rhenium is one of the most important mechanisms in human perception of the surrounding world. With the help of visual assessment, a person receives about 90% of the information coming from outside. Of course, with insufficient or completely absent vision, the body adapts, partially compensating for the loss with the help of other senses: hearing, smell and touch. Nevertheless, none of them can fill the gap that arises with a lack of visual analysis.
How does the most complex optical system of the human eye work? What is the visual assessment mechanism based on and what stages does it include? What happens to the eye when vision is lost? A review article will help you understand these issues.
Human eye anatomy
The visual analyzer includes 3 key components:
peripheral, represented directly by the eyeball and adjacent tissues;
conductive, consisting of optic nerve fibers;
central, concentrated in the cerebral cortex, where the formation and assessment of the visual image takes place.
Let’s consider the structure of the eyeball in order to understand what path the seen picture goes and what its perception depends on.
Eye structure: anatomy of the visual mechanism
The correct structure of the eyeball directly determines what the picture will be seen, what information will enter the brain cells and how it will be processed. Normally, this organ looks like a ball with a diameter of 24-25 mm (in an adult). Inside it are tissues and structures, thanks to which the picture is projected and transmitted to the part of the brain that is capable of processing the received information. The structures of the eye include several different anatomical units, which we will consider.
Integumentary membrane – cornea
The cornea is a special covering that protects the outside of the eye. Normally, it is completely transparent and uniform. Light rays pass through it, thanks to which a person can perceive a three-dimensional image. The cornea is bloodless because it does not contain a single blood vessel. It consists of 6 different layers, each of which has a specific function:
Epithelial layer . Epithelial cells are found on the outer surface of the cornea. They regulate the amount of moisture in the eye, which comes from the lacrimal glands and is saturated with oxygen due to the tear film. Microparticles – dust, debris, etc. – if they get into the eye, can easily disrupt the integrity of the cornea. However, this defect, if it has not affected the deeper layers, does not pose a danger to the health of the eye, since epithelial cells recover quickly and relatively painlessly.
Bowman’s membrane . This layer also belongs to the superficial, since it is located immediately behind the epithelial layer. He, unlike the epithelium, is not able to recover, so his injuries invariably lead to visual impairment. The membrane is responsible for nourishing the cornea and is involved in metabolic processes in cells.
Stroma . This rather voluminous layer consists of collagen fibers that fill the space.
Descemet’s membrane . A thin membrane at the border of the stroma separates it from the endothelial mass.
Endothelial layer . The endothelium provides ideal corneal permeability by removing excess fluid from the corneal layer. It recovers poorly, so it becomes less dense and functional with age. Normally, the density of the endothelium ranges from 3.5 to 1.5 thousand cells per 1 mm 2 , depending on age. If this figure falls below 800 cells, a person may develop corneal edema, as a result of which the clarity of vision is sharply reduced. Such a lesion is a natural result of deep trauma or serious inflammatory eye disease.
Tear film . The last stratum corneum is responsible for the sanitization, hydration and softening of the eyes. The lacrimal fluid entering the cornea washes away microparticles of dust, impurities and improves oxygen permeability.
Functions of the iris in the anatomy and physiology of the eye
Behind the anterior chamber of the eye, filled with fluid, is the iris. The color of a person’s eyes depends on its pigmentation: the minimum pigment content determines the blue color of the iris, the average value is typical for green eyes, and the maximum percentage is inherent in brown-eyed and black-eyed people. That is why most of the babies are born blue-eyed – their pigment synthesis has not yet been regulated, so the iris is most often light. With age, this characteristic changes, and the eyes become darker.
The anatomical structure of the iris is represented by muscle fibers. They contract and relax with lightning speed, regulating the penetrating light flux and changing the size of the passageway. In the very center of the iris, the pupil is located, which, under the action of muscles, changes its diameter depending on the degree of illumination: the more light rays hit the surface of the eye, the narrower the pupil lumen becomes. This mechanism can be disrupted by medication or disease. Short-term changes in the pupil’s response to light help diagnose the condition of the deep layers of the eyeball, but long-term dysfunction can lead to impaired vision.
The lens is responsible for focusing and clarity of vision. This structure is represented by a biconvex lens with transparent walls, which is held in place by a ciliary band. Due to its pronounced elasticity, the lens can almost instantly change its shape, adjusting the clarity of vision in the distance and near. In order for the picture to be seen to be correct, the lens must be absolutely transparent, however, with age or as a result of an illness, the lenses can become cloudy, causing the development of cataracts and, as a result, blurred vision. The possibilities of modern medicine make it possible to replace the human lens with an implant with a complete restoration of the functionality of the eyeball.
The vitreous humor helps maintain the spherical shape of the eyeball. It fills the free space of the posterior region and performs a compensatory function. Due to the dense structure of the gel, the vitreous body regulates intraocular pressure drops, neutralizing the negative consequences of its surges. In addition, the transparent walls retransmit the light rays directly to the retina, which creates a complete picture of what you see.
The role of the retina in the structure of the eye
The retina is one of the most complex and functional structures of the eyeball. Receiving light beams from the surface layers, it converts this energy into electrical energy and transmits impulses along nerve fibers directly to the cerebral section of vision. This process is ensured due to the coordinated work of photoreceptors – rods and cones:
Cones are receptors for detailed perception. For them to be able to perceive the light rays, the lighting must be sufficient. Thanks to this, the eye can distinguish shades and midtones, see small details and elements.
Rods belong to the group of hypersensitive receptors. They help the eye to see the picture in uncomfortable conditions: in low light or out of focus, that is, on the periphery. They support the lateral vision function, providing a person with a panoramic view.
The dorsum of the eyeball facing the orbit is called the sclera. It is denser than the cornea because it is responsible for moving and maintaining the shape of the eye. The sclera is opaque – it does not transmit light rays, completely enclosing the organ from the inside. Part of the vessels supplying the eye, as well as nerve endings, are concentrated here. Attached to the outer surface of the sclera are 6 oculomotor muscles that regulate the position of the eyeball in the orbit.
On the surface of the sclera there is a vascular layer that provides blood flow to the eye. The anatomy of this layer is imperfect: there are no nerve endings that could signal the appearance of dysfunction and other abnormalities. That is why ophthalmologists recommend examining the fundus of the eye at least once a year – this will reveal pathology in the early stages and avoid irreparable visual impairment.
Physiology of vision
To provide a mechanism for visual perception, one eyeball is not enough: the anatomy of the eye also includes conductors that transmit the information received to the brain for decoding and analysis. This function is performed by nerve fibers.
Light rays, reflected from objects, fall on the surface of the eye, penetrate through the pupil, focusing in the lens. Depending on the distance to the visible picture, the lens, with the help of the ciliary muscle ring, changes the radius of curvature: when assessing distant objects, it becomes flatter, and for viewing objects close up, on the contrary, convex. This process is called accommodation. It provides a change in refractive power and focal point, due to which the light fluxes are integrated directly on the retina.
In the photoreceptors of the retina – rods and cones – light energy is transformed into electrical energy, and in this form its flow is transmitted to the neurons of the optic nerve. Through its fibers, excitatory impulses move to the visual cortex, where information is read and analyzed. Such a mechanism provides the receipt of visual data from the surrounding world.
The structure of the eye of a person with visual impairment
According to statistics, more than half of the adult population is faced with visual impairment. The most common problems are hyperopia, myopia and a combination of these pathologies. The main cause of these diseases is various pathologies in the normal anatomy of the eye.
With farsightedness, a person does not see well objects located in the immediate vicinity, however, he can distinguish the smallest details of a distant picture. Long-range visual acuity is a permanent companion of age-related changes since in most cases it begins to develop after 45-50 years and gradually increases. There can be many reasons for this:
shortening of the eyeball, in which the image is projected not on the retina, but behind it;
flat cornea, incapable of adjusting the refractive power;
displacement of the lens in the eye, leading to incorrect focus;
a decrease in the size of the lens and, as a result, incorrect transmission of light fluxes to the retina.
In contrast to hyperopia, with myopia, a person distinguishes in detail the picture near, but he sees distant objects vaguely. This pathology often has hereditary causes and develops in school-age children when the eye is under stress during intensive learning. With such a visual impairment, the anatomy of the eye also changes: the size of the apple increases, and the image is focused in front of the retina, without falling on its surface. Another cause of myopia is excessive curvature of the cornea, due to which light rays are refracted too intensely.
p style=”background-color: white; box-sizing: border-box; color: #444444; font-family: "PT Serif", Georgia, Cambria, "Times New Roman", Times, serif; font-size: 18px; line-height: var(–line-article); margin: 0px 0px 24px; overflow-wrap: break-word; text-align: justify;”>There are often situations when signs of hyperopia and myopia are combined. In this case, changes in the structure of the eye affect both the cornea and the lens. Low accommodation does not allow a person to fully see the picture, which indicates the development of astigmatism. Modern medicine can correct most of the problems associated with visual impairment, but it is much easier and more logical to worry about the condition of the eyes in advance. A careful attitude to the organ of vision, regular gymnastics for the eyes and timely examination by an ophthalmologist will help to avoid many problems, which means preserve ideal vision for many years.