state of these zones, screening iridology of the pathology of internal organs is carried out. Light stimulation of these zones is at the heart of iridotherapy.
Iris functions:
- shielding the eye from excess light flux;
- reflex dosing of the amount of light depending on the degree of illumination of the retina (light diaphragm);
- dividing diaphragm: the iris, together with the lens, performs the function of the iridocular diaphragm, separating the anterior and posterior parts of the eye, keeping the vitreous from moving forward;
- the contractile function of the iris plays a positive role in the mechanism of the outflow of intraocular fluid and accommodation;
- trophic and thermoregulatory.
