Structure of the organ of vision. The organ of vision consists of the eyeball and the auxiliary apparatus. The eyeball contains the peripheral part of the visual analyzer. The human eye consists of the inner membrane (retina), the vascular and outer protein membrane.
The outer shell consists of two parts - the sclera and the cornea.
The opaque sclera occupies 5/6 of the surface of the outer shell, the transparent cornea is 1/6. The vascular membrane consists of three parts of the iris, the ciliated body and the vascular membrane proper. In the center of the iris is a hole - the pupil, through which the light rays penetrate into the eye. It contains pigments, which determine the color of the eyes. The iris goes into the body, and then, in turn, into the actual choroid. The retina is the inner shell of the eye. It has a complex layered structure - from nerve cells and their fibers.
There are ten layers of the retina. To the outer pigmented layer of the retina, sticks and cones are suitable, which are mutated processes of photosensitive visual cells. From the nerve cells of the retina is the optic nerve - the beginning of the leading part of the visual analyzer.
Scheme of the anatomical structure of the eye: 1 - retina, 2 ~ lens, 3 iris, 4 corneas, 5 - shell (sclera), 6 - vascular membrane, 7 - optic nerve.
Exercises for the eyes
The pupil contains a transparent dense lenticular body - lens. It lies in a transparent bag, from the edges of which elastic fibers emerge that connect it with the ciliary muscle.
When examining distant objects, the ciliary muscle is relaxed, and the zinn link, which is attached mainly to the anterior and posterior surface of the lens, is now taut. Tension of the zinn links causes the lens to be squeezed from front to back and its extension, i. E. the lens is flattened, and the clarity of the image increases. When objects approach the eyes, the ciliary muscle contraction occurs, the zinn links relax and the lens becomes convex, which also improves the image. Consequently, the expressiveness of the vision depends on the shape of the lens.
The body is a perfectly transparent substance that is contained in a very delicate capsule and fills most of the eyeball. It acts as a clammy environment and is part of the optical system of the eye. The anterior, slightly concave surface is adjacent to the posterior surface of the lens. His loss is not replenished.
In the upper lateral corner of the orbit, the lacrimal gland contains a lacrimal fluid (tear), moisturizing the surface of the eyeball, preventing its drying and supercooling. Tear, moistening the surface of the eye, drains off the exit channel in the nasal cavity. Eyelids and eyelashes protect the eyeball from the fact that foreign particles do not enter the eye, the eyebrows divert sweat from the forehead, and this also has a protective value.
Dimensions and mass of eyes in children
The size and weight of the eye in children of primary school age is almost the same as that of an adult.
Perceptions of visual stimuli. Light rays, which are specific stimuli for the visual receptors, pass inside the eyeball through several media, namely: through the cornea, watery moisture, the lens and the vicious body. Together they form an optical system of the eye, which refracts the rays and collects them on the retina.
All the media of the eye, except the lens, retain a constant refractive index of the rays. However, the refractive power of the eye can increase or decrease. It happens because the lens, due to the contraction or relaxation of the ciliary muscle, changes its bulge. With its increase in refraction, the rays in the eye increase, and with decreasing - it weakens. Therefore, in order to be more convenient to study the refractive power of the eye, often only take into account the refraction of the rays by the lens.
Images of objects on the retina arise due to the action of light on the retina, which leads to electrical phenomena in it. These are biocurrents that appear as a result of photochemical decomposition of rhodopsin in rods and iodopsin in cones. However, the rate of decay of rhodopsin in the world is much greater than the rate of decay of iodopsin, and therefore the sensitivity of the rods to light is 1000 times greater than cones.
The photochemical reaction of the decomposition of rhodopsin and iodopsin causes the appearance of impulses in the fibers of the optic nerve and is the beginning of visual perception. Sticks are the organ of twilight vision, which give colorless light sensations. The cones are the organ of day vision, which give color sensations. When the cones are functioning, the sticks are inhibited. And sticks give a sense of light, even in low light, if it hits the side of the retina, where only the sticks are located. The potential of the retina is one of the manifestations of the photochemical decay of rhodopsin.
Along with chemical changes in the visual receptors, there are also physical, in particular the occurrence of action currents.
Rod-shaped visual cells are sensitive to light visual receptors. They are irritated even by weak twilight light, but do not perceive the coloring of objects. That's why at night, when people see with the help of rod-shaped visual cells, they are not able to distinguish colors. Kolbochka-like cells are much less sensitive to light than rod-shaped cells. With the help of cone-shaped visual cells, day vision is formed. These are receptors that perceive not only light, but also color. An accumulation of a cone of similar cells is contained on the retina just opposite the pupil. And when the image of the object appears on this place, we see it brightly. This part of the retina is called a yellow spot. There are no visual receptors at the exit point of the optic nerve fibers from the retina. Therefore, the rays that fall on this part of the retina, called the blind spot, do not cause visual irritation.
From the retina, excitation proceeds along the fibers of the optic nerve and the conducting paths of the brain into the midbrain and into the visual bumps, and from them to the visual zone of the cerebral cortex. Here the final analysis of visual stimuli takes place.
The ability to distinguish colors in a child increases with age.
Adaptation of the eye
The development of the ability of the eye to see under different illumination is called adaptation. If you put out the light in the room in the evening, then at first the person completely does not distinguish the surrounding objects. However,
already after 1-2 minutes she begins to grasp the contours of objects, and a few minutes later sees objects quite clearly. This is due to a change in the sensitivity of the retina in the dark. Staying in the dark for one hour increases the sensitivity of the eye by about 200 times. And especially quickly increases sensitivity in the first minute.
This phenomenon is explained by the fact that in bright light, the visual purple of rod-shaped visual cells is completely destroyed. In the dark it is quickly restored, and the rod-shaped cells are very sensitive to light, they begin to perform their functions, while the cones are similar, insensitive to light, unable to perceive visual irritations. That's why a person does not distinguish colors in the dark.
However, when the light is switched on in a dark room, it seems to blind the person. She hardly distinguishes between surrounding objects, and after 1-2 minutes her eyes begin to see well. This is due to the fact that the visual purple in the rod-shaped cells has collapsed, the sensitivity to light has sharply decreased and visual irritations are now perceived only by the cone-shaped cells of the visual cells.
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Accomodation of the eye
The ability of the eye to see objects at different distances is called accommodation. The object is clearly visible when the rays reflected from it collect on the retina. This is achieved by changing the convexity of the lens. The change occurs reflexively - when considering objects that are at different distances from the eye. When we look at the objects located near the objects, the convexity of the lens increases. The refraction of the rays in the eye becomes larger, as a result of which an image appears on the retina. When we look into the distance, the lens is flattened.
In the state of rest of accommodation (a sight in the distance), the radius of curvature of the anterior surface of the lens is 10 mm, and with the maximum accommodation, when the object is all close to the eye, the radius of curvature of the anterior surface of the lens is 5.3 mm.
Loss of elasticity of the lens bag with age leads to a decrease in its cluttering capacity with the greatest accommodation. This increases the ability of older people to view objects at a great distance. The closest point of clear vision is removed with age. Thus, at the age of 10 it is located at a distance of less than 7 cm from the eye, at 20 years old it is 8.3 cm, at 30-11 cm, at 35-17 cm, and at 60-70 years approaches 80-100 cm .
With age, the lens becomes less elastic. The capacity for accommodation begins to decline from the age of ten, but in sight it affects only in old age (senile hyperopia).
Visual acuity - is the ability of the eye to separately perceive two points, spaced from each other at some distance. The vision of two points depends on the size of the image on the retina. If they are small, then both images merge and they can not be distinguished. The size of the image on the retina depends on the angle of view: the smaller it is with the perception of two images, the greater the visual acuity.
To determine visual acuity, lighting, color, pupil size, angle of vision, distance between objects, the places of the retina to which the image falls, and the state of adaptation are of great importance. Visual acuity is a simple indicator that characterizes the state of the visual analyzer in children and adolescents. Knowing visual acuity in children, it is possible to carry out an individual approach to students, placing them in a classroom, recommending an appropriate mode of teaching, corresponds to an adequate load on the visual analyzer.