Human eye

The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth. The human eye can distinguish about 10 million colors.

In common with the eyes of other mammals, the human eye's non-image-forming photosensitive ganglion cells in the retina receive the light signals which affect adjustment of the size of the pupil, regulation and suppression of the hormone melatonin and entrainment of the body clock.

General properties
The eye is not properly a sphere, rather it is a fused two-piece unit. The smaller frontal unit, more curved, called the cornea is linked to the larger unit called the sclera. The corneal segment is typically about 8 mm (0.3 in) in radius. The sclera constitutes the remaining five-sixths; its radius is typically about 12 mm. The cornea and sclera are connected by a ring called the limbus. The iris – the color of the eye – and its black center, the pupil, are seen instead of the cornea due to the cornea's transparency. To see inside the eye, an ophthalmoscope is needed, since light is not reflected out. The fundus (area opposite the pupil) shows the characteristic pale optic disk (papilla), where vessels entering the eye pass across and optic nerve fibers depart the globe.

Dimensions
The dimensions differ among adults by only one or two millimeters. The vertical measure, generally less than the horizontal distance, is about 24 mm among adults, at birth about 16–17 mm. (about 0.65 inch) The eyeball grows rapidly, increasing to 22.5–23 mm (approx. 0.89 in) by the age of three years. From then to age 13, the eye attains its full size. The volume is 6.5 ml (0.4 cu. in.) and the weight is 7.5 g. (0.25 oz.)

Components
The eye is made up of three coats, enclosing three transparent structures. The outermost layer is composed of the cornea and sclera. The middle layer consists of the choroid, ciliary body, and iris. The innermost is the retina, which gets its circulation from the vessels of the choroid as well as the retinal vessels, which can be seen in an ophthalmoscope.

Within these coats are the aqueous humor, the vitreous body, and the flexible lens. The aqueous humor is a clear fluid that is contained in two areas: the anterior chamber between the cornea and the iris and exposed area of the lens; and the posterior chamber, behind the iris and the rest. The lens is suspended to the ciliary body by the suspensory ligament (Zonule of Zinn), made up of fine transparent fibers. The vitreous body is a clear jelly that is much larger than the aqueous humor, and is bordered by the sclera, zonule, and lens. They are connected via the pupil.

Dynamic range
The retina has a static contrast ratio of around 100:1 (about 6½ f-stops). As soon as the eye moves (saccades) it re-adjusts its exposure both chemically and geometrically by adjusting the iris which regulates the size of the pupil. Initial dark adaptation takes place in approximately four seconds of profound, uninterrupted darkness; full adaptation through adjustments in retinal chemistry (the Purkinje effect) are mostly complete in thirty minutes. Hence, a dynamic contrast ratio of about 1,000,000:1 (about 20 f-stops) is possible. The process is nonlinear and multifaceted, so an interruption by light merely starts the adaptation process over again. Full adaptation is dependent on good blood flow; thus dark adaptation may be hampered by poor circulation, and vasoconstrictors like alcohol or tobacco.

The eye includes a lens not dissimilar to lenses found in optical instruments such as cameras and the same principles can be applied. The pupil of the human eye is its aperture; the iris is the diaphragm that serves as the aperture stop. Refraction in the cornea causes the effective aperture (the entrance pupil) to differ slightly from the physical pupil diameter. The entrance pupil is typically about 4 mm in diameter, although it can range from 2 mm in a brightly lit place to 8 mm  in the dark. The latter value decreases slowly with age, older people's eyes sometimes dilate to not more than 5-6mm.

Field of view
The approximate field of view of a human eye is 95° out, 75° down, 60° in, 60° up. About 12–15° temporal and 1.5° below the horizontal is the optic nerve or blind spot which is roughly 7.5° high and 5.5° wide.

Eye irritation


Eye irritation has been defined as “the magnitude of any stinging, scratching, burning, or other irritating sensation from the eye”. It is a common problem experienced by people of all ages. Related eye symptoms and signs of irritation are e.g. discomfort, dryness, excess tearing, itching, grating, sandy sensation, smarting, ocular fatigue, pain, scratchiness, soreness, redness, swollen eyelids, and tiredness, etc. These eye symptoms are reported with intensities from severe to less severe. It has been suggested that these eye symptoms are related to different causal mechanisms.

Several suspected causal factors in our environment have been studied so far. One hypothesis is that indoor air pollution may cause eye and airway irritation. Eye irritation depends somewhat on destabilization of the outer-eye tear film, in which the formation of dry spots results in such ocular discomfort as dryness. Occupational factors are also likely to influence the perception of eye irritation. Some of these are lighting (glare and poor contrast), gaze position, a limited number of breaks, and a constant function of accommodation, musculoskeletal burden, and impairment of the visual nervous system. Another factor that may be related is work stress. In addition, psychological factors have been found in multivariate analyses to be associated with an increase in eye irritation among VDU users. Other risk factors, such as chemical toxins/irritants, e.g. Amines, Formaldehyde, Acetaldehyde, Acrolein, N-decane, VOCs; Ozone, Pesticides and preservatives, Allergens, etc might cause eye irritation as well.

Certain volatile organic compounds that are both chemically reactive and airway irritants may cause eye irritation as well. Personal factors (e.g., use of contact lenses, eye make-up, and certain medications) may also affect destabilization of the tear film and possibly result in more eye symptoms. Nevertheless, if airborne particles alone should destabilize the tear film and cause eye irritation, their content of surface-active compounds must be high. An integrated physiological risk model with blink frequency, destabilization, and break-up of the eye tear film as inseparable phenomena may explain eye irritation among office workers in terms of occupational, climate, and eye-related physiological risk factors.

There are two major measures of eye irritation. One is blink frequency which can be observed by human behavior. The other measures are break up time, tear flow, hyperemia (redness, swelling), tear fluid cytology, and epithelial damage (vital stains) etc, which are human beings’ physiological reactions. Blink frequency is defined as the number of blinks per minute and it is associated with eye irritation. Blink frequencies are individual with mean frequencies of < 2-3 to 20-30 blinks/minute, and they depend on environmental factors including the use of contact lenses. Dehydration, mental activities, work conditions, room temperature, relative humidity, and illumination all influence blink frequency. Break-up time (BUT) is another major measure of eye irritation and tear film stability. It is defined as the time interval (in seconds) between blinking and rupture. BUT is considered to reflect the stability of the tear film as well. In normal persons, the break-up time exceeds the interval between blinks, and, therefore, the tear film is maintained. Studies have shown that blink frequency is correlated negatively with break-up time. This phenomenon indicates that perceived eye irritation is associated with an increase in blink frequency since the cornea and conjunctiva both have sensitive nerve endings that belong to the first trigeminal branch. Other evaluating methods, such as hyperemia, cytology etc have increasingly been used to assess eye irritation.

There are other factors that related to eye irritation as well. Three major factors that influence the most 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 comparisons with random samples of the general population. These research results might indicate that indoor air pollution has played an important role in causing eye irritation. There are more and more people wearing contact lens now and dry eyes appear to be the most common complaint among contact lens wearers. Although both contact lens wearers and spectacle wearers experience similar eye irritation symptoms, dryness, redness, and grittiness have been reported far more frequently among contact lens wearers and with greater severity than among spectacle wearers. Studies have shown that incidence of dry eyes increases with age. especially among women. Tear film stability (eg. break-up time) is significantly lower among women than among men. In addition, women have a higher blink frequency while reading. Several factors may contribute to gender differences. One is the use of eye make-up. Another reason could be that the women in the reported studies have done more VDU work than the men, including lower grade work. A third often-quoted explanation is related to the age-dependent decrease of tear secretion, particularly among women after 40 years of age.,

In a study conducted by UCLA, the frequency of reported symptoms in industrial buildings was investigated. The study's results were that eye irritation was the most frequent symptom in industrial building spaces, at 81%. Modern office work with use of office equipment has raised concerns about possible adverse health effects. Since the 1970s, reports have linked mucosal, skin, and general symptoms to work with self-copying paper. Emission of various particulate and volatile substances has been suggested as specific causes. These symptoms have been related to Sick Building Syndrome (SBS), which involves symptoms such as irritation to the eyes, skin, and upper airways, headache and fatigue.

Many of the symptoms described in SBS and multiple chemical sensitivity (MCS) resemble the symptoms known to be elicited by airborne irritant chemicals. A repeated measurement design was employed in the study of acute symptoms of eye and respiratory tract irritation resulting from occupational exposure to sodium borate dusts. The symptom assessment of the 79 exposed and 27 unexposed subjects comprised interviews before the shift began and then at regular hourly intervals for the next six hours of the shift, four days in a row. Exposures were monitored concurrently with a personal real time aerosol monitor. Two different exposure profiles, a daily average and short term (15 minute) average, were used in the analysis. Exposure-response relations were evaluated by linking incidence rates for each symptom with categories of exposure.

Acute incidence rates for nasal, eye, and throat irritation, and coughing and breathlessness were found to be associated with increased exposure levels of both exposure indices. Steeper exposure-response slopes were seen when short term exposure concentrations were used. Results from multivariate logistic regression analysis suggest that current smokers tended to be less sensitive to the exposure to airborne sodium borate dust.

Several actions can be taken to prevent eye irritation—
 * trying to maintain normal blinking by avoiding room temperatures that are too high; avoiding relative humidities that are too high or too low, because they reduce blink frequency or may increase water evaporation
 * trying to maintain an intact tear film by the following actions. 1) blinking and short breaks may be beneficial for VDU users. Increase these two actions might help maintain the tear film. 2) downward gazing is recommended to reduce the ocular surface area and water evaporation.   3) the distance between the VDU and keyboard should be kept as short as possible to minimize evaporation from the ocular surface area by a low direction of the gaze. And 4) blink training can be beneficial.

In addition, other measures are proper lid hygiene, avoidance of eye rubbing, and proper use of personal products and medication. Eye make-up should be used with care.

Eye movement


The visual system in the brain is too slow to process information if the images are slipping across the retina at more than a few degrees per second. Thus, for humans to be able to see while moving, the brain must compensate for the motion of the head by turning the eyes. Another complication for vision in frontal-eyed animals is the development of a small area of the retina with a very high visual acuity. This area is called the fovea, and covers about 2 degrees of visual angle in people. To get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Eye movements are thus very important for visual perception, and any failure to make them correctly can lead to serious visual disabilities.

Having two eyes is an added complication, because the brain must point both of them accurately enough that the object of regard falls on corresponding points of the two retinas; otherwise, double vision would occur. The movements of different body parts are controlled by striated muscles acting around joints. The movements of the eye are no exception, but they have special advantages not shared by skeletal muscles and joints, and so are considerably different.

Extraocular muscles
Each eye has six muscles that control its movements: the lateral rectus, the medial rectus, the inferior rectus, the superior rectus, the inferior oblique, and the superior oblique. When the muscles exert different tensions, a torque is exerted on the globe that causes it to turn, in almost pure rotation, with only about one millimeter of translation. Thus, the eye can be considered as undergoing rotations about a single point in the center of the eye.

Rapid eye movement
Rapid eye movement, or REM for short, typically refers to the sleep stage during which the most vivid dreams occur. During this stage, the eyes move rapidly. It is not in itself a unique form of eye movement.

Saccades
Saccades are quick, simultaneous movements of both eyes in the same direction controlled by the frontal lobe of the brain. Some irregular drifts, movements, smaller than a saccade and larger than a microsaccade, subtend up to six minutes of arc.

Microsaccades
Even when looking intently at a single spot, the eyes drift around. This ensures that individual photosensitive cells are continually stimulated in different degrees. Without changing input, these cells would otherwise stop generating output. Microsaccades move the eye no more than a total of 0.2° in adult humans.

Vestibulo-ocular reflex
The vestibulo-ocular reflex is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. For example, when the head moves to the right, the eyes move to the left, and vice versa.

Smooth pursuit movement
The eyes can also follow a moving object around. This tracking is less accurate than the vestibulo-ocular reflex, as it requires the brain to process incoming visual information and supply feedback. Following an object moving at constant speed is relatively easy, though the eyes will often make saccadic jerks to keep up. The smooth pursuit movement can move the eye at up to 100°/s in adult humans.

It is more difficult to visually estimate speed in low light conditions or while moving, unless there is another point of reference for determining speed.

Optokinetic reflex
The optokinetic reflex is a combination of a saccade and smooth pursuit movement. When, for example, looking out of the window at a moving train, the eyes can focus on a 'moving' train for a short moment (through smooth pursuit), until the train moves out of the field of vision. At this point, the optokinetic reflex kicks in, and moves the eye back to the point where it first saw the train (through a saccade).

Near response
The adjustment to close-range vision involves three processes to focus an image on the retina.

Vergence movement
When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both eyes. To look at an object closer by, the eyes rotate 'towards each other' (convergence), while for an object farther away they rotate 'away from each other' (divergence). Exaggerated convergence is called cross eyed viewing (focusing on the nose for example). When looking into the distance, or when 'staring into nothingness', the eyes neither converge nor diverge. Vergence movements are closely connected to accommodation of the eye. Under normal conditions, changing the focus of the eyes to look at an object at a different distance will automatically cause vergence and accommodation.

Pupil constriction
Lenses cannot refract light rays at their edges as well as they can closer to the center. The image produced by any lens is therefore somewhat blurry around the edges (spherical aberration). It can be minimized by screening out peripheral light rays and looking only at the better-focused center. In the eye, the pupil serves this purpose by constricting while the eye is focused on nearby objects. In this way the pupil has a dual purpose: to adjust the eye to variations in brightness and to reduce spherical aberration.

Accommodation of the lens
A change in the curvature of the lens, accommodation is carried out by the ciliary muscles surrounding the lens contracting. This narrows the diameter of the ciliary body, relaxes the fibers of the suspernsory ligament, and allows the lens to relax into a more convex shape. A more convex lens refracts light more strongly and focuses divergent light rays onto the retina allowing for closer objects to be brought into focus.

Effects of aging
There are many diseases, disorders, and age-related changes that may affect the eyes and surrounding structures.

As the eye ages certain changes occur that can be attributed solely to the aging process. Most of these anatomic and physiologic processes follow a gradual decline. With aging, the quality of vision worsens due to reasons independent of diseases of the aging eye. While there are many changes of significance in the nondiseased eye, the most functionally important changes seem to be a reduction in pupil size and the loss of accommodation or focusing capability (presbyopia). The area of the pupil governs the amount of light that can reach the retina. The extent to which the pupil dilates decreases with age, leading to a substantial decrease in light received at the retina. In comparison to younger people, it is as though older persons are constantly wearing medium-density sunglasses. Therefore, for any detailed visually guided tasks on which performance varies with illumination, older persons require extra lighting. Certain ocular diseases can come from sexually transmitted diseases such as herpes and genital warts. If contact between eye and area of infection occurs, the STD can be transmitted to the eye.

With aging a prominent white ring develops in the periphery of the cornea- called arcus senilis. Aging causes laxity and downward shift of eyelid tissues and atrophy of the orbital fat. These changes contribute to the etiology of several eyelid disorders such as ectropion, entropion, dermatochalasis, and ptosis. The vitreous gel undergoes liquefaction (posterior vitreous detachment or PVD) and its opacities — visible as floaters — gradually increase in number.

Various eye care professionals, including ophthalmologists, optometrists, and opticians, are involved in the treatment and management of ocular and vision disorders. A Snellen chart is one type of eye chart used to measure visual acuity. At the conclusion of an eye examination, an eye doctor may provide the patient with an eyeglass prescription for corrective lenses. Some disorders of the eyes for which corrective lenses are prescribed include myopia (near-sightedness) which affects one-third of the population, hyperopia (far-sightedness) which affects one quarter of the population, and presbyopia, a loss of focusing range due to aging.

Eye care professionals
The human eye contains enough complexity to warrant specialized attention and care beyond the duties of a general practitioner. These specialists, or eye care professionals, serve different functions in different countries. Each eye care professional can typically be categorized into one or a multiplicity (i.e. an ophthalmologist can perform surgery; and in some instances prescibe lenses, which is a duty often performed by optometrists) of duties of the following types of professionals:
 * Ophthalmologists
 * Orthoptists
 * Optometrists
 * Opticians