Choroid (choroid) – structure and functions. Choroid of the eyeball

Performing a transport function, the choroid supplies the retina with nutrients carried by the blood. It consists of a dense network of arteries and veins, which are closely intertwined, as well as loose fibrous connective tissue, rich in large pigment cells. Due to the fact that there are no sensory nerve fibers in the choroid, diseases associated with this organ are painless.

What is it and what is its structure?

The human eyes have three membranes that are closely interconnected, namely the sclera, the choroid or choroid, and the retina. The middle layer of the eyeball is an essential part of the blood supply to the organ. It contains the iris and ciliary body, from which the entire choroid extends and ends near the optic nerve head. Blood supply occurs through the ciliary vessels located posteriorly, and outflow through the vorticose veins of the eyes.

Due to the special structure of the blood flow and the small number of vessels, the risk of developing an infectious disease of the choroid increases.

An integral part of the middle layer of the eye is the iris, which contains pigment located in the chromatophores and is responsible for the color of the lens. It prevents direct light rays from entering and glare in the interior of the organ. Without the pigment, the sharpness and clarity of vision would be significantly reduced.

The choroid consists of the following components:

The shell is represented by several layers that perform specific functions.

• Perivascular space. It looks like a narrow gap located near the surface of the sclera and the vascular plate.
• Supravascular plate. Formed from elastic fibers and chromatophores. The more intense pigment is located in the center and decreases towards the sides.
• Vascular plate. It has the appearance of a brown membrane and a thickness of 0.5 mm. The size depends on the filling of the vessels with blood, since it is formed upward by a layering of large arteries, and downwards by medium-sized veins.
• Choriocapillary layer. It is a network of small vessels that turn into capillaries. Performs functions to ensure the functioning of the nearby retina.
• Bruch's membrane. The function of this layer is to allow oxygen into the retina.

Functions of the choroid

The most important task is the delivery of nutrients with the blood to the layer of the retina, which is located outward and contains cones and rods. The structural features of the membrane allow metabolic products to be removed into the bloodstream. Bruch's membrane limits the access of the capillary network to the retina, as metabolic reactions occur in it.

Anomalies and symptoms of diseases

Choroidal coloboma is one of the anomalies of this layer of the visual organ.

The nature of the disease can be acquired or congenital. The latter include anomalies of the choroid itself in the form of its absence; the pathology is called Choroidal Coloboma. Acquired diseases are characterized by degenerative changes and inflammation of the middle layer of the eyeball. Often, the inflammatory process of the disease involves the front part of the eye, which leads to partial loss of vision, as well as minor hemorrhages in the retina. When performing surgical operations to treat glaucoma, detachment of the choroid occurs due to pressure changes. The choroid can be subject to ruptures and hemorrhages due to injury, as well as the appearance of neoplasms.

Anomalies include:

• Polycoria. The iris contains several pupils. The patient's visual acuity decreases and he feels discomfort when blinking. Treated with surgery.
• Corectopia. Marked displacement of the pupil to the side. Strabismus and amblyopia develop, and vision sharply decreases.

The choroid proper (choroid) is the largest posterior section of the choroid (2/3 of the volume of the vascular tract), along the line from the dentate line to the optic nerve, formed by the posterior short ciliary arteries (6-12), which pass through the sclera at the posterior pole of the eye .

Between the choroid and the sclera there is a perichoroidal space filled with flowing intraocular fluid.

The choroid has a number of anatomical features:

• is devoid of sensitive nerve endings, therefore the pathological processes developing in it do not cause pain
• its vascular network does not anastomose with the anterior ciliary arteries; as a result, with choroiditis, the anterior part of the eye remains intact
• an extensive vascular bed with a small number of drainage vessels (4 vorticose veins) helps to slow down blood flow and settle pathogens of various diseases here
• limited connection with the retina, which in diseases of the choroid, as a rule, is also involved in the pathological process
• due to the presence of the perichoroidal space, it is quite easily exfoliated from the sclera. It is maintained in its normal position mainly due to the draining venous vessels that perforate it in the equator region. Vessels and nerves that penetrate the choroid from the same space also play a stabilizing role.

Functions

1. nutritional and metabolic- delivers food products with blood plasma to the retina to a depth of up to 130 microns (pigment epithelium, retinal neuroepithelium, outer plexiform layer, as well as the entire foveal retina) and removes metabolic reaction products from it, which ensures the continuity of the photochemical process. In addition, the peripapillary choroid nourishes the prelaminar region of the optic nerve head;
2. thermoregulation- removes with the blood flow excess thermal energy generated during the functioning of photoreceptor cells, as well as when light energy is absorbed by the retinal pigment epithelium during the visual work of the eye; the function is associated with a high blood flow rate in the choriocapillaris, and presumably with the lobular structure of the choroid and the predominance of the arteriolar component in the macular choroid;
3. structure-forming- maintaining the turgor of the eyeball due to the blood supply to the membrane, which ensures a normal anatomical relationship between the parts of the eye and the required level of metabolism;
4. maintaining the integrity of the external blood-retinal barrier- maintaining a constant outflow from the subretinal space and removing “lipid debris” from the retinal pigment epithelium;
5. regulation of ophthalmotonus, due to:
   • contraction of smooth muscle elements located in the layer of large vessels,
   • changes in the tension of the choroid and its blood supply,
   • influence on the rate of perfusion of the ciliary processes (due to the anterior vascular anastomosis),
   • heterogeneity in the size of venous vessels (volume regulation);
6. autoregulation- regulation by the foveal and peripapillary choroid of its volumetric blood flow with a decrease in perfusion pressure; the function is presumably associated with nitrergic vasodilator innervation of the central choroid;
7. stabilization of blood flow levels(shock-absorbing) due to the presence of two systems of vascular anastomoses, the hemodynamics of the eye are maintained in a certain unity;
8. light absorption- pigment cells located in the layers of the choroid absorb light flux, reduce light scattering, which helps obtain a clear image on the retina;
9. structural barrier- due to the existing segmental (lobular) structure, the choroid retains its functional usefulness when one or more segments are affected by the pathological process;
10. conductor and transport function- the posterior long ciliary arteries and long ciliary nerves pass through it, and carries out the uveoscleral outflow of intraocular fluid through the perichoroidal space.

The extracellular matrix of the choroid contains a high concentration of plasma proteins, which creates high oncotic pressure and ensures the filtration of metabolites through the pigment epithelium into the choroid, as well as through the supraciliary and suprachoroidal spaces. From the suprachoroid, the fluid diffuses into the sclera, scleral matrix and perivascular clefts of the emissaries and episcleral vessels. In humans, uveoscleral outflow is 35%.

Depending on fluctuations in hydrostatic and oncotic pressure, aqueous humor may be reabsorbed by the choriocapillaris layer. The choroid, as a rule, contains a constant amount of blood (up to 4 drops). An increase in choroidal volume by one drop can cause an increase in intraocular pressure of more than 30 mmHg. Art. The large volume of blood continuously passing through the choroid provides constant nutrition to the retinal pigment epithelium associated with the choroid. Choroidal thickness depends on blood supply and averages 256.3±48.6 µm in emmetropic eyes and 206.6±55.0 µm in myopic eyes, decreasing to 100 µm in the periphery.

The choroid becomes thinner with age. According to B. Lumbroso, the thickness of the choroid decreases by 2.3 microns per year. Thinning of the choroid is accompanied by impaired blood circulation in the posterior pole of the eye, which is one of the risk factors for the development of newly formed vessels. There was significant thinning of the choroid associated with increasing age in emmetropic eyes at all measurement points. In people under 50 years of age, the thickness of the choroid is on average 320 microns. In persons over 50 years of age, the thickness of the choroid decreases on average to 230 microns. In the group of people over 70 years of age, the average choroidal value is 160 µm. In addition, a decrease in choroidal thickness was noted with an increase in the degree of myopia. The average thickness of the choroid in emmetropes is 316 µm, in individuals with low and moderate myopia – 233 µm, and in individuals with high myopia – 96 µm. Thus, normally there are large differences in the thickness of the choroid depending on age and refraction.

Structure of the choroid

The choroid extends from the dentate line to the optic foramen. In these places it is tightly connected to the sclera. Loose attachment is present in the equator region and at the entry points of blood vessels and nerves into the choroid. For the rest of its length, it is adjacent to the sclera, separated from it by a narrow gap - suprachoroidal prowandering. The latter ends 3 mm from the limbus and at the same distance from the exit point of the optic nerve. Ciliary vessels and nerves pass through the suprachoroidal space, and fluid outflows from the eye.

The choroid is a formation consisting of five layers, the basis of which is a thin connective stroma with elastic fibers:

• suprachoroid;
• layer of large vessels (Haller);
• layer of middle vessels (Sattler);
• choriocapillaris layer;
• vitreous plate, or Bruch's membrane.

On a histological section, the choroid consists of lumens of vessels of various sizes, separated by loose connective tissue; process cells with a crumbly brown pigment, melanin, are visible in it. The number of melanocytes, as is known, determines the color of the choroid and reflects the nature of pigmentation of the human body. As a rule, the number of melanocytes in the choroid corresponds to the type of general pigmentation of the body. Thanks to the pigment, the choroid forms a kind of camera obscura, which prevents the reflection of rays entering the eye through the pupil and ensures a clear image on the retina. If there is little pigment in the choroid, for example, in fair-skinned people, or none at all, as is observed in albinos, its functionality is significantly reduced.

The vessels of the choroid make up its bulk and represent branches of the posterior short ciliary arteries that penetrate the sclera at the posterior pole of the eye around the optic nerve and give further dichotomous branching, sometimes before the arteries penetrate the sclera. The number of posterior short ciliary arteries ranges from 6 to 12.

The outer layer is formed by large vessels , between which there is loose connective tissue with melanocytes. The layer of large vessels is formed mainly by arteries, which are distinguished by the unusual width of the lumen and the narrowness of the intercapillary spaces. An almost continuous vascular bed is created, separated from the retina only by the lamina vitrea and a thin layer of pigment epithelium. In the layer of large vessels of the choroid there are 4-6 vorticose veins (v. vorticosae), through which venous outflow occurs mainly from the posterior part of the eyeball. Large veins are located close to the sclera.

Layer of middle vessels goes behind the outer layer. It contains much less melanocytes and connective tissue. Veins in this layer predominate over arteries. Behind the middle vascular layer is located layer of small vessels , from which branches extend into the innermost is the choriocapillaris layer (lamina choriocapillaris).

Choriocapillaris layer In terms of diameter and number of capillaries per unit area, it dominates over the first two. It is formed by a system of precapillaries and postcapillaries and has the appearance of wide lacunae. The lumen of each such lacuna can accommodate up to 3-4 red blood cells. In terms of diameter and number of capillaries per unit area, this layer is the most powerful. The densest vascular network is located in the posterior part of the choroid, less intense - in the central macular region and poor - in the area of ​​exit of the optic nerve and near the dentate line.

The arteries and veins of the choroid have the usual structure characteristic of these vessels. Venous blood flows from the choroid through the vorticose veins. The venous branches of the choroid flowing into them connect with each other within the choroid, forming a bizarre system of whirlpools and an expansion at the confluence of the venous branches - an ampulla, from which the main venous trunk departs. Vorticose veins exit the eyeball through oblique scleral canals on the sides of the vertical meridian behind the equator - two above and two below, sometimes their number reaches 6.

The inner lining of the choroid is vitreous plate, or Bruch's membrane , separating the choroid from the retinal pigment epithelium. Electron microscopic studies show that Bruch's membrane has a layered structure. The vitreous plate contains retinal pigment epithelial cells firmly connected to it. On the surface they have the shape of regular hexagons; their cytoplasm contains a significant amount of melanin granules.

From the pigment epithelium, the layers are distributed in the following order: the basement membrane of the pigment epithelium, the inner collagen layer, the elastic fiber layer, the outer collagen layer and the basement membrane of the choriocapillaris endothelium. Elastic fibers are distributed across the membrane in bundles and form a network-like layer, slightly shifted to the outside. In the anterior sections it is denser. The fibers of Bruch's membrane are immersed in a substance (amorphous substance), which is a mucoid gel-like medium, which includes acidic mucopolysaccharides, glycoproteins, glycogen, lipids and phospholipids. The collagen fibers of the outer layers of Bruch's membrane extend between the capillaries and are woven into the connective structures of the choriocapillaris layer, which promotes tight contact between these structures.

Suprachoroidal space

The outer border of the choroid is separated from the sclera by a narrow capillary gap, through which suprachoroidal plates, consisting of elastic fibers covered with endothelium and chromatophores, go from the choroid to the sclera. Normally, the suprachoroidal space is almost not expressed, but under conditions of inflammation and edema, this potential space reaches significant sizes due to the accumulation of exudate here, pushing apart the suprachoroidal plates and pushing the choroid inwards.

The suprachoroidal space begins at a distance of 2-3 mm from the exit of the optic nerve and ends approximately 3 mm short of the insertion of the ciliary body. Long ciliary arteries and ciliary nerves, enveloped in the delicate tissue of the suprachoroid, pass through the suprachoroidal space to the anterior part of the vascular tract.

The choroid easily moves away from the sclera along its entire length, with the exception of its posterior section, where the dichotomously dividing vessels included in it fasten the choroid to the sclera and prevent its detachment. In addition, choroidal detachment can be prevented by vessels and nerves along the rest of its length, penetrating into the choroid and ciliary body from the suprachoroidal space. With expulsive hemorrhage, the tension and possible separation of these nerve and vascular branches causes a reflex disturbance in the general condition of the patient - nausea, vomiting, and a drop in pulse.

Structure of choroidal vessels

Arteries

The arteries do not differ from arteries of other localizations and have a middle muscle layer and adventitia containing collagen and thick elastic fibers. The muscle layer is separated from the endothelium by an internal elastic membrane. The fibers of the elastic membrane are intertwined with the fibers of the basement membrane of endothelial cells.

As the caliber decreases, the arteries become arterioles. In this case, the continuous muscle layer of the vessel wall disappears.

Vienna

The veins are surrounded by a perivascular membrane, outside of which there is connective tissue. The lumen of veins and venules is lined with endothelium. The wall contains unevenly distributed smooth muscle cells in small numbers. The diameter of the largest veins is 300 µm, and the smallest, precapillary venules, are 10 µm.

Capillaries

The structure of the choriocapillary network is very unique: the capillaries that form this layer are located in the same plane. There are no melanocytes in the choriocapillaris layer.

The capillaries of the choriocapillary layer of the choroid have a fairly large lumen, allowing the passage of several red blood cells. They are lined with endothelial cells, on the outside of which lie pericytes. The number of pericytes per endothelial cell of the choriocapillaris layer is quite large. So, if in the capillaries of the retina this ratio is 1:2, then in the choroid it is 1:6. There are more pericytes in the foveal region. Pericytes are contractile cells and are involved in the regulation of blood supply. A feature of choroidal capillaries is that they are fenestrated, making their wall permeable to small molecules, including fluoroscein and some proteins. The pore diameter ranges from 60 to 80 microns. They are covered with a thin layer of cytoplasm, thickened in the central areas (30 μm). Fenestrae are located in the choriocapillaris on the side facing Bruch's membrane. Typical closure zones are revealed between the endothelial cells of arterioles.

Around the optic nerve head there are numerous anastomoses of the choroidal vessels, in particular, the capillaries of the choriocapillary layer, with the capillary network of the optic nerve, that is, the central retinal artery system.

The wall of arterial and venous capillaries is formed by a layer of endothelial cells, a thin basal layer and a wide adventitial layer. The ultrastructure of the arterial and venous sections of the capillaries has certain differences. In arterial capillaries, those endothelial cells that contain the nucleus are located on the side of the capillary facing the large vessels. The cell nuclei with their long axis are oriented along the capillary.

On the side of Bruch's membrane, their wall is sharply thinned and fenestrated. The connections of endothelial cells on the scleral side are presented in the form of complex or semi-complex joints with the presence of obliteration zones (classification of joints according to Shakhlamov). On the side of Bruch's membrane, cells are connected by simply touching two cytoplasmic processes, between which there is a wide gap (backlash junction).

In venous capillaries, the perikaryon of endothelial cells is often located on the sides of flattened capillaries. The peripheral part of the cytoplasm on the side of Bruch's membrane and large vessels is greatly thinned and fenestrated, i.e. venous capillaries may have thinned and fenestrated endothelium on both sides. The organoid apparatus of endothelial cells is represented by mitochondria, lamellar complex, centrioles, endoplasmic reticulum, free ribosomes and polysomes, as well as microfibrils and vesicles. In 5% of the studied endothelial cells, communication between the channels of the endoplasmic reticulum and the basal layers of blood vessels was established.

In the structure of the capillaries of the anterior, middle and posterior sections of the membrane, slight differences are revealed. In the anterior and middle sections, capillaries with a closed (or semi-closed) lumen are quite often recorded; in the posterior sections, capillaries with a wide open lumen predominate, which is typical for vessels in different functional states. Information accumulated to date allows us to consider the endothelial cells of capillaries to be dynamic structures that continuously change their shape, diameter and length of intercellular spaces.

The predominance of capillaries with a closed or semi-closed lumen in the anterior and middle sections of the membrane may indicate the functional ambiguity of its sections.

Innervation of the choroid

The choroid is innervated by sympathetic and parasympathetic fibers emanating from the ciliary, trigeminal, pterygopalatine and superior cervical ganglia; they enter the eyeball with the ciliary nerves.

In the stroma of the choroid, each nerve trunk contains 50-100 axons that lose their myelin sheath when they penetrate it, but retain the Schwann sheath. Postganglionic fibers arising from the ciliary ganglion remain myelinated.

The vessels of the supravascular plate and stroma of the choroid are extremely abundantly supplied with both parasympathetic and sympathetic nerve fibers. Sympathetic adrenergic fibers emanating from the cervical sympathetic nodes have a vasoconstrictor effect.

Parasympathetic innervation of the choroid comes from the facial nerve (fibers coming from the pterygopalatine ganglion), as well as from the oculomotor nerve (fibers coming from the ciliary ganglion).

Recent studies have significantly expanded knowledge regarding the characteristics of the innervation of the choroid. In various animals (rat, rabbit) and in humans, the arteries and arterioles of the choroid contain a large number of nitrergic and peptidergic fibers, forming a dense network. These fibers come with the facial nerve and pass through the pterygopalatine ganglion and unmyelinated parasympathetic branches from the retroocular plexus. In humans, in addition, in the stroma of the choroid there is a special network of nitrergic ganglion cells (positive for the detection of NADP-diaphorase and nitroxide synthetase), whose neurons are connected with each other and with the perivascular network. It is noted that such a plexus is determined only in animals that have a foveola.

Ganglion cells are concentrated mainly in the temporal and central regions of the choroid, adjacent to the macular region. The total number of ganglion cells in the choroid is about 2000. They are unevenly distributed. The greatest number of them is found on the temporal side and centrally. Cells of small diameter (10 µm) are located along the periphery. The diameter of ganglion cells increases with age, possibly due to the accumulation of lipofuscin granules in them.

In some organs, such as the choroid, nitrergic neurotransmitters are detected simultaneously with peptidergic ones, which also have a vasodilating effect. Peptidergic fibers probably arise from the pterygopalatine ganglion and pass into the facial and greater petrosal nerves. It is likely that nitro- and peptidergic neurotransmitters mediate vasodilation when the facial nerve is stimulated.

The perivascular ganglion plexus dilates choroidal vessels, possibly regulating blood flow as intra-arterial blood pressure changes. It protects the retina from damage from thermal energy released when it is illuminated. Flugel et al. proposed that ganglion cells located at the foveola protect from the damaging effects of light precisely the area where the greatest focusing of light occurs. It was revealed that when the eye is illuminated, blood flow in the areas of the choroid adjacent to the foveola increases significantly.

Average, or choroid, membrane of the eye-tunica vasculosa oculi-located between the fibrous and retinal membranes. It consists of three sections: the choroid proper (23), ciliary body (26) and iris (7). The latter is located in front of the lens. The choroid itself makes up the largest part of the tunica media in the area of ​​the sclera, and the ciliary body lies between them, in the area of ​​the lens.

SENSE ORGAN SYSTEM

The choroid proper, or choroid,-chorioidea - in the form of a thin membrane (up to 0.5 mm), rich in vessels, dark brown in color, located between the sclera and the retina. The choroid is connected to the sclera rather loosely, with the exception of the places where the vessels and the optic nerve pass, as well as the area of ​​​​the transition of the sclera to the cornea, where the connection is stronger. It connects to the retina rather tightly, especially with the pigment layer of the latter. After removing this pigment, the choroid protrudes noticeably reflective shell, or tapetum, - tape-turn fibrosum, occupying a place in the form of an isosceles triangular blue-green, with a strong metallic sheen, field dorsal from the optic nerve, up to the ciliary body.

Rice. 237. The front half of the horse's left eye is from behind.

Rear view (lens removed);1 - tunica albuginea;2 -eyelash crown;3 -pigment-~ layer of the iris;3" -grape grains;4 -pupil.

Ciliary body - corpus ciliare (26) - is a thickened, vessel-rich section of the middle tunic, located in the form of a belt up to 10 mm wide on the border between the choroid itself and the iris. Radial folds in the form of scallops in the amount of 100-110 are clearly visible on this belt. Together they form eyelash crown- corona ciliaris (Fig. 237-2). Towards the choroid, i.e. behind, the ciliary ridges decrease, and in front they end ciliary processes-processus ciliares. Thin fibers - fibrae zonulares - are attached to them, forming eyelash belt, or lens ligament of Zinn - zonula ciliaris (Zinnii) (Fig. 236- 13),- or ligament that suspends the lens - lig. suspensoriumlentis. Lymphatic gaps remain between the bundles of fibers of the ciliary girdle - spatia zonularia s. canalis Petiti, - made by lymph.

Contained in the ciliary body ciliary muscle-m. ciliaris - made of smooth muscle fibers, which, together with the lens, constitutes the accommodative apparatus of the eye. It is innervated only by the parasympathetic nerve.

Rainbow shell-iris (7) - part of the middle membrane of the eye located directly in front of the lens. In its center there is a transverse oval-shaped hole - pupil-pupilla (Fig. 237-4), occupying up to 2/6 of the transverse diameter of the iris. On the iris, there is a front surface - facies anterior - facing the cornea, and a posterior surface - facies posterior - adjacent to the lens; the iris part of the retina grows to it. Delicate folds - plicae iridis - are noticeable on both surfaces.

The edge framing the pupil is called pupillary m-margo pu-pillaris. From its dorsal area hang grapevines on stalks. grains- granula iridis (Fig. 237-3") - in the form 2- 4 rather dense black-brown formations.

The edge of the attachment of the iris, or the ciliary edge - margo ciliaris r-connects with the ciliary body and the cornea, with the latter via the pectineal ligament-ligamentum pectinatum iridis, -consisting from separate crossbars, between which there are lymphatic gaps - fountain spaces A-spatia anguli iridis (Fontanae).

VISUAL ORGANS OF THE HORSE 887

The iris contains scattered pigment cells, which determine the “color” of the eyes. It can be brownish-yellowish, less often light brown. As an exception, the pigment may not be absent.

Smooth muscle fibers embedded in the iris form the pupillary sphincter-m. sphincter pupillae - from circular fibers and dila - tator pupil-m. dilatator pupillae - made of radial fibers. With their contractions, they cause the pupil to contract and dilate, which regulates the flow of rays into the eyeball. In strong light, the pupil narrows; in weak light, on the contrary, it expands and becomes more rounded.

The blood vessels of the iris run radially from the arterial ring located parallel to the ciliary edge - circulus arteriosus iridis maior.

The sphincter of the pupil is innervated by the parasympathetic nerve, and the dilator by the sympathetic.

Retina of the eye

The retina of the eye, or retina, -retina (Fig. 236- 21) -is the inner lining of the eyeball. It is divided into the visual part, or the retina itself, and the blind part. The latter breaks up into ciliary and iridescent parts.

The 3rd part of the retina - pars optica retinae - consists of a pigment layer (22), tightly fused with the choroid proper, and from the retina itself, or retina (21), easily separated from the pigment layer. The latter extends from the entrance of the optic nerve to the ciliary body, at which it ends with a fairly smooth edge. During life, the retina is a delicate transparent shell of a pinkish color, which becomes cloudy after death.

The retina is tightly attached at the entrance of the optic nerve. This place, which has a transverse oval shape, is called the visual nipple - papilla optica (17) -with a diameter of 4.5-5.5 mm. In the center of the nipple protrudes a small (up to 2 mm high) process - processus hyaloideus - a rudiment of the vitreous artery.

In the center of the retina on the optical axis, the central field is faintly visible in the form of a light stripe - area centralis retinae. It is the site of the best vision.

The ciliary part of the retina and pars ciliaris retinae (25) - and the iris part of the retina and pars iridis retinae (8) - are very thin; they are built from two layers of pigment cells and grow together. the first with the ciliary body, the second with the iris. On the pupillary edge of the latter, the retina forms the grape seeds mentioned above.

Optic nerve

Optic nerve opticus (20), -up to 5.5 mm in diameter, pierces the choroid and albuginea and then exits the eyeball. In the eyeball, its fibers are pulpless, but outside the eye they are pulpy. Externally, the nerve is covered with dura and pia maters, forming the optic nerve sheath a-vaginae nervi optici (19). The latter are separated by lymphatic slits communicating with the subdural and subarachnoid spaces. Inside the nerve are the central retinal artery and vein, which in the horse supply only the nerve.

Lens

Lens-lens crystalline (14,15) - has the shape of a biconvex lens with a flatter anterior surface - facies anterior (radius 13-15 mm) - and a more convex posterior surface - facies posterior (radius 5.5-

SENSE ORGAN SYSTEM

10.0 mm). The lens is distinguished by the anterior and posterior poles and the equator.

The horizontal diameter of the lens can be up to 22 mm long, the vertical diameter up to 19 mm, the distance between the poles along the crystal axis and the a-axis lentis is up to 13.25 mm.

On the outside, the lens is dressed in a capsule - capsula lentis {14). Parenchyma lens a-substantia lentis (16)- disintegrates into a soft consistency cortical part-substantia corticalis-and dense lens nucleus-nucleus lentis. The parenchyma consists of flat cells in the form of plates - laminae lentis - located concentrically around the nucleus; one end of the plates is directed forward, A the other back. The dried and compacted lens can be divided into sheets like an onion. The lens is completely transparent and quite dense; after death, it gradually becomes cloudy and adhesions of plate cells become noticeable on it, forming three rays a - radii lentis - converging in the center on the front and back surfaces of the lens.

The human eye is an amazing biological optical system. In fact, lenses enclosed in several shells allow a person to see the world around him in color and volume.

Here we will look at what the shell of the eye can be, how many shells the human eye is enclosed in and find out their distinctive features and functions.

The eye consists of three membranes, two chambers, and a lens and vitreous body, which occupies most of the inner space of the eye. In fact, the structure of this spherical organ is in many ways similar to the structure of a complex camera. Often the complex structure of the eye is called the eyeball.

The membranes of the eye not only hold the internal structures in a given shape, but also take part in the complex process of accommodation and supply the eye with nutrients. It is customary to divide all layers of the eyeball into three layers of the eye:

1. Fibrous or outer membrane of the eye. Which consists of 5/6 opaque cells - the sclera and 1/6 of transparent cells - the cornea.
2. Choroid. It is divided into three parts: the iris, the ciliary body and the choroid.
3. Retina. It consists of 11 layers, one of which will be cones and rods. With their help, a person can distinguish objects.

Now let's look at each of them in more detail.

Outer fibrous membrane of the eye

This is the outer layer of cells that covers the eyeball. It is a support and at the same time a protective layer for the internal components. The anterior part of this outer layer is the cornea, which is strong, transparent and strongly concave. This is not only a shell, but also a lens that refracts visible light. The cornea refers to those parts of the human eye that are visible and are formed from clear, special transparent epithelial cells. The back part of the fibrous membrane - the sclera - consists of dense cells to which 6 muscles that support the eye are attached (4 straight and 2 oblique). It is opaque, dense, white in color (reminiscent of the white of a boiled egg). Because of this, its second name is the tunica albuginea. At the boundary between the cornea and sclera there is a venous sinus. It ensures the outflow of venous blood from the eye. There are no blood vessels in the cornea, but in the back of the sclera (where the optic nerve exits) there is a so-called lamina cribrosa. Through its openings pass blood vessels that supply the eye.

The thickness of the fibrous layer ranges from 1.1 mm at the edges of the cornea (in the center it is 0.8 mm) to 0.4 mm of the sclera in the area of ​​the optic nerve. At the border with the cornea, the sclera is slightly thicker, up to 0.6 mm.

Damage and defects of the fibrous membrane of the eye

Among the diseases and injuries of the fibrous layer, the most common are:

• Damage to the cornea (conjunctiva), this can be a scratch, burn, hemorrhage.
• Contact with a foreign body (eyelash, grain of sand, larger objects) on the cornea.
• Inflammatory processes - conjunctivitis. Often the disease is infectious.
• Among diseases of the sclera, staphyloma is common. With this disease, the ability of the sclera to stretch is reduced.
• The most common will be episcleritis - redness, swelling caused by inflammation of the surface layers.

Inflammatory processes in the sclera are usually secondary in nature and are caused by destructive processes in other structures of the eye or from the outside.

Diagnosis of corneal disease is usually not difficult, since the degree of damage is determined visually by an ophthalmologist. In some cases (conjunctivitis), additional tests are required to detect infection.

Middle, choroid of the eye

Inside, between the outer and inner layers, the middle choroid is located. It consists of the iris, ciliary body and choroid. The purpose of this layer is defined as nutrition and protection and accommodation.

1. Iris. The iris of the eye is a kind of diaphragm of the human eye; it not only takes part in the formation of the image, but also protects the retina from burns. In bright light, the iris narrows the space, and we see a very small point of the pupil. The less light, the larger the pupil and narrower the iris.

   The color of the iris depends on the number of melanocyte cells and is determined genetically.

2. Ciliary or ciliary body. It is located behind the iris and supports the lens. Thanks to it, the lens can quickly stretch and react to light and refract rays. The ciliary body takes part in the production of aqueous humor for the internal chambers of the eye. Another purpose is to regulate the temperature inside the eye.
3. Choroid. The rest of this membrane is occupied by the choroid. Actually, this is the choroid itself, which consists of a large number of blood vessels and performs the functions of feeding the internal structures of the eye. The structure of the choroid is such that there are larger vessels on the outside, and smaller ones on the inside, and capillaries at the very border. Another of its functions will be the depreciation of internal unstable structures.

The choroid of the eye is equipped with a large number of pigment cells; it prevents the passage of light into the eye and thereby eliminates the scattering of light.

The thickness of the vascular layer is 0.2–0.4 mm in the area of ​​the ciliary body and only 0.1–0.14 mm near the optic nerve.

Damage and defects of the choroid of the eye

The most common disease of the choroid is uveitis (inflammation of the choroid). Choroiditis is often encountered, which is combined with various types of retinal damage (chorioreditinitis).

More rare diseases such as:

• choroidal dystrophy;
• detachment of the choroid, this disease occurs when intraocular pressure changes, for example during ophthalmological operations;
• ruptures as a result of injuries and impacts, hemorrhage;
• tumors;
• nevi;
• Colobomas are the complete absence of this membrane in a certain area (this is a congenital defect).

Diagnosis of diseases is carried out by an ophthalmologist. The diagnosis is made as a result of a comprehensive examination.

The retina of the human eye is a complex structure of 11 layers of nerve cells. It does not include the anterior chamber of the eye and is located behind the lens (see picture). The topmost layer consists of light-sensitive cone and rod cells. Schematically, the arrangement of layers looks approximately as in the figure.

All these layers represent a complex system. Here the perception of light waves occurs, which are projected onto the retina by the cornea and lens. With the help of nerve cells in the retina, they are converted into nerve impulses. And then these nerve signals are transmitted to the human brain. This is a complex and very fast process.

The macula plays a very important role in this process; its second name is the yellow spot. Here the transformation of visual images and the processing of primary data occurs. The macula is responsible for central vision in daylight.

This is a very heterogeneous shell. So, near the optic disc it reaches 0.5 mm, while in the fovea of ​​the macula it is only 0.07 mm, and in the central fovea up to 0.25 mm.

Damage and defects of the inner retina of the eye

Among injuries to the human retina, at the everyday level, the most common burn is from skiing without protective equipment. Diseases such as:

• retinitis is an inflammation of the membrane, which occurs as an infectious disease (purulent infections, syphilis) or of an allergic nature;
• retinal detachments, which occur when the retina is depleted and torn;
• age-related macular degeneration, which affects the cells of the center - the macula. It is the most common cause of vision loss in patients over 50 years of age;
• retinal dystrophy - this disease most often affects older people; it is associated with thinning of the layers of the retina; at first, its diagnosis is difficult;
• retinal hemorrhage also occurs as a result of aging in older people;
• diabetic retinopathy. It develops 10–12 years after diabetes and affects the nerve cells of the retina.
• Tumor formations on the retina are also possible.

Diagnosis of retinal diseases requires not only special equipment, but also additional examinations.

Treatment of diseases of the retinal layer of the eye of an elderly person usually has a cautious prognosis. At the same time, diseases caused by inflammation have a more favorable prognosis than those associated with the aging process of the body.

Why is the mucous membrane of the eye needed?

The eyeball is located in the eye orbit and securely fixed. Most of it is hidden, only 1/5 of the surface—the cornea—transmits light rays. From above, this part of the eyeball is closed by eyelids, which, when opened, form a gap through which light passes. The eyelids are equipped with eyelashes that protect the cornea from dust and external influences. Eyelashes and eyelids are the outer layer of the eye.

The mucous membrane of the human eye is the conjunctiva. The inside of the eyelids is lined with a layer of epithelial cells that form the pink layer. This layer of delicate epithelium is called the conjunctiva. The cells of the conjunctiva also contain lacrimal glands. The tears they produce not only moisturize the cornea and prevent it from drying out, but also contain bactericidal and nutrient substances for the cornea.

The conjunctiva has blood vessels that connect to the vessels of the face and has lymph nodes that serve as outposts for infection.

Thanks to all the membranes, the human eye is reliably protected and receives the necessary nutrition. In addition, the membranes of the eye take part in the accommodation and transformation of the information received.

The onset of the disease or other damage to the membranes of the eye can cause loss of visual acuity.

The structures of the eyeball need constant blood supply. The most vascular-dependent structure of the eye is the one that performs receptor functions.

Even short-term blockage of the blood vessels of the eye can lead to serious consequences. The so-called choroid of the eye is responsible for the blood supply.

Choroid - choroid of the eye

In the literature, the choroid of the eye is usually called the choroid proper. It is part of the uveal tract of the eye. The uveal tract consists of the following three parts:

• – colored structure surrounding . The pigment components of this structure are responsible for the color of human eyes. Inflammation of the iris is called iritis or anterior uveitis.
• . This structure is located behind the iris. The ciliary body contains muscle fibers that regulate the focusing of vision. Inflammation of this structure is called cyclitis or intermediate uveitis.
• Choroid. This is the layer of the uveal tract containing blood vessels. The vasculature is located at the back of the eye, between the retina and sclera. Inflammation of the choroid itself is called choroiditis or posterior uveitis.

The uveal tract is called the choroid, but only the choroid is a vasculature.

Features of the choroid

Choroidal melanoma of the eye

The choroid is formed by a large number of vessels necessary for feeding the photoreceptors and epithelial tissues of the eye.

Choroidal vessels are characterized by extremely fast blood flow, which is provided by the internal capillary layer.

The capillary layer of the choroid itself is located under Bruch's membrane; it is responsible for metabolism in photoreceptor cells. The large arteries are located in the outer layers of the posterior choroidal stroma.

The long posterior ciliary arteries are located in the suprachoroidal space. Another feature of the choroid itself is the presence of unique lymphatic drainage.

This structure is capable of reducing the thickness of the choroid several times with the help of smooth muscle fibers. The drainage function is controlled by sympathetic and parasympathetic nerve fibers.

The choroid has several main functions:

• The choroidal vasculature is the main source of nutrition.
• By changing the blood flow of the choroid, the temperature of the retina is regulated.
• The choroid contains secretory cells that produce tissue growth factors.

Changing the thickness of the choroid allows the retina to move. This is necessary so that the photoreceptors fall into the plane of focus of the light rays.

Weakened blood supply to the retina can cause age-related macular degeneration.

Pathologies of the choroid

Pathology of the choroid of the eye

The choroid is susceptible to a large number of pathological conditions. These may be inflammatory diseases, malignant neoplasms, hemorrhages and other disorders.

The particular danger of such diseases is that pathologies of the choroid itself also affect the retina.

Main diseases:

1. Hypertensive choroidopathy. Systemic hypertension, associated with high blood pressure, affects the functioning of the vasculature of the eye. The anatomical and histological features of the choroid make it especially susceptible to the damaging effects of high pressure. This disease is also called non-diabetic vascular eye disease.
2. Detachment of the choroid proper. The choroid is located quite freely relative to the adjacent layers of the eye. When the choroid detaches from the sclera, hemorrhage occurs. This pathology can form due to low intraocular pressure, blunt trauma, inflammatory disease and oncological process. When choroidal detachment occurs, visual impairment occurs.
3. Rupture of the choroid. Pathology occurs due to dullness. Rupture of the choroid may be accompanied by quite severe bleeding. The disease may be asymptomatic, but some patients complain of decreased vision and a feeling of pulsation in the eye.
4. Dystrophy of the choroid. Almost all dystrophic lesions of the choroid are associated with genetic disorders. Patients may complain of axial loss of visual fields and inability to see in fog. Most of these disorders cannot be treated.
5. Choroidopathy. This is a heterogeneous group of pathological conditions characterized by inflammation of the choroid itself. Some conditions may be associated with systemic infection of the body.
6. Diabetic retinopathy. The disease is characterized by metabolic disorders of the vascular network of the eye.
   Malignant neoplasms of the choroid. These are various tumors of the choroid. Melanoma is the most common type of such formations. Older people are more susceptible to such diseases.

Most diseases of the choroid itself have a positive prognosis.

Diagnosis and treatment

Anatomy of the eye: schematically

The vast majority of diseases of the choroid itself are asymptomatic. Early diagnosis is possible in rare cases - usually the detection of certain pathologies is associated with a routine examination of the visual apparatus.

Basic diagnostic methods:

• Retinoscopy is an examination method that allows you to study the condition of the retina in detail.
• – a method for detecting diseases of the fundus of the eyeball. Using this method, most vascular pathologies of the eye can be detected.
• . This procedure allows visualization of the vasculature of the eye.
• Computed and magnetic resonance imaging. Using these methods, you can obtain a detailed picture of the state of the eye structures.
• – a method of visualizing blood vessels using contrast agents.

Treatment methods are different for each disease. The main treatment regimens can be distinguished:

1. Steroids and medications that lower blood pressure.
2. Surgical interventions.
3. Cyclosporines are powerful immunosuppressants.
4. Pyridoxine (vitamin B6) for certain genetic disorders.

Timely treatment of vascular pathologies will prevent retinal damage.

Prevention methods

Eye surgery

Prevention of choroidal diseases is largely related to the prevention of vascular diseases. It is important to observe the following measures:

• Control of blood cholesterol composition to avoid the development of atherosclerosis.
• Control of pancreatic functions to avoid the development of diabetes mellitus.
• Regulation of blood sugar in diabetes.
• Treatment of vascular hypertension.

Compliance with hygiene measures will prevent some infectious and inflammatory lesions of the choroid itself. It is also important to treat systemic infectious diseases in a timely manner, since they often become a source of choroidal pathology.

Thus, the choroid of the eye is the vascular network of the visual apparatus. Diseases of the choroid also affect the condition of the retina.

Video about the structure and functions of the choroid (choroid):