Light refractive media of the eye. Refraction and its anomalies

Opaque media appear more or less dark in transmitted light, depending on their ability to reflect light. Formations with a highly reflective surface may appear not only light, but also shiny.

It must also be borne in mind that when examined in transmitted light, some areas of transparent media may appear more or less dark, as if clouded, but in reality there is no clouding in this place. The reason for this phenomenon may be the fact that in the indicated places the rays emanating from the bottom of the eye, due to reflection or refraction, are so deflected to the side that they either do not reach the observer’s eye at all, or only a small part of them reaches it.

A distinctive feature of such dark areas is often that... that when changing the direction of view, as well as when illuminating the eye with an ophthalmoscope from different positions, an unusual play of shadows is noted in the area of ​​\u200b\u200bapparent opacities. To completely eliminate opacities, it is necessary to resort to side lighting, in which, in such cases, gray inclusions will not be visible against a dark background.

Opacities in the media of the eye can be mobile or immobile. A mobile cloudiness is one that continues to move in the eye after the eye, having made a small movement, again assumes a calm position. Mobile opacities can only be found in liquid media - in the moisture of the anterior chamber or in the liquefied vitreous body. Opacities in the moisture of the anterior chamber are easily recognized, since they are already detected when examined using lateral illumination.

The location of many opacities in the media of the anterior segment of the eye (cornea, aqueous humor of the anterior chamber, lens), as is known, can be established with lateral illumination. Examination in transmitted light also makes it possible to accurately localize opacities based on parallax phenomena, i.e., by observing the change in the position of opacities relative to the pupil or the light reflex of the cornea at different turns of the eye.

Localization of opacities relative to the pupil.

a - clouding of the cornea,
c - on the anterior capsule of the lens,
c - on the posterior capsule of the lens,
d - in the vitreous body.

If such an eye looks directly into the ophthalmoscope mirror, then all these opacities, located along the visual line one after another, will merge into one point located in the center of the pupil (Fig. 30 - top).

Cloudiness a, lying on the cornea, will move in the direction of eye movement when turning: when turning the eye upward, it will approach the upper edge of the pupil and vice versa.

Opacities c and d, located behind the plane of the pupil, c. in the substance of the lens or in the vitreous body they move in the direction opposite to the movement of the eye: when you speak your eyes upward, they approach the lower edge of the pupil, when you turn downward, they are positioned eccentrically upward. The turbidity makes a greater excursion, the further it is located from the plane of the pupil.

Localization of opacities relative to the light reflex of the cornea. Here, in fact, we are talking about the localization of opacities relative to the center of rotation of the eye, located slightly behind the posterior pole of the lens (about 1.5 mm behind the volar capsule of the lens).

Obviously, when the eyeball rotates, the clouding located at the center of rotation of the eye will not change its position.
Opacities located anterior to the center of rotation of the eye will move in the direction of movement of the anterior segment of the eye, and opacities localized behind the center of rotation will move in the opposite direction. This is clearly visible in Fig. 31 - at the top, where a number of opacities are located along the optical axis: a opacification on the cornea, c - on the anterior capsule of the lens, c - behind the lens, in the center of rotation: the eye, d - in the vitreous body, behind the center of rotation of the eye. When the subject looks straight ahead, all the opacities will merge into one point.

According to the laws of optics, the reflex reflected by the surface of a convex mirror always lies on the straight line connecting the light source and the center of curvature of the mirror. Therefore, at. In any position of the eye, the light reflex of the cornea will always be on the line connecting the center of curvature of the cornea and the center of the ophthalmoscope mirror, i.e. the reflex will cover the center of curvature of the cornea, which almost coincides with the center of rotation of the eye. It is therefore obvious that the light reflex of the cornea at any position of the eyeball indicates the location of the center of rotation of the eye. That is why, when localizing opacities relative to the center of rotation of the eye, they monitor the movement of opacities when the eye turns to the light reflex of the cornea.

Localization of opacification relative to the corneal reflex makes it possible to draw the following practical conclusions. If the opacities are located in the anterior part of the vitreous or in the lens, near the posterior capsule, when the eye turns, it almost does not move in relation to the corneal reflex. If the clouding is located in the anterior parts of the lens or in the cornea, it will noticeably mix, and the movement occurs in the direction of eye movement; when the cloudiness moves in the direction opposite to the movement of the eye, it is located in the vitreous body, the further the lens is from the posterior capsule, the faster its movement.

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Article from the book.

Refractive media of the eyeball: cornea, fluid of the eye chambers, lens, vitreous body.

The inner nucleus of the eye consists of transparent light-refracting media: the vitreous body, the lens, and the aqueous humor of the chambers of the eye.

The vitreous body is located in the vitreous chamber. Its volume for an adult is 4 ml. In composition, it is a gel-like medium with the presence of special proteins in the framework: vitrosin and mucin, with which hyaluronic acid is associated, which ensures the viscosity and elasticity of the body. The primary vitreous develops from the mesoderm, the secondary - from the mesoderm and ectoderm. The formed vitreous body is a permanent environment of the eye, which is not restored if lost. It is covered along the perimeter with a limiting membrane, which is firmly connected with the ciliary epithelium (the base is a ring-shaped base protruding anterior to the serrated edge) and with the posterior part of the lens capsule (hyaloid-lens ligament).

The lens is located between the iris and the vitreous body, in a recess (vitreous fossa) and is held in place by the fibers of the ciliary girdle.

In the lens there are different types:

1. the anterior surface of the capsule (epithelium and fibers) with the most protruding point - the pole;
2. posterior surface of the capsule (epithelium and fibers) with a more convex posterior pole;
3. equator - transition of the front surface to the back;
4. lens substance made of lens fibers and the formation that glues them together; lens nucleus - lens fibers without nuclei: sclerotic, compacted;
5. ciliary girdle, the fibers of which begin from the anterior and posterior surfaces of the capsule in the equator region.

The axis of the lens is the distance between the poles, the refractive power of the lens is 18 diopters (D).

The anterior chamber is located between the cornea and the iris, and the posterior chamber is located between the iris and the anterior surface of the lens capsule. Both are filled with moisture, capable of slight refraction of light.

The anterior chamber is bounded along the perimeter by the pectineal ligament, between the bundles of fibers of which there are spaces of the iris-corneal angle lined with flat cells (fountain spaces) - the path for the outflow of moisture into the venous sinus of the sclera. Damage to the angle underlies the development of angular glaucoma.

The posterior chamber exchanges moisture due to the slit-like spaces between the fibers of the ciliary girdle, which, in the form of a common circular slit (Petite canal), cover the lens along the periphery.

The cornea is located in the outer shell of the eye, making up its anterior part and participating with its convexity in the formation of the anterior pole of the eyeball. It is transparent, has a round shape with a diameter of 12 mm in an adult, and a thickness of 1 mm. In the sagittal plane it is smoothly curved. On the outer surface the cornea is convex, and on the inner surface it is concave. The radius of curvature is up to 7.5-8 mm, which ensures light refraction of up to 40 diopters. The cornea grows into the circular groove of the sclera, forming with its peripheral edge a small thickening - the limbus.

There are five layers in the cornea:

1. anterior epithelium up to 50 µm thick with numerous free nerve endings; characterized by high regeneration and permeability to drugs;
2. anterior border plate 6-9 µm thick;
3. its own substance from fibrous plates, including bundles of collagen fibers, branched flat fibroblasts and an amorphous medium of keratin sulfates, glycosaminoglycans and water;
4. posterior border plate 5-10 µm thick; both plates: anterior and posterior consist of collagen fibers and amorphous substance;
5. posterior epithelium of flat polygonal cells of various shapes.

The cornea has no vessels; it receives diffuse nutrition from the fluid of the anterior chamber and the vessels of the circular groove of the sclera.

The inner core of the eye consists of transparent light-refracting media: the vitreous body, the lens, intended for constructing an image on the retina, and aqueous humor, which fills the eye chambers and serves to nourish the avascular formations of the eye.

A. The vitreous body, corpus vitreum, forms the cavity of the eyeball medially from the retina and is a completely transparent mass, similar to jelly, lying behind the lens. Thanks to the depression from the latter, a fossa is formed on the anterior surface of the vitreous body - fossa hyaloidea, the edges of which are connected to the lens capsule through a special ligament.

B. The lens, or lens, is a very significant light-refracting medium of the eyeball. It is completely transparent and has the appearance of lentils or biconvex glass. The central points of the anterior and posterior surfaces are called poles (polus anterior et posterior), and the peripheral edge of the lens, where both surfaces meet each other, is called the equator. The axis of the lens connecting both poles is 3.7 mm when looking at distance and 4.4 mm during accommodation, when the lens becomes more convex. Equatorial diameter 9 mm. The lens, with the plane of its equator, stands at a right angle to the optical axis, adjoining its anterior surface to the iris, and its posterior surface to the vitreous body.

The lens is enclosed in a thin, also completely transparent, structureless capsule, capsula lentis, and is held in its position by a special ligament - the ciliary girdle, zonula ciliaris, which is made up of many thin fibers running from the lens capsule to the ciliary body, where they lie mainly between the ciliary processes . Between the fibers of the ligament there are fluid-filled spaces of the girdle, spatia zonularia, communicating with the chambers of the eye.

Thanks to the elasticity of its capsule, the lens easily changes its curvature depending on whether we look far or near. This phenomenon is called accommodation. In the first case, the lens is somewhat flattened due to the tension of the ciliary band; in the second, when the eye must be placed at a close distance, the ciliary band, under the influence of contraction of the m.ciliaris, weakens along with the lens capsule and the latter becomes more convex. Thanks to this, rays coming from a nearby object are refracted by the lens more strongly and can connect on the retina. The lens, like the vitreous body, does not have blood vessels.

B. Chambers of the eye. The space located between the anterior surface of the iris and the posterior side of the cornea is called the anterior chamber of the eyeball, camera anterior bulbi. The anterior and posterior walls of the chamber come together along its circumference in the angle formed by the transition of the cornea into the sclera, on the one hand, and the ciliary edge of the iris, on the other. This angle, angulus iridocornealis, is rounded by a network of crossbars.

Between the crossbars there are slot-like spaces. Angulus iridocornealis has an important physiological significance in terms of the circulation of fluid in the chamber, which, through the indicated spaces, is emptied into the venous sinus located nearby in the thickness of the sclera.

Behind the iris there is a narrower posterior chamber of the eye, camera posterior bulbi, which also includes the spaces between the fibers of the ciliary girdle; behind it is limited by the lens, and on the side by the corpus ciliare. Through the pupil, the posterior chamber communicates with the anterior one. Both chambers of the eye are filled with a transparent liquid - aqueous humor, humor aquosus, the outflow of which occurs into the venous sinus of the sclera.

Ticket number 7

Human anatomy and age. Peculiarities of the structure of organs and bodies in adolescent children, in adolescence, adulthood, old age and senility. Examples.

Age anatomy studies the structure of a person at different age periods. Under the influence of age and external factors, the structure and shape of human organs changes with a certain pattern. In children of the first years of life, adults and elderly people, there are significant differences in the anatomical structure of the body. In clinical practice, independent disciplines have even emerged, for example, pediatrics - the science of the child, gerontology - the science of the elderly.

Examples:

Three main periods can be traced in the growth of the skull after birth. The first period - up to 7 years of age - is characterized by vigorous growth of the skull, especially in the occipital part.

In the 1st year of a child’s life, the thickness of the skull bones increases approximately 3 times, the outer and inner plates begin to form in the bones of the vault, with diploe between them. The mastoid process of the temporal bone develops and in it - mastoid cells. In growing bones, ossification points continue to merge, forming a bony external auditory canal, which closes into a bone ring by the age of 5. By the age of 7, the fusion of parts of the frontal bone ends, parts of the ethmoid bone grow together.

In the second period, from 7 years to the onset of puberty, slow but uniform growth of the skull occurs, especially in the area of ​​its base. The volume of the cerebral part of the skull reaches 1300 cm 3 by the age of 10. At this age, the fusion of individual parts of the skull bones, developing from independent ossification points, is basically complete.

The third period - from 13 to 20-23 years - is characterized by intensive growth, mainly of the facial part of the skull, and the appearance of sexual differences. After 13 years, further thickening of the skull bones occurs; Pneumatization of the bones continues, as a result of which the mass of the skull is relatively reduced while maintaining its strength. By the age of 20, the sutures between the sphenoid and occipital bones ossify. The growth of the base of the skull in length ends by this period.

After 20 years, especially after 30 years, the sutures of the cranial vault heal. The sagittal suture in its posterior part begins to heal first (22-35 years), then the coronal suture in the middle part (24-42 years), mastoid-occipital (30-81 years); scaly rarely overgrows. In old age, the bones of the skull become thinner and more fragile.

Visual system– a set of protective, optical, receptor and nervous structures that perceive and analyze light stimuli. Ability to see related objects Light refractive media of the eye.

The light-refracting apparatus of the eye includes: the cornea, aqueous humor, lens, and vitreous body.

· The cornea is a transparent plate convex outward, thickening from the center to the periphery. The curvature of its surface determines the characteristics of light refraction. When the curvature of the cornea is abnormal, distortion of visual images occurs, called astigmatism.

· Between the cornea and the iris there is an anterior chamber filled with fluid - aqueous humor, which is produced by the ciliary body.

· The lens is a biconvex lens that is suspended and held in place by the fibers of the ciliary girdle. The lens changes its curvature depending on the tension of the fibers of the zonule, thereby providing the ability to focus objects located at different distances from the eye on the retina. Change in lens curvature – accommodation.

· Vitreous body is a colloidal solution (jelly-like mass) of hyaluronic acid in extracellular fluid. Fills the space between the lens and retina. The vitreous body ensures the passage of light rays, maintains the position of the lens, participates in the metabolism of the retina, and presses the inner layers of the retina to the pigment epithelium.

related to the reflection of light from the surface of objects.

Refraction- the refractive power of the optical system of the eye, which is measured by a conventional unit - diopter. One diopter is the refractive power of glass with a main focal length of 1 meter. The average refractive power of a normal eye can vary from 52 to 68 diopters.

The normal state of refraction of the eye is called emmetropia. With emmetropia, the focus of the optical system of the eye coincides with the retina, i.e. Parallel rays from objects falling on the eye are collected on the retina.

Myopia (myopia) is a condition in which the focus of the optical system of the eye does not coincide with the retina, but is located in front of it (i.e., the distance between the lens and the retina is greater than the focal length of the lens). Such people see well up close, but poorly at a distance. Myopia is corrected with diverging lenses.

Farsightedness (hyperopia) is a condition in which the focus of the optical system of the eye does not coincide with the retina, but is located behind it (that is, the retina is located too close to the lens). Such people see well in the distance and poorly near. Correction occurs through collective lenses.

Anisometropia is a condition in which the refraction of the left and right eyes is different.

The concept of visual acuity. Mechanisms of accommodation.

Visual acuity– the minimum angular distance between two objects (points) visible to the eye.

Sharpness is determined using special tables of letters and rings and is measured by the value I/a, where a is the angle corresponding to the minimum distance between two adjacent break points in the ring. Visual acuity depends on the general illumination of surrounding objects. In daylight it is maximum, in twilight light the severity decreases.

The lens is suspended and held in place by the fibers of the ciliary girdle. Next to the ciliary girdle is the ciliary muscle. It consists of two bundles of smooth muscle cells, lying circularly on the inside and radially on the outside. By contracting, it weakens the tension of the fibers of the ciliary girdle, increasing the curvature of the lens and focusing the eye on close objects.

The refractive (dioptric) apparatus of the eye includes the cornea, lens, vitreous body, fluids of the anterior and posterior chambers of the eye.

The cornea occupies 1/16 of the area of ​​the fibrous membrane of the eye and, performing a protective function, is characterized by high optical homogeneity, transmits and refracts light rays and is an integral part of the light-refracting apparatus of the eye. The plates of collagen fibrils, which make up the main part of the cornea, have the correct location, the same refractive index with the nerve branches and interstitial substance, which, together with the chemical composition, determines its transparency.

Microscopically, 5 layers are distinguished in the cornea: 1) anterior multilayered squamous non-keratinizing epithelium; 2) anterior limiting membrane (Bowman's membrane); 3) own substance of the cornea; 4) posterior limiting elastic membrane (Descemet's membrane); 5) posterior epithelium (“endothelium”).

The cells of the anterior epithelium of the cornea are tightly adjacent to each other, arranged in 5 layers, connected by desmosomes. The basal layer is located on Bowman's membrane. Under pathological conditions (if the connection between the basal layer and Bowman's membrane is not strong enough), detachment from the basal layer of Bowman's membrane occurs. The cells of the basal layer of the epithelium (germinative, germinal layer) have a prismatic shape and an oval nucleus located close to the top of the cell. Adjacent to the basal layer are 2-3 layers of polyhedral cells. Their laterally elongated processes are embedded between neighboring epithelial cells, like wings (winged, or spiny, cells). The nuclei of winged cells are round. The two superficial epithelial layers consist of sharply flattened cells and have no signs of keratinization. The elongated narrow nuclei of the cells of the outer layers of the epithelium are located parallel to the surface of the cornea. The epithelium contains numerous free nerve endings, which determine the high tactile sensitivity of the cornea. The surface of the cornea is moistened with the secretion of the lacrimal and conjunctival glands, which protects the eye from the harmful physical and chemical effects of the outside world and bacteria. The corneal epithelium has a high regenerative capacity. Under the corneal epithelium there is a structureless anterior limiting membrane - Bowman's membrane with a thickness of 6-9 microns. It is a modified hyalinized part of the stroma, is difficult to distinguish from the latter and has the same composition as the cornea's own substance. The boundary between Bowman's membrane and the epithelium is well defined, and the fusion of Bowman's membrane with the stroma occurs imperceptibly.

The proper substance of the cornea - the stroma - consists of homogeneous thin connective tissue plates, intersecting at an angle, but regularly alternating and located parallel to the surface of the cornea. Processed flat cells, which are types of fibroblasts, are located in the plates and between them. The plates consist of parallel bundles of collagen fibrils with a diameter of 0.3-0.6 microns (1000 in each plate). Cells and fibrils are immersed in an amorphous substance rich in glycosaminoglycans (mainly keratin sulfates), which ensures the transparency of the cornea's own substance. In the area of ​​the iridocorneal angle, it continues into the opaque outer shell of the eye - the sclera. The cornea itself does not have blood vessels.

The posterior border plate - Descemet's membrane - 5-10 microns thick, is represented by collagen fibers with a diameter of 10 nm, immersed in an amorphous substance. This is a glassy membrane that strongly refracts light. It consists of 2 layers: the outer - elastic, the inner - cuticular and is a derivative of cells of the posterior epithelium ("endothelium"). The characteristic features of Descemet's membrane are strength, resistance to chemical agents and the melting effect of purulent exudate in corneal ulcers.

When the anterior layers die, Desmet's membrane protrudes into a transparent vesicle (descemetocele). At the periphery it thickens, and in elderly people, round warty formations - Hassall-Henle bodies - can form in this place.

At the limbus, Descemet's membrane, becoming thinner and becoming more fibrous, passes into the trabeculae of the sclera.

The "corneal endothelium", or posterior epithelium, consists of a single layer of flat polygonal cells. It protects the corneal stroma from exposure to anterior chamber moisture. The nuclei of endothelial cells are round or slightly oval, their axis is parallel to the surface of the cornea. Endothelial cells often contain vacuoles. At the periphery, the “endothelium” passes directly onto the fibers of the trabecular meshwork, forming the outer cover of each trabecular fiber, stretching in length.

Bowman's and Descemet's membranes play a role in the regulation of water metabolism, and metabolic processes in the cornea are ensured by the diffusion of nutrients from the anterior chamber of the eye due to the marginal looped network of the cornea, numerous terminal capillary branches forming a dense perilimbal plexus.

The lymphatic system of the cornea is formed from narrow lymphatic slits communicating with the ciliary venous plexus. The cornea is highly sensitive due to the presence of nerve endings in it.

The long ciliary nerves, representing branches of the nasociliary nerve extending from the first branch of the trigeminal nerve, penetrate into its thickness at the periphery of the cornea, lose myelin at some distance from the limbus, dividing dichotomously. The nerve branches form the following plexuses: in the substance of the cornea, preterminal and under Bowman's membrane - terminal, subbasal (Riser's plexus).

During inflammatory processes, blood capillaries and cells (leukocytes, macrophages, etc.) penetrate from the limbus into the cornea's own substance, which leads to its clouding and keratinization, the formation of a cataract.

The anterior chamber of the eye is formed by the cornea (outer wall) and the iris (posterior wall), in the pupil area - by the anterior capsule of the lens. At its extreme periphery, in the corner of the anterior chamber, there is a chamber, or iridocorneal, angle with a small portion of the ciliary body. The chamber (also called filtration) corner borders the drainage apparatus - the Schlemm canal. The state of the chamber angle plays a large role in the exchange of intraocular fluid and in changes in intraocular pressure. Corresponding to the apex of the angle, a ring-shaped groove passes through the sclera. The posterior edge of the groove is somewhat thickened and forms a scleral ridge formed by circular fibers of the sclera (posterior limiting ring of Schwalbe). The scleral ridge serves as the attachment point for the suspensory ligament of the ciliary body and the iris - the trabecular apparatus that fills the anterior part of the scleral groove. In the posterior part it covers Schlemm's canal.

The trabecular apparatus, previously mistakenly called the pectineal ligament, consists of 2 parts: the sclerocorneal part, which occupies most of the trabecular apparatus, and the second, more delicate, uveal part, which is located on the inside and is the pectineal ligament itself; the sclerocorneal section of the trabecular apparatus is attached to the scleral spur, partially merges with the ciliary muscle (muscle of Brücke). The sclerocorneal part of the trabecular apparatus consists of a network of interwoven trabeculae with a complex structure. In the center of each trabecula, which is a flat thin cord, there passes a collagen fiber, entwined, reinforced with elastic fibers and covered on the outside with a case of a homogeneous vitreous membrane, which is a continuation of Descemet’s membrane. Between the complex interweaving of corneoscleral fibers there remain numerous free slit-like openings - fountain spaces, lined with “endothelium” passing from the posterior surface of the cornea. The Fontan spaces are directed to the wall of the venous sinus of the sclera - Schlemm's canal, located in the lower part of the scleral groove 0.25 cm wide. In some places it is divided into a number of tubules, then merging into one trunk. The inside of Schlemm's canal is lined with endothelium. Wide, sometimes varicose vessels extend from its outer side, forming a complex network of anastomoses, from which veins originate, draining chamber moisture into the deep scleral venous plexus.

Lens, This is a transparent biconvex lens, the shape of which changes during the accommodation of the eye to see near or distant objects. Together with the cornea and vitreous body, the lens constitutes the main light-refracting medium. The radius of curvature of the lens varies from 6 to 10 mm, the refractive index is 1.42. The lens is covered with a transparent capsule 11-18 microns thick. Its anterior wall consists of a single-layer squamous epithelium of the lens.

Towards the equator, epithelial cells become taller and form the growth zone of the lens. This zone “supplies” new cells throughout life to both the anterior and posterior surfaces of the lens. New epithelial cells are transformed into so-called lens fibers. Each fiber is a transparent hexagonal prism. In the cytoplasm of the lens fibers there is a transparent protein - crystallin. The fibers are glued together with a special substance that has the same refractive index as them. The centrally located fibers lose their nuclei and, overlapping each other, form the nucleus of the lens.

The lens is supported in the eye by the fibers of the ciliary girdle, formed by radially arranged bundles of inextensible fibers attached on one side to the ciliary body, and on the other to the lens capsule, due to which the contraction of the muscles of the ciliary body is transmitted to the lens.

Vitreous body. This is a transparent jelly-like mass that fills the cavity between the lens and the retina. On fixed preparations, the vitreous body has a mesh structure. At the periphery it is denser than in the center. A canal passes through the vitreous body - a remnant of the embryonic vascular system of the eye - from the retinal papilla to the posterior surface of the lens. The vitreous contains the protein vitrein and hyaluronic acid. The refractive index of the vitreous body is 1.33.