Eye

An eye is an organ that detects light. Different kinds of light-sensitive organs are found in a variety of creatures. The simplest eyes do nothing but detect whether the surroundings are light or dark. More complex eyes are used to provide the sense of vision. Many complex organisms 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; or on different planes producing two separate "images" (monocular vision), such as in rabbits and chameleons.

Varieties of eyes

Diagram of a human eye. Note that not all eyes have the same anatomy as a human eye.

Color Cross Section Illustration of the Human Eye courtesy of Classroom Clip Art (http://classroomclipart.com)

In most vertebrates and some mollusks the eye works by projecting images onto a light-sensitive retina, where the light is detected and signals are transmitted to the brain via the optic nerve. 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. Although they are quite similar in function and appearance once fully developed, vertebrate eyes grow outward from brain cells during embryonic development, while mollusk eyes grow inward from skin cells.

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). Each sensor has its own lens and photosensitive cell(s). Some eyes have up to 28,000 such sensors, which are arranged hexagonally, and which can give a full 360 degree field of vision. Compound eyes are very sensitive to motion.

Trilobites, which are now extinct, had unique compound eyes. They used clear calcite crystals to form the lenses of their eyes. In this, they differ from most other arthropods, which have soft eyes. The number of lenses in such an eye varies, 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 can't actually 'see' in the common sense. They do have photosensitive cells, but no lens and no other means of projecting an image onto these cells. They can distinguish between light and dark (day and night), but no more. This enables snails to keep out of direct sunlight. Jumping spiders have simple eyes that are so large, supported by an array of other smaller eyes, that they can get enough visual inputs to hunt and pounce on their prey. Some insect larvae like caterpillars have a different type of single eye (stemmata) which gives a rough image.

Eye of a Komodo Dragon.Image provided by Classroom Clip Art (http://classroomclipart.com)

Eye Anatomy

The structure of the mammalian eye owes itself completely to the task of focusing light onto 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. The cornea and lens help to focus (converge) light rays 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.

Light enters the eye from an external medium such as air or water, passes through the cornea, into the first of two humours, the aqueous humour. Most of the light refraction occurs at the cornea which has a fixed curvature. 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. The iris, between the lens and the first humour, is a coloured ring of muscle fibres. 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, too little light would be blinding. The lens, behind the iris, is a convex, springy disk which focuses light, through the second humour, onto the retina.

Light from a single point of a distant object and light from a single point of a near object being brought to a focus.

To clearly see an object far away, the circularly arranged ciliary muscles will pull on the lens, flattening it. Without muscles pulling on it, the lens will spring back into a thicker, more convex, form. As we age we gradually lose this ability to spring back, resulting in the inability to focus on nearby objects, which is known as presbyopia. There are other refraction errors arising from the shape of the cornea and lens, and from the length of the eyeball. These include myopia, hyperopia, and astigmatism.

On the other side of the lens is the second humour, the vitreous humour, which is bounded on all sides: by the lens, ciliary body, suspensory ligaments and by the retina. It lets light through without refraction, helps maintain the shape of the eye and suspends the delicate lens.

Wrapped around these tissues are three layers of tissue surrounding the vitreous humour. The outermost is the sclera which gives most of they eye its white colour. It consists of fibrin connective tissue and both protects the inner components of the eye and maintains its shape. On the inner side of the sclera is the choroid which contains blood vessels which supply the retinal cells with necessary oxygen and removes the waste products of respiration. The sclera and ciliary muscles contain blood vessels, the rest of the eye does not. The choroid gives the inner eye a dark colour, which prevents disruptive reflections within the eye. The inner most layer of the eye is the retina, containing of the photosensitive rod and cone cells, and neurons.

To maximise vision and light absorption, the retina is a relatively smooth (but curved) layer. It does however have two points at which it is different; the fovea and blind spot. The fovea is a dip in the retina directly opposite the lens, which is densely packed with cone cells. It is largely responsible for colour vision in humans, and enables high acuity, such as is necessary in reading. The blind spot is a point on the retina where the optic nerve pierces the retina to connect to the nerve cells on its inside. No photosensitive cells exist at this point, it is thus "blind". In some animals, the retina contains a reflective layer which increases the amount of light each photosensitive cell perceives, which allows the animal to see better under low light conditions.

Diagram of the human eye.Image provided by [http://classroomclipart.com Classroom Clip Art

Rods and Cones Cytology

The retina contains two forms of photosensitive cells - rods and cones. Though structurally and metabolically similar, their function is quite different, though they are equally important to vision. 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). However, they do not distinguish between colours, and have low visual acuity (a measure of detail). This is why the darker conditions become, the less colour objects seem to have. Cone cells, conversely, need high light intensities to respond and have high visual acuity. Different cone cells respond to different colours (wavelengths) of light, which allows an organism to see colour.

The differences are useful; apart from enabling sight in both dim and light conditions, humans have given them further application. The fovea, directly behind the lens, consists of mostly densely-packed cone cells. This gives humans a highly detailed central vision, allowing reading, bird watching, or any other task which primarily requires looking at things. 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" 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 differently to different frequencies of light. They both contain different pigmented photoreceptor proteins. Rod cells contain the protein rhodopsin and cone cells contain different proteins for each colour-range. The process through which these proteins go is quite similar - upon being subjected to electromagnetic radiation of a particular wavelength and intensity (ie. a colour visible light) the protein breaks down into two constituent products. Rhodopsin, of rods, breaks down into opsin and retinal; iodopsin of cones breaks down into photopsin and retinal. The opsin in both opens ion channels on the cell membrane which leads to the generation of an action potential (an impulse which will eventually get to the visual cortex in the brain).

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. Whereas, a single cone cell is connected to a single bipolar cell. Thus, action potentials from rods share neurons, where those from cones are given their own. This results in the high visual acuity, or the high ability to distinguish between detail, of cone cells and not rods. If a ray of light were to reach just one rod cell this may not be enough to stimulate an action potential. Because several "converge" onto a bipolar cell, enough transmitter molecules reach the synapse of the bipolar cell to attain the threshold level to generate an action potential.

Furthermore, colour 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. If all three forms of cones are stimulated equally, then white is seen. If none are stimulated, black is seen. Most of the time however, the three forms are stimulated to different extents - resulting in different colours being seen. If, for example, the red and green cones are stimulated to the same extent, and no blue cones are stimulated, yellow is seen. For this reason we call red, green and blue primary colours and the products of mixing two secondary colours. The secondary colours can be further complimented with primary colours to see tertiary colours.

Peripherals of the eye

The eye is, for many animals (and humans), an important but delicate organ.

Eyesockets

In many animals, including humans, with binocular vision, the eyes are inset in the skull, in the eyesockets. This way, a stick or plate striking the head will not damage the eye, unless it breaks the skull.

Reflexes

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. Blinking the eyes is, of course, also a reflex.

Eyebrows

In humans, the eyebrows redirect flowing substances (usually rainwater) away from the eye. Water in the eye can alter the refractive properties of the eye and blur vision. It can also wash away the tear fluid, and its beneficial effects, and can damage the cornea, due to osmotic differences between tear fluid and freshwater.

Eyelids

In many animals, including humans, eyelids wipe the eye and prevent the eyes from dehydration. They spread tear fluid on the eyes, which contains substances which help fight bacterial infection as part of the immune system. Some aquatic animals have a second eyelid in each eye which refracts the light and helps them see clearly both above water and below it.

Eyelashes

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.

Eye movement

This sections deals with the movement of projection eyes within their sockets, and with accommodation. Animals with compound eyes have a wide field of vision, allowing them to look in many directions. To see more, they have to move their entire head or even body. Compound eyes can't accommodate either.

Rapid eye movement is a term usually used to refer to the stage during sleep 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 rapid refocussing actions of the eyes. Many animals are able to quickly look at a point in space (prompted by memory, peripheral vision or an audio cue) without actively looking at anything in between. The eyes simply jerk into a new position. Saccades move the eye at up to 900?/s in adult humans.

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

Many animals can look at something while turning their heads. The eyes are automatically rotated to remain fixed on the object, directed by input from the organs of balance near the ears.

Smooth pursuit movement

The eyes can also follow a moving object around. This is less accurate than the vestibulo occular 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.

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. To look at an object closer by, the eyes rotate 'towards each other' (convergence), while for an object farther away they rotate 'away from eachother' (divergence). Exaggerated convergence is called cross eyed viewing (focussing 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.

Accommodation reflex

To see clearly, the lens will be pulled flatter or allowed to regain its thicker form.

Main article: accommodation reflex

History of ophthalmology

Eye related problems

Achromotropsia aka Maskun — a low cone count or lack of function in cone cells
Age-related macular degeneration — the photosensitive cells in the macula malfunction and over time cease to work
Aniridia — a rare congenital eye condition leading to underdevelopment or even absence of the iris of the eye
Amblyopia aka lazy eye — poor or blurry vision due to either no transmission or poor transmission of the visual image to the brain
Anisometropia — the lenses of the two eyes have different focal lengths
Arc eye aka snow blindness — a painful condition caused by exposure of unprotected eyes to bright light
Astigmatism — the cornea or the lens of the eye is not perfectly spherical, resulting in different focal points in different planes.
Blindness — the brain does not receive optical information, through various causes
Cataracts — the lens becomes opaque
Color blindness — the inability to perceive differences between some or all colors that other people can distinguish
Conjunctivitis — inflammation of the conjunctiva (the outermost layer of the eye and the inner surface of the eyelids)
Esotropia — the tendency for eyes to become cross-eyed
Exotropia — the tendency for eyes to look outward
Farsightedness aka Hyperopia —
Floaters — shadow-like shapes which appear singly or together with several others in the field of vision
Glaucoma — increased intraocular pressure causes damage to the optic nerve
Hyperopia aka Farsightedness — the inability to focus on near objects, often caused by the eyeball being too short
Keratoconus — the cornea thins and changes shape to be more like a cone than a parabola
Maskun aka Achromotropsia — a low cone count or lack of function in cone cells
Myopia — also known as nearsightedness. It is a refractive error in which the image focuses in front of the retina, leading to far away objects to become blurred
Night blindness aka Nyctalopia
Nyctalopia aka Nightblindness — a condition making it difficult or impossible to see in the dark
Opthalmoplegia — the partial or total paralysis of the eye muscles
Presbyopia — a condition that occurs with growing age and results in the inability of the human eye to focus on objects up close
Retinal detachment Maskun aka the retina detaches from the choroid, leading to blurred and distorted vision
Retinopathy — general term referring to non-inflammatory damage to the retina of the eye
Scotoma aka Blind spot
Snow blindness aka Arc eye — a painful condition caused by exposure of unprotected eyes to bright light
Strabismus aka Crossed eye or Wandering eye (or, for one outward-turned eye, walleye) — a disorder of the eyes involving a lack of coordination between the muscles of the eyes
Uveitis — inflammatory process involving the interior of the eye.

Anatomy Clipart and Pictures

Clip Art (https://classroomclipart.com/image/category/clipart.htm)
Anatomy Clip Art (https://classroomclipart.com/image/category/anatomy-clipart.htm)
Anatomy Illustrations (https://classroomclipart.com/image/category/anatomy-illustrations.htm)

Sensory system - Visual system

Eye - Optic nerve - Optic chiasm - Optic tract - Lateral geniculate nucleus - Optic radiations - Visual cortex

Sensory system - Visual system - Eye	Edit (https://academickids.com/encyclopedia/index.php?title=Template:eye&action=edit)
Optic disc - Retina - Cornea - Iris - Pupil - Lens - Macula - Sclera - Optic fovea - Blind spot - Vitreous humour - Aqueous humour - Choroid - Ciliary body - Conjunctiva - Angle structure - Tapetum lucidum

Nervous system - Sensory system

Visual system - Auditory system - Olfactory system - Gustatory system - Somatosensory system

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Eye