A specialized receptor organ responding to light stimuli. Various forms exist, such as the stigmata of certain protozoa, the ocelli of annelids, and the compound eye of insects. In land-based vertebrates, such as humans, the eyeball is composed of two parts: the transparent corneal part at the front, and the opaque scleral part at the back. Three concentric coats form the wall of the eyeball: an outer fibrous coat, consisting of the cornea and sclera; a middle vascular coat, consisting of the choroid, ciliary body, and iris; and an inner nervous coat (the retina). The coats surround and partly divide the contents of the eyeball. The vitreous body (a jelly-like substance containing a meshwork of fine fibres) lies between the lens and the retina. The space between the lens and the cornea contains a watery fluid (the aqueous humour), and is further partly divided by the iris into front and rear chambers. The lens is transparent and biconvex, lying between the iris and the vitreous body; changes in its convexity alter its focal length (the greater the convexity, the shorter the focal length), and are brought about by the action of the ciliary muscles. The lens is soft and elastic in the fetus, but becomes hardened and flatter with increasing age, making focusing on close objects more difficult (presbyopia); it may also become opaque in the elderly (cataract). Light entering the eye is refracted by the cornea, and passes through the lens, which focuses it to form an inverted image on the retina. This image is coded by the retina and sent to the visual areas of the cerebral cortex for interpretation, thus enabling the original pattern of light stimuli to be seen. The amount of light entering the eye is determined by the size of the pupil, which in turn is determined by the degree of contraction of the iris. If the image from distant objects is focused in front of the retina, the condition is known as myopia, with vision being better for near objects (short-sightedness). If the image is focused beyond the retina, the condition is known as hypermetropia, with vision being better for distant objects (long-sightedness). Glasses using concave lenses are used to correct short sight; a convex lens is used for long sight.
An eye is an organ of vision that detects light. The simplest eyes do nothing but detect whether the surroundings are light or dark, while more complex eyes can distinguish shapes and colors. Many animals, including some mammals, birds, reptiles and fish, have two eyes which may be placed on the same plane to be interpreted as a single three-dimensional "image" (binocular vision), as in humans;
Varieties of eyes
In most vertebrates and some mollusks, the eye works by allowing light to enter it and project onto a light-sensitive panel of cells known as the retina at the rear of the eye, where the light is detected and converted into electrical signals. Such eyes are typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often an iris which regulates the intensity of the light that enters the eye. The eyes of cephalopods, fish, amphibians, and snakes usually have fixed lens shapes, and focusing vision is achieved by telescoping the lens — similar to how a camera focuses.
Compound eyes are found among the arthropods and are composed of many simple facets which give a pixelated image (not multiple images, as is often believed). Some arthropods, including many Strepsiptera, have compound eye composed of a few facets each, with a retina capable of creating an image, which does provide multiple-image vision. With each eye viewing a different angle, a fused image from all the eyes is produced in the brain, providing a very wide-angle, high-resolution image.
Possessing detailed hyperspectral color vision, the Mantis shrimp has been reported to have the world's most complex color vision system. The number of lenses in such an eye varied, however: some trilobites had only one, and some had thousands of lenses in one eye.
Some of the simplest eyes, called ocelli, can be found in animals like snails, who cannot actually "see" in the normal sense. Some insect larvae, like caterpillars, have a different type of single eye (stemmata) which gives a rough image.
Evolution of eyes
The common origin (monophyly) of all animal eyes is now widely accepted as fact based on shared anatomical and genetic features of all eyes; Hence multiple eye types and subtypes developed in parallel.
Eyes in various animals show adaptation to their requirements. For example, birds of prey have much greater visual acuity than humans, and some can see ultraviolet light.
The earliest eyes, called "eyespots", were simple patches of photoreceptor cells, physically similar to the receptor patches for taste and smell. This gradually changed as the eyespot depressed into a shallow "cup" shape, granting the ability to slightly discriminate directional brightness by using the angle at which the light hit certain cells to identify the source.
The thin overgrowth of transparent cells over the eye's aperture, originally formed to prevent damage to the eyespot, allowed the segregated contents of the eye chamber to specialize into a transparent humour that optimized color filtering, blocked harmful radiation, improved the eye's refractive index, and allowed functionality outside of water.
The gap between tissue layers naturally formed a bioconvex shape, an ideal structure for a normal refractive index. Formation of a nontransparent ring allows more blood vessels, more circulation, and larger eye sizes.
Anatomy of the mammalian eye
Three layers
The structure of the mammalian eye can be divided into three main layers or tunics whose names reflect their basic functions: the fibrous tunic, the vascular tunic, and the nervous tunic. The sclera gives the eye most of its white color. It consists of dense connective tissue filled with the protein collagen to both protect the inner components of the eye and maintain its shape. The choroid gives the inner eye a dark color, which prevents disruptive reflections within the eye. To maximise vision and light absorption, the retina is a relatively smooth (but curved) layer.
Anterior and posterior segments
The mammalian eye can also be divided into two main segments: the anterior segment and the posterior segment.
Anterior segment
The anterior segment is the front third of the eye that includes the structures in front of the vitreous humour: the cornea, iris, ciliary body, and lens.
The cornea and lens help to converge light rays to focus onto the retina. The lens, behind the iris, is a convex, springy disk which focuses light, through the second humour, onto the retina. Light must first pass though the centre of the iris, the pupil. The size of the pupil is actively adjusted by the circular and radial muscles to maintain a relatively constant level of light entering the eye. Too much light being let in could damage the retina;
All of the individual components through which light travels within the eye before reaching the retina are transparent, minimising dimming of the light. Light enters the eye from an external medium such as air or water, passes through the cornea, and into the first of two humours, the aqueous humour. The first humour is a clear mass which connects the cornea with the lens of the eye, helps maintain the convex shape of the cornea (necessary to the convergence of light at the lens) and provides the corneal endothelium with nutrients.
Posterior segment
The posterior segment is the back two-thirds of the eye that includes the anterior hyaloid membrane and all structures behind it: the vitreous humor, retina, choroid, and optic nerve. It lets light through without refraction, helps maintain the shape of the eye and suspends the delicate lens. In some animals, the retina contains a reflective layer (the tapetum lucidum) which increases the amount of light each photosensitive cell perceives, allowing the animal to see better under low light conditions.
Extraocular anatomy
In many species, the eyes are inset in the portion of the skull known as the orbits or eyesockets.
In humans, the eyebrows redirect flowing substances (such as rainwater or sweat) away from the eye. Water in the eye can alter the refractive properties of the eye and blur vision. It can be reversed by irrigating the eye with hypertonic saline which osmotically draws the excess water out of the eye.
In many animals, including humans, eyelids wipe the eye and prevent dehydration. Some aquatic animals have a second eyelid in each eye which refracts the light and helps them see clearly both above and below water. Most creatures will automatically react to a threat to its eyes (such as an object moving straight at the eye, or a bright light) by covering the eyes, and/or by turning the eyes away from the threat.
In many animals, including humans, eyelashes prevent fine particles from entering the eye. Fine particles can be bacteria, but also simple dust which can cause irritation of the eye, and lead to tears and subsequent blurred vision.
Other articles regarding eye anatomy
Annulus of Zinn, Conjunctiva, Macula, Nictitating membrane, Schlemm's canal, Trabecular meshwork.
Cytology
The structure of the mammalian eye owes itself completely to the task of focusing light onto the retina. This light causes chemical changes in the photosensitive cells of the retina, the products of which trigger nerve impulses which travel to the brain.
The retina contains two forms of photosensitive cells important to vision — rods and cones. Rod cells are highly sensitive to light allowing them to respond in dim light and dark conditions. These are the cells which allow humans and other animals to see by moonlight, or with very little available light (as in a dark room). Cone cells, conversely, need high light intensities to respond and have high visual acuity. Different cone cells respond to different wavelengths of light, which allows an organism to see color.
The differences are useful; apart from enabling sight in both dim and light conditions, humans have given them further application. Its requirement for high intensity light does cause problems for astronomers, as they cannot see dim stars, or other objects, using central vision because the light from these is not enough to stimulate cone cells. Because cone cells are all that exist directly in the fovea, astronomers have to look at stars through the "corner of their eyes" (averted vision) where rods also exist, and where the light is sufficient to stimulate cells, allowing the individual to observe distant stars.
Rods and cones are both photosensitive, but respond differently to different frequencies of light. Rod cells contain the protein rhodopsin and cone cells contain different proteins for each color-range.
This is the reason why cones and rods enable organisms to see in dark and light conditions — each of the photoreceptor proteins requires a different light intensity to break down into the constituent products. Further, synaptic convergence means that several rod cells are connected to a single bipolar cell, which then connects to a single ganglion cell and information is relayed to the visual cortex. If a ray of light were to reach just one rod cell this may not be enough to stimulate an action potential.
Furthermore, color is distinguishable when breaking down the iodopsin of cone cells because there are three forms of this protein. One form is broken down by the particular EM wavelength that is red light, another green light, and lastly blue light. In simple terms, this allows human beings to see red, green and blue light.
Acuity
Visual acuity can be measured with several different metrics.
Cycles per degree (CPD) measures how much an eye can differentiate one object from another in terms of degree angles.
A diopter is the unit of measure of optical power.
Dynamic range
At any given instant, the retina can resolve a contrast ratio of around 100:1 (about 6 1/2 stops). As soon as your eye moves (saccades) it re-adjusts its exposure both chemically and by adjusting the iris. The process is nonlinear and multifaceted, so an interruption by light nearly starts the adaptation process over again.
Eye movement
Animals with compound eyes have a wide field of vision, allowing them to look in many directions.
The visual system in the brain is too slow to process that information if the images are slipping across the retina at more than a few degrees per second (Westheimer and McKee, 1954). 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; 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.
How we see an object
The steps of how we see an object:
The light rays enter the eye through the cornea (transparent front portion of eye to focus the light rays) Then, light rays move through the pupil, which is surrounded by Iris to keep out extra light Then, light rays move through the crystalline lens (Clear lens to further focus the light rays ) Then, light rays move through the vitreous humor (clear jelly like substance) Then, light rays fall on the retina, which processes and converts incident light to neuron signals using special pigments in rod and cone cells. These neuron signals are transmitted through the optic nerve, Then, the neuron signals move through the visual pathway: Optic nerve → Optic Chiasm → Optic Tract → Optic Radiations → Cortex Then, the neuron signals reach the occipital (visual) cortex and its radiations for the brain's processing.Color vision
What is seen as color is essentially different combinations of certain ranges of wavelengths in the electromagnetic spectrum. In humans at least, there are three different kinds of cones for three ranges of wavelengths, roughly red, green and blue light. Each color of cone picks up the intensity of light in its range of wavelengths, and the combination is translated by the brain to a perceived color.
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. 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 typically refers to the stage during sleep during which the most vivid dreams occur. 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.
Microsaccades
Even when looking intently at a single spot, the eyes drift around. Microsaccades move the eye no more than a total of 0.2° in adult humans.
Vestibulo-ocular reflex
Smooth pursuit movement
The eyes can also follow a moving object around. The smooth pursuit movement can move the eye at up to 100°/s in adult humans.
While still, the eye can measure relative speed with high accuracy, however under movement relative speed is highly distorted.
When an observer views an object in motion moving away or towards himself, there is no eye movement occurring as in the examples above, however the ability to discern speed and speed difference is still present;
Optokinetic reflex
The optokinetic reflex is a combination of a saccade and smooth pursuit movement. When, for example, looking out of the window in a moving train, the eyes can focus on a 'moving' tree for a short moment (through smooth pursuit), until the tree 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 tree (through a saccade).
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.
Vergence movements are closely connected to accommodation of the eye.
Accommodation
To see clearly, the lens will be pulled flatter or allowed to regain its thicker form.
Diseases, disorders, and age-related changes
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. 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. 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.
With aging a prominent white ring develops in the periphery of the cornea- called arcus senilis.
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.
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