Human eye

The human eye
1:posterior chamber 2:ora serrata 3:ciliary muscle 4:ciliary zonules 5:canal of Schlemm 6:pupil 7:anterior chamber 8:cornea 9:iris 10:lens cortex 11:lens nucleus 12:ciliary process 13:conjunctiva 14:inferior oblique muscle 15:inferior rectus muscle 16:medial rectus muscle 17:retinal arteries and veins 18:optic disc 19:dura mater 20:central retinal artery 21:central retinal vein 22:optic nerve 23:vorticose vein 24:bulbar sheath 25:macula 26:fovea 27:sclera 28:choroid 29:superior rectus muscle 30:retina
Identifiers
MeSH	D005123
TA98	A01.1.00.007 A15.2.00.001
TA2	113, 6734
FMA	54448
Anatomical terminology

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 colour differentiation and the perception of depth. The human eye can distinguish about 10 million colours.

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, less curved unit called the cornea, is linked to the larger unit called the sclera. The cornea and sclera are connected by a ring called the limbus. The iris 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

A human eye

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. 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 opthalmoscope.

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 6 1/2 f-stops). As soon as the eye moves (saccades) it re-adjusts its exposure both chemically and by adjusting the iris. 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 (f/8.3) in a brightly lit place to 8 mm (f/2.1) in the dark.

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° in height and 5.5° in width.

Eye irritation

Eye irritation is a common problem experienced by people of all ages. There are numerous causes in which some can be prevented and treated properly. However, in order to take precaution it is important to have some basic knowledge regarding what eye irritants are and where they can be found in our environments. Eye irritation depends somewhat on destabilization of the outer-eye tear film. Certain volatile organic compounds that are both chemically reactive and airway irritants may cause eye irritation as well. Personal factors (eg, use of contact lenses, eye make-up, and certain medication) 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.

In a study conducted by NIOSH, 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, which involves symptoms such as irritation to the eyes, skin, and upper airways, headache and fatigue.

Many of the symptoms described in Sick Building Syndrome (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.

Eye movement

MRI scan of human eye

Main article: Eye movements

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

Main article: 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

Main article: Rapid eye movement (sleep)

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

Main article: Saccade

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 cascade and larger than a microscopical, subtend up to six minutes of arc.

Microsaccades

Main article: Microsaccade

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

Main article: 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

Main article: 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).

Vergence movement

Main article: Vergence

The two eyes converge to point to the same object.

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.

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 aging eye diseases. 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 also decreases with age. Because of the smaller pupil size, older eyes receive much less light at the retina. In comparison to younger people, it is as though older persons wear medium-density sunglasses in bright light and extremely dark glasses in dim light. 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.

See also

• Mammalian eye

External links

Media related to human eyes at Wikimedia Commons