An area of the retina in the eye that has the highest concentration of light-sensitive cells. It is responsible for detailed vision. (See also colour vision.)
The outer coat consists of the sclera and the cornea; their junction is called the limbus. SCLERA This is white, opaque, and constitutes the posterior ?ve-sixths of the outer coat. It is made of dense ?brous tissue. The sclera is visible anteriorly, between the eyelids, as the ‘white of the eye’. Posteriorly and anteriorly it is covered by Tenons capsule, which in turn is covered by transparent conjunctiva. There is a hole in the sclera through which nerve ?bres from the retina leave the eye in the optic nerve. Other smaller nerve ?bres and blood vessels also pass through the sclera at di?erent points. CORNEA This constitutes the transparent, colourless anterior one-sixth of the eye. It is transparent in order to allow light into the eye and is more steeply curved than the sclera. Viewed from in front, the cornea is roughly circular. Most of the focusing power of the eye is provided by the cornea (the lens acts as the ‘?ne adjustment’). It has an outer epithelium, a central stroma and an inner endothelium. The cornea is supplied with very ?ne nerve ?bres which make it exquisitely sensitive to pain. The central cornea has no blood supply – it relies mainly on aqueous humour for nutrition. Blood vessels and large nerve ?bres in the cornea would prevent light from entering the eye. LIMBUS is the junction between cornea and sclera. It contains the trabecular meshwork, a sieve-like structure through which aqueous humour leaves the eye.
The middle coat (uveal tract) consists of the choroid, ciliary body and iris. CHOROID A highly vascular sheet of tissue lining the posterior two-thirds of the sclera. The network of vessels provides the blood supply for the outer half of the retina. The blood supply of the choroid is derived from numerous ciliary vessels which pierce the sclera in front and behind. CILIARY BODY A ring of tissue extending 6 mm back from the anterior limitation of the sclera. The various muscles of the ciliary body by their contractions and relaxations are responsible for changing the shape of the lens during ACCOMMODATION. The ciliary body is lined by cells that secrete aqueous humour. Posteriorly, the ciliary body is continuous with the choroid; anteriorly it is continuous with the iris. IRIS A ?attened muscular diaphragm that is attached at its periphery to the ciliary body, and has a round central opening – the pupil. By contraction and relaxation of the muscles of the iris, the pupil can be dilated or constricted (dilated in the dark or when aroused; constricted in bright light and for close work). The iris forms a partial division between the anterior chamber and the posterior chamber of the eye. It lies in front of the lens and forms the back wall of the anterior chamber. The iris is visible from in front, through the transparent cornea, as the ‘coloured part of the eye’. The amount and distribution of iris pigment determine the colour of the iris. The pupil is merely a hole in the centre of the iris and appears black.
The inner layer The retina is a multilayered tissue (ten layers in all) which extends from the edges of the optic nerve to line the inner surface of the choroid up to the junction of ciliary body and choroid. Here the true retina ends at the ora serrata. The retina contains light-sensitive cells of two types: (i) cones – cells that operate at high and medium levels of illumination; they subserve ?ne discrimination of vision and colour vision; (ii) rods – cells that function best at low light intensity and subserve black-and-white vision.
The retina contains about 6 million cones and about 100 million rods. Information from them is conveyed by the nerve ?bres which are in the inner part of the retina, and leave the eye in the optic nerve. There are no photoreceptors at the optic disc (the point where the optic nerve leaves the eye) and therefore there is no light perception from this small area. The optic disc thus produces a physiological blind spot in the visual ?eld.
The retina can be subdivided into several areas: PERIPHERAL RETINA contains mainly rods and a few scattered cones. Visual acuity from this area is fairly coarse. MACULA LUTEA So-called because histologically it looks like a yellow spot. It occupies an area 4·5 mm in diameter lateral to the optic disc. This area of specialised retina can produce a high level of visual acuity. Cones are abundant here but there are few rods. FOVEA CENTRALIS A small central depression at the centre of the macula. Here the cones are tightly packed; rods are absent. It is responsible for the highest levels of visual acuity.
The chambers of the eye There are three: the anterior and posterior chambers, and the vitreous cavity. ANTERIOR CHAMBER Limited in front by the inner surface of the cornea, behind by the iris and pupil. It contains a transparent clear watery ?uid, the aqueous humour. This is constantly being produced by cells of the ciliary body and constantly drained away through the trabecular meshwork. The trabecular meshwork lies in the angle between the iris and inner surface of the cornea. POSTERIOR CHAMBER A narrow space between the iris and pupil in front and the lens behind. It too contains aqueous humour in transit from the ciliary epithelium to the anterior chamber, via the pupil. VITREOUS CAVITY The largest cavity of the eye. In front it is bounded by the lens and behind by the retina. It contains vitreous humour.
Lens Transparent, elastic and biconvex in cross-section, it lies behind the iris and in front of the vitreous cavity. Viewed from the front it is roughly circular and about 10 mm in diameter. The diameter and thickness of the lens vary with its accommodative state. The lens consists of: CAPSULE A thin transparent membrane surrounding the cortex and nucleus. CORTEX This comprises newly made lens ?bres that are relatively soft. It separates the capsule on the outside from the nucleus at the centre of the lens. NUCLEUS The dense central area of old lens ?bres that have become compacted by new lens ?bres laid down over them. ZONULE Numerous radially arranged ?bres attached between the ciliary body and the lens around its circumference. Tension in these zonular ?bres can be adjusted by the muscles of the ciliary body, thus changing the shape of the lens and altering its power of accommodation. VITREOUS HUMOUR A transparent jelly-like structure made up of a network of collagen ?bres suspended in a viscid ?uid. Its shape conforms to that of the vitreous cavity within which it is contained: that is, it is spherical except for a shallow concave depression on its anterior surface. The lens lies in this depression.
Eyelids These are multilayered curtains of tissue whose functions include spreading of the tear ?lm over the front of the eye to prevent desiccation; protection from injury or external irritation; and to some extent the control of light entering the eye. Each eye has an upper and lower lid which form an elliptical opening (the palpebral ?ssure) when the eyes are open. The lids meet at the medial canthus and lateral canthus respectively. The inner medial canthus is ?xed; the lateral canthus more mobile. An epicanthus is a fold of skin which covers the medial canthus in oriental races.
Each lid consists of several layers. From front to back they are: very thin skin; a sheet of muscle (orbicularis oculi, whose ?bres are concentric around the palpebral ?ssure and which produce closure of the eyelids); the orbital septum (modi?ed near the lid margin to form the tarsal plates); and ?nally, lining the back surface of the lid, the conjunctiva (known here as tarsal conjunctiva). At the free margin of each lid are the eyelashes, the openings of tear glands which lie within the lid, and the lacrimal punctum. Toward the medial edge of each lid is an elevation known as the papilla: the lacrimal punctum opens into this papilla. The punctum forms the open end of the cannaliculus, part of the tear-drainage mechanism.
Orbit The bony cavity within which the eye is held. The orbits lie one on either side of the nose, on the front of the skull. They a?ord considerable protection for the eye. Each is roughly pyramidal in shape, with the apex pointing backwards and the base forming the open anterior part of the orbit. The bone of the anterior orbital margin is thickened to protect the eye from injury. There are various openings into the posterior part of the orbit – namely the optic canal, which allows the optic nerve to leave the orbit en route for the brain, and the superior orbital and inferior orbital ?ssures, which allow passage of nerves and blood vessels to and from the orbit. The most important structures holding the eye within the orbit are the extra-ocular muscles, a suspensory ligament of connective tissue that forms a hammock on which the eye rests and which is slung between the medial and lateral walls of the orbit. Finally, the orbital septum, a sheet of connective tissue extending from the anterior margin of the orbit into the lids, helps keep the eye in place. A pad of fat ?lls in the orbit behind the eye and acts as a cushion for the eye.
Conjunctiva A transparent mucous membrane that extends from the limbus over the anterior sclera or ‘white of the eye’. This is the bulbar conjunctiva. The conjunctiva does not cover the cornea. Conjunctiva passes from the eye on to the inner surface of the eyelid at the fornices and is continuous with the tarsal conjunctiva. The semilunar fold is the vertical crescent of conjunctiva at the medial aspect of the palpebral ?ssure. The caruncle is a piece of modi?ed skin just within the inner canthus.
Eye muscles The extra-ocular muscles. There are six in all, the four rectus muscles (superior, inferior, medial and lateral rectus muscles) and two oblique muscles (superior and inferior oblique muscles). The muscles are attached at various points between the bony orbit and the eyeball. By their combined action they move the eye in horizontal and vertical gaze. They also produce torsional movement of the eye (i.e. clockwise or anticlockwise movements when viewed from the front).
Lacrimal apparatus There are two components: a tear-production system, namely the lacrimal gland and accessory lacrimal glands; and a drainage system.
Tears keep the front of the eye moist; they also contain nutrients and various components to protect the eye from infection. Crying results from excess tear production. The drainage system cannot cope with the excess and therefore tears over?ow on to the face. Newborn babies do not produce tears for the ?rst three months of life. LACRIMAL GLAND Located below a small depression in the bony roof of the orbit. Numerous tear ducts open from it into predominantly the upper lid. Accessory lacrimal glands are found in the conjunctiva and within the eyelids: the former open directly on to the surface of the conjunctiva; the latter on to the eyelid margin. LACRIMAL DRAINAGE SYSTEM This consists of: PUNCTUM An elevated opening toward the medial aspect of each lid. Each punctum opens into a canaliculus. CANALICULUS A ?ne tube-like structure run-ning within the lid, parallel to the lid margin. The canaliculi from upper and lower lid join to form a common canaliculus which opens into the lacrimal sac. LACRIMAL SAC A small sac on the side of the nose which opens into the nasolacrimal duct. During blinking, the sac sucks tears into itself from the canaliculus. Tears then drain by gravity down the nasolacrimal duct. NASOLACRIMAL DUCT A tubular structure which runs down through the wall of the nose and opens into the nasal cavity.
Visual pathway Light stimulates the rods and cones of the retina. Electrochemical messages are then passed to nerve ?bres in the retina and then via the optic nerve to the optic chiasm. Here information from the temporal (outer) half of each retina continues to the same side of the brain. Information from the nasal (inner) half of each retina crosses to the other side within the optic chiasm. The rearranged nerve ?bres then pass through the optic tract to the lateral geniculate body, then the optic radiation to reach the visual cortex in the occipital lobe of the brain.... eye
Astigmatism (See ASTIGMATISM.)
Blepharitis A chronic in?ammation of the lid margins. SEBORRHOEA and staphylococcal infection are likely contributors. The eyes are typically intermittently red, sore and gritty over months or years. Treatment is di?cult and may fail. Measures to reduce debris on the lid margins, intermittent courses of topical antibiotics, steroids or systemic antibiotics may help the sufferer.
Blepharospasm Involuntary closure of the eye. This may accompany irritation but may also occur without an apparent cause. It may be severe enough to interfere with vision. Treatment involves removing the source of irritation, if present. Severe and persistent cases may respond to injection of Botulinum toxin into the orbicularis muscle.
Cataract A term used to describe any opacity in the lens of the eye, from the smallest spot to total opaqueness. The prevalence of cataracts is age-related: 65 per cent of individuals in their sixth decade have some degree of lens opacity, while all those over 80 are affected. Cataracts are the most important cause of blindness worldwide. Symptoms will depend on whether one or both eyes are affected, as well as the position and density of the cataract(s). If only one eye is developing a cataract, it may be some time before the person notices it, though reading may be affected. Some people with cataracts become shortsighted, which in older people may paradoxically ‘improve’ their ability to read. Bright light may worsen vision in those with cataracts.
The extent of visual impairment depends on the nature of the cataracts, and the ?rst symptoms noticed by patients include di?culty in recognising faces and in reading, while problems watching television or driving, especially at night, are pointers to the condition. Cataracts are common but are not the only cause of deteriorating vision. Patients with cataracts should be able to point to the position of a light and their pupillary reactions should be normal. If a bright light is shone on the eye, the lens may appear brown or, in advanced cataracts, white (see diagram).
While increasing age is the commonest cause of cataract in the UK, patients with DIABETES MELLITUS, UVEITIS and a history of injury to the eye can also develop the disorder. Prolonged STEROID treatment can result in cataracts. Children may develop cataracts, and in them the condition is much more serious as vision may be irreversibly impaired because development of the brain’s ability to interpret visual signals is hindered. This may happen even if the cataracts are removed, so early referral for treatment is essential. One of the physical signs which doctors look for when they suspect cataract in adults as well as in children is the ‘red re?ex’. This is observable when an ophthalmoscopic examination of the eye is made (see OPHTHALMOSCOPE). Identi?cation of this red re?ex (a re?ection of light from the red surface of the retina –see EYE) is a key diagnostic sign in children, especially young ones.
There is no e?ective medical treatment for established cataracts. Surgery is necessary and the decision when to operate depends mainly on how the cataract(s) affect(s) the patient’s vision. Nowadays, surgery can be done at any time with limited risk. Most patients with a vision of 6/18 – 6/10 is the minimum standard for driving – or worse in both eyes should
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bene?t from surgery, though elderly people may tolerate visual acuity of 6/18 or worse, so surgery must be tailored to the individual’s needs. Younger people with a cataract will have more demanding visual requirements and so may opt for an ‘earlier’ operation. Most cataract surgery in Britain is now done under local anaesthetic and uses the ‘phaco-emulsi?cation’ method. A small hole is made in the anterior capsule of the lens after which the hard lens nucleus is liqui?ed ultrasonically. A replacement lens is inserted into the empty lens bag (see diagram). Patients usually return to their normal activities within a few days of the operation. A recent development under test in the USA for children requiring cataract operations is an intra-ocular ?exible implant whose magnifying power can be altered as a child develops, thus precluding the need for a series of corrective operations as happens now.
Chalazion A ?rm lump in the eyelid relating to a blocked meibomian gland, felt deep within the lid. Treatment is not always necessary; a proportion spontaneously resolve. There can be associated infection when the lid becomes red and painful requiring antibiotic treatment. If troublesome, the chalazion can be incised under local anaesthetic.
Conjunctivitis In?ammation of the conjunctiva (see EYE) which may affect one or both eyes. Typically the eye is red, itchy, sticky and gritty but is not usually painful. Redness is not always present. Conjunctivitis can occasionally be painful, particularly if there is an associated keratitis (see below) – for example, adenovirus infection, herpetic infection.
The cause can be infective (bacteria, viruses or CHLAMYDIA), chemical (e.g. acids, alkalis) or allergic (e.g. in hay fever). Conjunctivitis may also be caused by contact lenses, and preservatives or even the drugs in eye drops may cause conjunctival in?ammation. Conjunctivitis may addtionally occur in association with other illnesses – for example, upper-respiratory-tract infection, Stevens-Johnson syndrome (see ERYTHEMA – erythema multiforme) or REITER’S SYNDROME. The treatment depends on the cause. In many patients acute conjunctivitis is self-limiting.
Dacryocystitis In?ammation of the lacrimal sac. This may present acutely as a red, painful swelling between the nose and the lower lid. An abscess may form which points through the skin and which may need to be drained by incision. Systemic antibiotics may be necessary. Chronic dacryocystitis may occur with recurrent discharge from the openings of the tear ducts and recurrent swelling of the lacrimal sac. Obstruction of the tear duct is accompanied by watering of the eye. If the symptoms are troublesome, the patient’s tear passageways need to be surgically reconstructed.
Ectropion The lid margin is everted – usually the lower lid. Ectropion is most commonly associated with ageing, when the tissues of the lid become lax. It can also be caused by shortening of the skin of the lids such as happens with scarring or mechanical factors – for example, a tumour pulling the skin of the lower lid downwards. Ectropion tends to cause watering and an unsightly appearance. The treatment is surgical.
Entropion The lid margin is inverted – usually the lower lid. Entropion is most commonly associated with ageing, when the tissues of the lid become lax. It can also be caused by shortening of the inner surfaces of the lids due to scarring – for example, TRACHOMA or chemical burns. The inwardly directed lashes cause irritation and can abrade the cornea. The treatment is surgical.
Episcleritis In?ammation of the EPISCLERA. There is usually no apparent cause. The in?ammation may be di?use or localised and may affect one or both eyes. It sometimes recurs. The affected area is usually red and moderately painful. Episcleritis is generally not thought to be as painful as scleritis and does not lead to the same complications. Treatment is generally directed at improving the patient’s symptoms. The in?ammation may respond to NON-STEROIDAL ANTI-INFLAMMATORY DRUGS (NSAIDS) or topical CORTICOSTEROIDS.
Errors of refraction (Ametropia.) These will occur when the focusing power of the lens and cornea does not match the length of the eye, so that rays of light parallel to the visual axis are not focused at the fovea centralis (see EYE). There are three types of refractive error: HYPERMETROPIA or long-sightedness. The refractive power of the eye is too weak, or the eye is too short so that rays of light are brought to a focus at a point behind the retina. Longsighted people can see well in the distance but generally require glasses with convex lenses for reading. Uncorrected long sight can lead to headaches and intermittent blurring of vision following prolonged close work (i.e. eye strain). As a result of ageing, the eye becomes gradually long-sighted, resulting in many people needing reading glasses in later life: this normal process is known as presbyopia. A particular form of long-sightedness occurs after cataract extraction (see above). MYOPIA(Short sight or near sight.) Rays of light are brought to a focus in front of the retina because the refractive power of the eye is too great or the eye is too short. Short-sighted people can see close to but need spectacles with concave lenses in order to see in the distance. ASTIGMATISMThe refractive power of the eye is not the same in each meridian. Some rays of light may be focused in front of the retina while others are focused on or behind the retina. Astigmatism can accompany hypermetropia or myopia. It may be corrected by cylindrical lenses: these consist of a slice from the side of a cylinder (i.e. curved in one meridian and ?at in the meridian at right-angles to it).
Keratitis In?ammation of the cornea in response to a variety of insults – viral, bacterial, chemical, radiation, or mechanical trauma. Keratitis may be super?cial or involve the deeper layers, the latter being generally more serious. The eye is usually red, painful and photophobic. Treatment is directed at the cause.
Nystagmus Involuntary rhythmic oscillation of one or both eyes. There are several causes including nervous disorders, vestibular disorders, eye disorders and certain drugs including alcohol.
Ophthalmia In?ammation of the eye, especially the conjunctiva (see conjunctivitis, above). Ophthalmia neonatorum is a type of conjunctivitis that occurs in newborn babies. They catch the disease when passing through an infected birth canal during their mother’s labour (see PREGNANCY AND LABOUR). CHLAMYDIA and GONORRHOEA are the two most common infections. Treatment is e?ective with antibiotics: untreated, the infection may cause permanent eye damage.
Pinguecula A benign degenerative change in the connective tissue at the nasal or temporal limbus (see EYE). This is visible as a small, ?attened, yellow-white lump adjacent to the cornea.
Pterygium Overgrowth of the conjunctival tissues at the limbus on to the cornea (see EYE). This usually occurs on the nasal side and is associated with exposure to sunlight. The pterygium is surgically removed for cosmetic reasons or if it is thought to be advancing towards the visual axis.
Ptosis Drooping of the upper lid. May occur because of a defect in the muscles which raise the lid (levator complex), sometimes the result of ageing or trauma. Other causes include HORNER’S SYNDROME, third cranial nerve PALSY, MYASTHENIA GRAVIS, and DYSTROPHIA MYOTONICA. The cause needs to be determined and treated if possible. The treatment for a severely drooping lid is surgical, but other measures can be used to prop up the lid with varying success.
Retina, disorders of The retina can be damaged by disease that affects the retina alone, or by diseases affecting the whole body.
Retinopathy is a term used to denote an abnormality of the retina without specifying a cause. Some retinal disorders are discussed below. DIABETIC RETINOPATHY Retinal disease occurring in patients with DIABETES MELLITUS. It is the commonest cause of blind registration in Great Britain of people between the ages of 20 and 65. Diabetic retinopathy can be divided into several types. The two main causes of blindness are those that follow: ?rst, development of new blood vessels from the retina, with resultant complications and, second, those following ‘water logging’ (oedema) of the macula. Treatment is by maintaining rigid control of blood-sugar levels combined with laser treatment for certain forms of the disease – in particular to get rid of new blood vessels. HYPERTENSIVE RETINOPATHY Retinal disease secondary to the development of high blood pressure. Treatment involves control of the blood pressure (see HYPERTENSION). SICKLE CELL RETINOPATHY People with sickle cell disease (see under ANAEYIA) can develop a number of retinal problems including new blood vessels from the retina. RETINOPATHY OF PREMATURITY (ROP) Previously called retrolental ?broplasia (RLF), this is a disorder affecting low-birth-weight premature babies exposed to oxygen. Essentially, new blood vessels develop which cause extensive traction on the retina with resultant retinal detachment and poor vision. RETINAL ARTERY OCCLUSION; RETINAL VEIN OCCLUSION These result in damage to those areas of retina supplied by the affected blood vessel: the blood vessels become blocked. If the peripheral retina is damaged the patient may be completely symptom-free, although areas of blindness may be detected on examination of ?eld of vision. If the macula is involved, visual loss may be sudden, profound and permanent. There is no e?ective treatment once visual loss has occurred. SENILE MACULAR DEGENERATION (‘Senile’ indicates age of onset and has no bearing on mental state.) This is the leading cause of blindness in the elderly in the western world. The average age of onset is 65 years. Patients initially notice a disturbance of their vision which gradually progresses over months or years. They lose the ability to recognise ?ne detail; for example, they cannot read ?ne print, sew, or recognise people’s faces. They always retain the ability to recognise large objects such as doors and chairs, and are therefore able to get around and about reasonably well. There is no e?ective treatment in the majority of cases. RETINITIS PIGMENTOSAA group of rare, inherited diseases characterised by the development of night blindness and tunnel vision. Symptoms start in childhood and are progressive. Many patients retain good visual acuity, although their peripheral vision is limited. One of the characteristic ?ndings on examination is collections of pigment in the retina which have a characteristic shape and are therefore known as ‘bone spicules’. There is no e?ective treatment. RETINAL DETACHMENTusually occurs due to the development of a hole in the retina. Holes can occur as a result of degeneration of the retina, traction on the retina by the vitreous, or injury. Fluid from the vitreous passes through the hole causing a split within the retina; the inner part of the retina becomes detached from the outer part, the latter remaining in contact with the choroid. Detached retina loses its ability to detect light, with consequent impairment of vision. Retinal detachments are more common in the short-sighted, in the elderly or following cataract extraction. Symptoms include spots before the eyes (?oaters), ?ashing lights and a shadow over the eye with progressive loss of vision. Treatment by laser is very e?ective if caught early, at the stage when a hole has developed in the retina but before the retina has become detached. The edges of the hole can be ‘spot welded’ to the underlying choroid. Once a detachment has occurred, laser therapy cannot be used; the retina has to be repositioned. This is usually done by indenting the wall of the eye from the outside to meet the retina, then making the retina stick to the wall of the eye by inducing in?ammation in the wall (by freezing it). The outcome of surgery depends largely on the extent of the detachment and its duration. Complicated forms of detachment can occur due to diabetic eye disease, injury or tumour. Each requires a specialised form of treatment.
Scleritis In?ammation of the sclera (see EYE). This can be localised or di?use, can affect the anterior or the posterior sclera, and can affect one or both eyes. The affected eye is usually red and painful. Scleritis can lead to thinning and even perforation of the sclera, sometimes with little sign of in?ammation. Posterior scleritis in particular may cause impaired vision and require emergency treatment. There is often no apparent cause, but there are some associated conditions – for example, RHEUMATOID ARTHRITIS, GOUT, and an autoimmune disease affecting the nasal passages and lungs called Wegener’s granulomatosis. Treatment depends on severity but may involve NON-STEROIDAL ANTI-INFLAMMATORY DRUGS (NSAIDS), topical CORTICOSTEROIDS or systemic immunosuppressive drugs.
Stye Infection of a lash follicle. This presents as a painful small red lump at the lid margin. It often resolves spontaneously but may require antibiotic treatment if it persists or recurs.
Sub-conjunctival haemorrhage Haemorrhage between the conjunctiva and the underlying episclera. It is painless. There is usually no apparent cause and it resolves spontaneously.
Trichiasis Inward misdirection of the lashes. Trichiasis occurs due to in?ammation of or trauma to the lid margin. Treatment involves removal of the patient’s lashes. Regrowth may be prevented by electrolysis, by CRYOTHERAPY to the lid margin, or by surgery.
For the subject of arti?cial eyes, see under PROSTHESIS; also GLAUCOMA, SQUINT and UVEITIS.... eye, disorders of
between about 400 and 700 nanometres. Different wavelengths produce sensations of violet, indigo, blue, green, yellow, orange, and red when they fall on the retina and stimulate nerve signals, which are processed in the brain.
As light falls on the retina, it strikes light-sensitive cells called rods and cones. The rods can detect all visible light, but only the cones can distinguish colour. There are 3 types of cones: red-sensitive, blue-sensitive, and green-sensitive. Each responds more strongly to a particular part of the light spectrum. Because the cones are most concentrated in a central area of the retina called the fovea, colour vision is most accurate for objects viewed directly and is poor at the edges of vision. When light hits a cone, it causes the cone to emit an electrical signal, which passes to the brain via the optic nerve. Colour perception requires a minimum level of light, below which everything is seen as shades of grey. (See also colour vision deficiency; eye; perception; vision.)... colour vision
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