The human eye is the organ that reacts to light and pressure. As sensory organs, the eyes of mammals allow vision. The human eye helps provide three-dimensional moving images, usually colored during the day. The rod and conical cells in the retina allow perceptions and visions of conscious light including color differentiation and depth perception. The human eye can distinguish between about 10 million colors and may be able to detect a single photon.
Similar to other mammalian eyes, photosensitive ganglion ganglion cells that do not form eye images in the retina receive light signals that affect the pupil size adjustment, regulation and suppression of the hormone melatonin and entrainment of the body clock.
Video Human eye
Structure
The eye is not shaped like a perfect ball; it is a unitary two-piece unit, consisting of the anterior segment and the posterior segment. The anterior segment consists of cornea, iris and lens. The cornea is transparent and more curved, and is associated with a larger posterior segment, consisting of vitreous, retina, choroid and outer white skin called sclera. The cornea usually has a diameter of about 11.5 mm (0.3 inches), and a thickness of 1/2 mm (500 m) near its center. The posterior space is the remaining five-sixths; Its diameter is usually about 24 mm. The cornea and sclera are connected by an area called the limbus. The iris is a pigmented circular structure concentrically around the center of the eye, pupils, which appear to be black. The size of the pupil, which controls the amount of light entering the eye, is adjusted by the iris dilator and sphincter muscle.
Light energy enters the eye through the cornea, through the pupil and then through the lens. The shape of the lens is altered for close focus (accommodation) and is controlled by ciliary muscles. Light photons falling on the light-sensitive cells of the retina (cones and photoreceptor rods) are converted into electrical signals transmitted to the brain by the optic nerve and interpreted as sight and vision.
Size
Dimensions are usually different among adults with just one or two millimeters, very consistent across different ethnicities. Vertical size, generally less than horizontal, about 24 mm. The transverse size of adult human eye is about 24.2 mm and the sagittal size is 23.7 mm with no significant difference between sex and age group. Strong correlations have been found between transverse diameter and orbit width (r = 0.88). The typical adult eye has an anterior to posterior diameter of 24 millimeters, a volume of six cubic centimeters (0.4 cu. Di.), And a mass of 7.5 grams (weight 0.25 oz.)..
The eyeball grows rapidly, rising from about 16-17 millimeters (about 0.65 inches) at birth to 22.5-23 mm (about 0.89 inches) at the age of three. At the age of 13, the eye reaches its full size.
Components
The eye consists of three layers, or layers, that line the various anatomical structures. The outer layer, known as the fibrous tunic, is composed of the cornea and sclera. The middle layer, known as the vascular or uvea tunic, consists of the choroid, ciliary body, pigment epithelium and iris. The deepest is the retina, which gets its oxygenation from the coroid (posterior) blood vessels as well as the retinal (anterior) vessels.
The eye spaces are filled with anterior aqueous humor, between the cornea and the lens, and the vitreous body, the jelly-like substance, behind the lens, filling the entire posterior cavity. Aqueous humor is a clear aqueous liquid contained in two areas: the anterior space between the cornea and the iris, and the posterior space between the iris and the lens. The lens is suspended into the body of sili by the suspensory ligament (Zonule of Zinn), consisting of hundreds of transparent smooth fibers that transmit muscle strength to change the shape of the lens for accommodation (focus). The vitreous body is a clear substance made up of water and protein, which gives it a composition like jelly and sticky.
Maps Human eye
Vision
Field of view
The field of estimate of individual human eye views (measured from the point of fixation, ie, the point at which a person's vision is directed) varies with the facial anatomy, but is usually 30 ° superior (top, delimited by the eyebrow), 45 ° nasal (constrained by the nose) 70 Â ° inferior (down), and 100 Â ° temporal (towards the temple). For both eyes the combined visual field (binocular) is 135 Â ° vertical and 200 Â ° horizontal. When viewed at a large angle from the side, the iris and pupil may still be visible to the viewer, indicating the person has a possible edge vision at that angle.
Approximately 15 Â ° temporal and 1.5 Â ° below the horizontal are blind spots made by nasally optical nerve, which is approximately 7.5 Â ° high and 5.5 Â ° wide.
Dynamic range
The retina has a static contrast ratio of about 100: 1 (about 6.5 f-stops). As soon as the eye moves quickly to get the target (saccade), it adjusts its exposure by adjusting the iris, which adjusts the pupil size. The initial dark adaptation occurs in about four seconds of deep and undisturbed darkness; Full adaptation through adjustment in the retinal rod photoreceptor is 80% complete in thirty minutes. The process is nonlinear and diverse, so disturbance by light exposure necessitates restarting the dark adaptation process again. Full adaptation depends on good blood flow; thus dark adaptation can be inhibited by retinal disease, poor blood vessel circulation and high altitude exposure.
The human eye can detect a luminance range of 10 14 , or one hundred trillion (100,000,000,000,000) (about 46.5 f-stops), from 10 -6 cd/m 2 , or a millionth of a (0.000001) of candela per square meter up to 10 8 cd/m 2 or a hundred million (100,000,000) candelas per square meter. This range does not include viewing the midday sun (10 9 cd/m 2 ) or lightning discharge.
At the lower end of the range is the absolute vision threshold for stable light in the wide viewing field, about 10 -6 cd/m2 (.000001 candela per square meter). The upper end of the range is given in the form of normal visual performance as 10 8 cd/m 2 (100,000,000 or hundred million candelas per square meter).
Eyes include lenses similar to lenses found in optical instruments such as cameras and similar physics principles can be applied. The human eye pupil is the opening; iris is a diaphragm that functions as a stop aperture. Refraction in the cornea causes the effective aperture (incoming pupil) slightly different from the diameter of the physical pupil. The incoming pupil is usually about 4 mm in diameter, although it can range from 2 mm ( f /8.3) in brightly lit up to 8 mm ( f /2.1) in the dark. The last value decreases slowly with age; the eyes of the parents sometimes widen to no more than 5-6mm in the dark, and may be as small as 1mm in light.
Eye movements
The visual system in the human brain is too slow to process information if images slip across the retina more than a few degrees per second. Thus, to be able to see while moving, the brain must balance the head movement by rolling eyes. The front-edged animal has a small area of ​​the retina with very high visual acuity, the center of the fovea. It covers about 2 degrees of visual angle in humans. To get a clear view of the world, the brain must rotate the eye so that the image of the object seen falls on the fovea. Failure to make eye movement properly can lead to serious visual degradation.
Having two eyes allows the brain to determine the depth and distance of an object, called stereovision, and give a three-dimensional sense to the vision. Both eyes must show quite accurately that the object of respect falls at the appropriate point of two retinas to stimulate stereovision; otherwise, double vision may occur. Some people with crossed eyes tend to ignore one eye, so as not to suffer double vision, and do not have stereovision. The movement of the eye is controlled by the six muscles attached to each eye, and allows the eye to lift, press, unite, distort, and roll over. These muscles are controlled voluntarily and unconsciously to track objects and correct head movement simultaneously.
Extraocular muscle
Each eye has six muscles that control its motion: the lateral rectus, the medial rectus, the inferior rectus, the superior rectus, the inferior oblique, and the oblique superior. When the muscle exerts a different tension, a torque is given to the globe causing it to spin, in almost pure rotation, with only about a millimeter of translation. Thus, the eye can be regarded as a rotation undergoing about a point in the middle of the eye.
Fast eye movement
Rapid eye movement, REM, usually refers to the stage of sleep in which the most obvious dreams occur. During this stage, the eye moves quickly. That alone is not a unique form of eye movement.
Saccades
Saccade is a rapid and simultaneous movement of both eyes in the same direction that is controlled by the frontal lobe of the brain. Some irregular movements, movements, are smaller than saccade and are larger than microsaccade, which reach up to a tenth of a degree.
Microphone
Even when looking intently in one place, the eyes float. This ensures individual photosensitive cells are continuously stimulated in different degrees. Without changing the input, these cells would otherwise stop generating output. Microscopically move the eye no more than a total of 0.2 Â ° in adult humans.
Vestibulo-ocular reflex
Vestibulo-ocular reflexes are the eye reflex movements that stabilize the image on the retina during head movement by producing eye movement in the opposite direction of the head movement in response to the nerve input of the inner ear vestibular system, thus maintaining the image within the visual field center. For example, when the head moves to the right, the eye moves to the left. This applies to upward and downward, left and right head movements, and tilts to the right and left, all of which provide input to the ocular muscles to maintain visual stability.
Smooth pursuit
The eye can also follow the moving object around it. This tracking is less accurate than the vestibulo-ocular reflex, as it requires the brain to process incoming visual information and feedback. Following a moving object with a constant velocity is relatively easy, though the eye often makes a saccadic jerk to follow it. Fine pursuit movement can move the eye up to 100 Â °/s in adult humans.
It is more difficult to estimate visually the speed in low light conditions or when moving, unless there is another reference point to determine the speed.
Optokinetic reflex
Main article: Optocinetic Response
Optokinetic reflexes (or optokinetic nystagmus) stabilize the image on the retina through visual feedback. This is induced when the entire visual scene hovers in the retina, bringing the eye rotation in the same direction and at a speed that minimizes the motion of the image on the retina. When the direction of the view deviates too far from the front, saccade compensation is induced to reset the view to the center of the visual field.
For example, when looking out the window on the moving car, the eye can focus on the moving car for a moment (stabilizing the retina), until the train moves out of the field of vision. At this point, the eye is moved back to the point where he first sees the train (via saccade).
Close response
The close-up vision adjustment involves three processes to focus the image on the retina.
Vergence Movement
When creatures with binocular vision see objects, the eyes must rotate around the vertical axis so that the projection of the image is at the center of the retina in both eyes. To see objects nearby, the eyes rotate 'against each other' (convergence), while for more distant objects they rotate 'away from each other' (divergence).
Narrow pupil
The lenses can not refract the rays of light at the edges and they can get closer to the center. Therefore, the image produced by any lens is somewhat opaque around the edges (ball aberration). This can be minimized by filtering out the rays of peripheral light and only seeing the center more focused. In the eyes, the pupil serves this purpose with constriction while the eye is focused on nearby objects. Small holes also provide increased depth of field, allowing a wider range of vision "in focus". In this way the pupil has a dual purpose for near vision: to reduce spherical aberrations and increase depth of field.
Lens accommodation
Changing the curvature of the lens is done by the ciliary muscles that surround the lens; this process is called "accommodation". The accomodation constricts the inner diameter of the ciliary body, which actually relaxes the suspensory ligament fibers attached to the periphery of the lens, and allows the lens to relax into a more convex or round shape. Convex lenses that refract more light and focus different rays of light from near the object to the retina, allowing the closer objects to become better focus.
Clinical interests
Professional eye care
The human eye contains enough complexity to ensure attention and special care beyond the duties of a general practitioner. This specialist, or eye care professional, serves different functions in different countries. Professional eye care can have overlaps in their patient care rights. For example, ophthalmologists (M.D.) and optometrists (OD.) are professionals who diagnose eye diseases and can prescribe lenses to improve vision. However, usually only ophthalmologists have permission to perform surgical procedures. Optometrists can also specialize in surgical areas, such as corneas, cataracts, lasers, retinas, or oculoplastics. Other eye care professionals include:
- Ophthalmologists
- Opticians
- Opticians
- Vision Therapists and Orthoptists
- Okularis
Eye irritation
Eye irritation has been defined as "the amount of stinging, scratching, burning, or other irritating sensations from the eye". This is a common problem experienced by people of all ages. Eye-related symptoms and signs of irritation are discomfort, dryness, excessive tearing, itching, lattice, foreign body sensation, eyestrain, pain, itching, pain, redness, swollen eyelids, and fatigue, etc. These eye symptoms are reported with intensity from mild to severe. It has been suggested that these eye symptoms are associated with different causal mechanisms, and the symptoms associated with certain ocular anatomy are involved.
Some of the suspected causal factors in our environment have been studied so far. One hypothesis is that indoor air pollution can cause eye irritation and respiratory tract. Eye irritation is somewhat dependent on destabilization of the outer tear film, where the formation of dry spots on the cornea, resulting in ocular discomfort. Occupational factors also tend to affect perceptions of eye irritation. Some of them are lighting (blinding light and poor contrast), eye-level position, reduced blink rate, limited number of breaks from visual assignment, and constant combination of accommodation, muscle and bone load, and visual nervous system disorders. Another factor that may be related is work stress. In addition, psychological factors have been found in multivariate analysis to be associated with increased eye irritation among VDU users. Other risk factors, such as toxic/chemical irritants (eg amines, formaldehyde, acetaldehyde, acrolein, N-decane, VOC, ozone, pesticides and preservatives, allergens, etc.) may cause eye irritation as well.
Certain volatile organic compounds that are chemically reactive and airway irritants can cause eye irritation. Personal factors (eg contact lens wear, eye makeup, and certain medications) may also affect the destabilization of tear films and may result in more eye symptoms. However, if airborne particles alone must destabilize the tear film and cause eye irritation, the content of the surface active compound should be high. An integrated physiological risk model with flashing, destabilization, and breaking of film tears as an inherent phenomenon can explain the eye irritation among office workers in terms of physiological risk factors related to work, climate and eyes.
There are two main measures of eye irritation. One is the flickering frequency that can be observed by human behavior. Other steps are breaking time, tear flow, hyperemia (redness, swelling), cytology of tear fluid, and epithelial damage (vital stains) etc., which are human physiological reactions. The blinking frequency is defined as the number of blinks per minute and is associated with eye irritation. Blink individual frequency with average frequency & lt; 2-3 to 20-30 blink/min, and they depend on environmental factors including contact lens wear. Dehydration, mental activity, working conditions, room temperature, relative humidity, and illumination of all the effects of flashing frequencies. Break-up time (BUT) is another major measure of eye irritation and tear film stability. This is defined as the time interval (in seconds) between blinking and breaking. BUT it is considered to reflect the stability of the tear film as well. In normal people, break-up time exceeds the interval between blinks, and, therefore, the tear film is maintained. Studies have shown that flashing frequencies are negatively correlated with break times. This phenomenon suggests that perceived eye irritation is associated with an increase in blinking frequency because the cornea and conjunctiva have sensitive nerve endings that belong to the first trigeminal branch. Other evaluation methods, such as hyperemia, cytology, etc. It has been used to assess eye irritation.
There are other factors associated with eye irritation as well. The three most influencing main factors are indoor air pollution, contact lenses and gender differences. Field studies have found that the prevalence of objective eye signs is often significantly altered among office workers in comparison with random samples from the general population. The results of this study may indicate that indoor air pollution has played an important role in causing eye irritation. There are more and more people wearing contact lenses now and dry eyes seem to be the most common complaint among contact lens wearers. Although both contact lens wearers and eyewear users experience the same symptoms of eye irritation, dryness, redness, and tidiness have been reported much more frequently among contact lens wearers and with a higher severity than among glasses wearers. Research has shown that the incidence of dry eyes increases with age, especially among women. Tear film stability (eg rest periods) is significantly lower among women than men. In addition, women have higher frequency flicker when reading. Several factors can contribute to gender differences. One is the use of eye make-up. Another reason could be that women in the reported study had done more VDU work than men, including lower-class jobs. The third frequently cited explanation is associated with decreased age-dependent tears secretion, especially among women after the age of 40.
In a study conducted by UCLA, the frequency of symptoms reported in industrial buildings was investigated. The results of the study were that eye irritation was the most common symptom in industrial building space, at 81%. Modern office work using office equipment has raised concerns about the potential adverse health effects. Since the 1970s, reports have linked mucosal, skin, and general symptoms to work with self-copying papers. Emissions of various particulates and volatiles have been suggested as specific causes. These symptoms are associated with sick building syndrome (SBS), which involves symptoms such as irritation of the eyes, skin, and upper airways, headaches and fatigue.
Many of the symptoms described in SBS and dual chemical sensitivity (MCS) resemble symptoms known to be caused by irritant chemicals in the air. A repeatable measurement design is used in studies of acute symptoms of the eye and respiratory irritation resulting from occupational exposure to sodium borate dust. Symptom assessment of 79 exposed subjects and 27 unexposed ones consisted of interviews before the shift began and then at regular hour intervals for the next six hours of shift, four consecutive days. Exposure is monitored simultaneously with a personal real time aerosol monitor. Two different profile exposures, average daily and short-term (15 minutes), were used in the analysis. Exposure-response relationships are evaluated by correlating incidence rates for each symptom by exposure category.
Acute incidence rates for nasal, eye, and throat irritation, and cough and breathlessness were found to be associated with increased exposure levels from both exposure indices. A sharp exposure response slope is seen when short-term exposure concentrations are used. Results from a multivariate logistic regression analysis showed that current smokers tend to be less sensitive to exposure to airborne sodium borate dust.
Some measures can be taken to prevent eye irritation -
- try to keep normal flickering by avoiding too much room temperature; avoid too high or too low relative humidity, as it reduces the flicker frequency or can increase water evaporation
- try to keep the tear film intact with the following actions:
- 1) Blinking and pausing may be useful for VDU users. Increasing these two actions can help keep the tear films.
- 2) Gazing down is recommended to reduce the ocular surface area and water evaporation.
- 3) The distance between the VDU and the keyboard should be kept as short as possible to minimize evaporation from the ocular surface area by the low sight direction. And 4) blink training can be useful.
In addition, other measures are good hygiene cover, avoid eye rubbing, and proper use of products and personal medicines. Eye makeup should be used with care.
The practice of paraphilic oculolinctus, or licking of the eyeball, can also cause irritation, infection, or damage to the eyes.
Eye disease
There are many diseases, disorders, and age-related changes that can affect the surrounding eyes and structures.
As the eye ages, certain changes occur that can be attributed only to the aging process. Most of these anatomical and physiological processes follow a gradual decline. With aging, vision quality worsens for reasons independent of aging eye disease. Although there are many significant changes in the non-painful eye, the most functionally functional changes appear to be pupillary diminution and loss of accommodation or focusing ability (presbyopia). The pupil area regulates the amount of light that can reach the retina. The extent to which the pupils dilate decreases with age, causing a substantial decrease in light received in the retina. Compared to younger people, it is as if older people are constantly wearing medium-density sunglasses. Therefore, for any visually guided task where performance varies with lighting, older people need additional lighting. Certain eye diseases may originate from sexually transmitted diseases such as herpes and genital warts. If contact between eye and area of ​​infection occurs, STD can be transmitted to the eye.
With aging, a prominent white ring develops on the periphery of the cornea called the senile arcus. Aging causes weakness, downward shifting of eyelid tissue and orbital fat atrophy. These changes contribute to the etiology of some eyelid disorders such as ectropion, entropion, dermatochalasis, and ptosis. The vitreous gel undergoes liquefaction (posterior vitreous release or PVD) and its turbidity - seen as floaters - gradually increases in number.
Various eye care specialists, including ophthalmologists (optometrists/surgeons), ophthalmologists, and eye doctors, are involved in the care and management of eye and vision disorders. The Snellen chart is one type of eye chart used to measure visual acuity. At the end of a complete eye exam, the ophthalmologist may provide patients with prescription glasses for corrective lenses. Some eye disorders whose corrective lenses are prescribed include myopia (near-sightedness) affecting about one-third of the human population, hyperopia (farsightedness) affecting about a quarter of the population, astigmatism, and presbyopia (loss of focusing range during aging).
Macular degeneration
Macular degeneration mainly occurs in the US and affects about 1.75 million Americans each year. Having lower levels of lutein and zeaxanthin in the macula may be associated with an increased risk of age-related macular degeneration. Lutein and zeaxanthin act as antioxidants that protect the retina and macula from oxidative damage from high-energy light waves. When light waves enter the eye, they excrete electrons that can cause damage to the cells in the eye, but before they can cause oxidative damage that can cause macular degeneration or cataracts. Lutein and zeaxanthin bind to free radical electrons and reduce safe electron rendering. There are many ways to ensure a diet rich in lutein and zeaxanthin, it is best to eat dark green vegetables including kale, spinach, broccoli, and turnip greens. Nutrition is an important aspect of the ability to achieve and maintain proper eye health. Lutein and zeaxanthin are two major carotenoids, found in the eye macula, which are being investigated to identify their role in the pathogenesis of eye disorders such as age-related macular degeneration and cataracts.
Additional images
See also
References
External links
- 3D Interactive Human Eyes
- How to Get Rid of Styles Before Shaping
- Eyes - Hilzbook
- Retina - Hilzbook
- How to Get a Better View
Source of the article : Wikipedia
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