Eyeball Health Dictionary

Exophthalmos, or PROPTOSIS, refers to forward displacement of the eyeball and must be distinguished from retraction of the eyelids, which causes an illusion of exophthalmos. Lid retraction usually results from activation of the autonomic nervous sytem. Exophthalmos is a more serious disorder caused by in?ammatory and in?ltrative changes in the retro-orbital tissues and is essentially a feature of Graves’ disease, though it has been described in chronic thyroiditis (see THYROID GLAND, DISEASES OF). Exophthalmos commonly starts shortly after the development of thyrotoxicosis but may occur months or even years after hyperthyroidism has been successfully treated. The degree of exophthalmos is not correlated with the severity of hyperthyroidism even when their onset is simultaneous. Some of the worst examples of endocrine exophthalmos occur in the euthyroid state and may appear in patients who have never had thyrotoxicosis; this disorder is named ophthalmic Graves’ disease. The exophthalmos of Graves’ disease is due to autoimmunity (see IMMUNITY). Antibodies to surface antigens on the eye muscles are produced and this causes an in?ammatory reaction in the muscle and retroorbital tissues.

Exophthalmos may also occur as a result of OEDEMA, injury, cavernous venous THROMBOSIS or a tumour at the back of the eye, pushing the eyeball forwards. In this situation it is always unilateral.... exophthalmos

Otherwise known as the blind spot of the EYE, the disc is the beginning of the optic nerve – the point where nerve ?bres from the retina’s rods and cones (the light- and colour-sensitive cells) leave the eyeball.... optic disc

The mucus membrane which covers the underside of the eyelids and the front surfaces of the eyeball.... conjunctiva

Double vision. It is due to some irregularity in action of the muscles which move the eyeballs, in consequence of which the eyes are placed so that rays of light from one object do not fall upon corresponding parts of the two retinae, and two images are produced. It is a symptom of several nervous diseases, and often a temporary attack follows an injury to the eye, intoxication, or some febrile disease like DIPHTHERIA.... diplopia

Extrusion of the abdominal VISCERA or internal organs, usually as the result of serious injury. (Usually described as disembowelment when deliberately carried out by one person on another.) In surgery the term refers to part-removal of the viscera, and in OPHTHALMOLOGY it is an operation to remove the contents of the eyeball (see also EYE).... evisceration

The eye is the sensory organ of sight. It is an elaborate photoreceptor detecting information, in the form of light, from the environment and transmitting this information by a series of electrochemical changes to the BRAIN. The visual cortex is the part of the brain that processes this information (i.e. the visual cortex is what ‘sees’ the environment). There are two eyes, each a roughly spherical hollow organ held within a bony cavity (the orbit). Each orbit is situated on the front of the skull, one on each side of the nose. The eye consists of an outer wall of three main layers and a central cavity divided into three.

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

A large, prominent eyeball in an infant as a result of increased

pressure inside the eyeball due to congenital glaucoma.

Treatment of the condition usually involves surgery to reduce the pressure, otherwise the child’s sight is progressively damaged.... buphthalmos

The 3rd cranial nerve, controlling most of the muscles that move the eye. The oculomotor nerve also supplies the muscle that constricts the pupil, that which raises the upper eyelid, and the ciliary muscle, which focuses the eye. The nerve may be damaged due to a fracture to the base of the skull or a tumour. Symptoms include ptosis, squint, dilation of the pupil, inability to focus the eye, double vision, and slight protrusion of the eyeball. (See also trochlear nerve; abducent nerve.)... oculomotor nerve

The transparent thin-walled dome that forms the front of the eyeball. The cornea is joined at its circumference to the sclera (white of the eye); the black pupil and the coloured iris are visible beneath it. The main functions of the cornea are to help focus light-rays on to the retina at the back of the eye and to protect the front of the eye. It is kept moist by tears produced by the lacrimal gland and the mucus- and fluid-secreting cells in the eyelids and conjunctiva. cornea, disorders of Injuries or diseases affecting the cornea, the outer shell of the eyeball. Injuries include corneal abrasions, which sometimes become infected and progress to a corneal ulcer. Penetrating corneal injuries can cause scarring, which may lead to impairment of vision. Chemical injuries can result from contact with a corrosive substance and require immediate flushing of the eye with water.

In actinic keratopathy, the outer layer of the cornea is damaged by ultraviolet light. In exposure keratopathy, damage is due to reduced protection by the tear film and blink reflex. The cornea can also be infected by viruses, bacteria, and fungi, the herpes simplex virus being especially dangerous. True inflammation of the cornea (called keratitis) is uncommon as the cornea contains no blood vessels.

Other disorders include: keratomalacia as a result of vitamin A deficiency; keratoconjunctivitis sicca (dry eye); corneal dystrophies such as keratoconus; and oedema, in which fluid builds up in the cornea and impairs vision.

Rare congenital defects include microcornea (smaller cornea than normal) or megalocornea (bigger than normal) and buphthalmos, or “ox-eye’’, in which the entire eyeball is distended as a result of glaucoma.

Degenerative conditions of the cornea such as calcium deposition, thinning, and spontaneous ulceration occur mainly in the elderly, and are more common in previously damaged eyes.... cornea

An alteration in the direction of eyelash growth, in which the lashes grow inwards towards the eyeball.

They can rub against the eye, causing severe discomfort and sometimes damage to the cornea.

Trachoma is a cause.... trichiasis

Also called beta blockers, these drugs interrupt the transmission of neuronal messages via the body’s adrenergic receptor sites. In the HEART these are called beta1 (cardioselective) receptors. Another type – beta2 (non-cardioselective) receptors – is sited in the airways, blood vessels, and organs such as the eye, liver and pancreas. Cardioselective beta blockers act primarily on beta1 receptors, whereas non-cardioselective drugs act on both varieties, beta1 and beta2. (The neurotransmissions interrupted at the beta-receptor sites through the body by the beta blockers are initiated in the ADRENAL GLANDS: this is why these drugs are sometimes described as beta-adrenergic-blocking agents.)

They work by blocking the stimulation of beta adrenergic receptors by the neurotransmitters adrenaline and noradrenaline, which are produced at the nerve endings of that part of the SYMPATHETIC NERVOUS SYSTEM – the autonomous (involuntary) network

– which facilitates the body’s reaction to anxiety, stress and exercise – the ‘fear and ?ight’ response.

Beta1 blockers reduce the frequency and force of the heartbeat; beta2 blockers prevent vasodilation (increase in the diameter of blood vessels), thus in?uencing the patient’s blood pressure. Beta1 blockers also affect blood pressure, but the mechanism of their action is unclear. They can reduce to normal an abnormally fast heart rate so the power of the heart can be concomitantly controlled: this reduces the oxygen requirements of the heart with an advantageous knock-on e?ect on the respiratory system. These are valuable therapeutic effects in patients with ANGINA or who have had a myocardial infarction (heart attack – see HEART, DISEASES OF), or who suffer from HYPERTENSION. Beta2 blockers reduce tremors in muscles elsewhere in the body which are a feature of anxiety or the result of thyrotoxicosis (an overactive thyroid gland – see under THYROID GLAND, DISEASES OF). Noncardioselective blockers also reduce the abnormal pressure caused by the increase in the ?uid in the eyeball that characterises GLAUCOMA.

Many beta-blocking drugs are now available; minor therapeutic di?erences between them may in?uence the choice of a drug for a particular patient. Among the common drugs are:

Primarily cardioselective	Non-cardioselective
Acebutolol	Labetalol
Atenolol	Nadolol
Betaxolol	Oxprenolol
Celiprolol	Propanolol
Metoprolol	Timolol

These powerful drugs have various side-effects and should be prescribed and monitored with care. In particular, people who suffer from asthma, bronchitis or other respiratory problems may develop breathing diffculties. Long-term treatment with beta blockers should not be suddenly stopped, as this may precipitate a severe recurrence of the patient’s symptoms – including, possibly, a sharp rise in blood pressure. Gradual withdrawal of medication should mitigate untoward effects.... beta-adrenoceptor-blocking drugs

The normal condition of the EYE as regards refraction of light rays. When the muscles in the eyeball are completely relaxed, the focusing power is accurately adjusted for parallel rays, so that vision is perfect for distant objects.... emmetropia

Inadequate drainage of tears in the eyes with the result that they ‘over?ow’ down the cheeks. The condition is caused by an abnormality of the tear ducts which drain away the normal secretions that keep the eyeball moist (see EYE).... epiphora

Also known as a proptometer. An instrument used to measure the extent of protrusion of the eyeball – a development that occurs in certain disorders such as GOITRE, TUMOUR, OEDEMA, injuries, orbital in?ammation or cavernous venous thrombosis (a blood clot in the cavernous sinus in the base of the skull behind each eye). (See EXOPHTHALMOS.)... exophthalmometer

(1) Originating outside the body.

(2) An extrinsic muscle is one whose origin is some way from the part of the body it acts upon

– for example, the muscles controlling the movement of the eyeball which are attached to the bony orbit in which the eye sits.... extrinsic

Particles that appear to be ?oating in a person’s ?eld of vision. They move quickly as the eye moves, but when the eye is still they seem to drift. Vision is not usually affected. Most ?oaters are shadows on the retina from minute particles in the vitreous humour (see EYE) which lies in the main part of the eyeball behind the lens. As a person ages, the jelly-like vitreous humour usually shrinks a little and becomes detached from the retina; this produces ?oaters which vanish over time. If a person notices a sudden cloud of ?oaters, sometimes accompanied by ?ashes of light, it is likely that a tear in or detachment of the retina has occurred. This requires prompt medical attention (see EYE, DISORDERS OF – Retinal detachment).... floaters

An eye disease, usually chronic and slow, with increased pressure of fluid within the eye causing degrees of impairment to the optic nerve, and slowing circulation between the eye chambers sufficient to also contribute to lens deposits and corneal opacities. When under adrenalin stress or under the effect of most stimulants, pupils dilate, the eyeball changes shape, and pressure within the eye increases. This may not itself start glaucoma, but adrenergic stress will surely make it worse.... glaucoma

Abnormal protrusion of the eyeballs. May be a symptom of hyperthryroid states. A rare cause is a tumour at back of the eye. Abnormal exposure of the white of the eye, with double vision. Treatment. The underlying condition should be treated – overactive thyroid. Many cases arise from infection.

“Internal remedies should be carefully selected, because each case is different,” writes Margaret Wilkenloh, MD, Chicago (Ellingwood). “The best remedies to my mind are Echinacea, Pulsatilla, Skullcap and Hawthorn.” These are available as herbs, tablets, powders, liquid extracts or tinctures. Specimen combination: Liquid extracts: Echinacea 2; Pulsatilla half; Skullcap 1; Hawthorn 1. Mix. One to two 5ml teaspoons in water thrice daily. ... exophthalmus

A number of causes including reflex pain from inflammation of the middle ear or decayed teeth. Eyeball tender to touch.

Alternatives. Plantain, Ginkgo. Teas, tablets, etc.

Topical. Cold compress: Witch Hazel.

Supplements. Daily. Vitamins C (500mg); E (400iu). Beta-carotene. Palming. ... eyes – pain

Inflammation of the iris.

Causes: juvenile polyarthritis, ankylosing spondylitis, sexually transmitted diseases, tuberculosis, injury, etc.

Symptoms. Eyeballs stuck down in the mornings from exudate, contraction of the pupil, pain, photophobia, discoloration of the iris. If exudate is with pus: Echinacea, Goldenseal, Poke root. Salmon- coloured zone around the cornea. Pupil fails to respond to light.

There is a type of eye inflammation associated with arthritic change in the body and which should not be mistaken for conjunctivitis but can be damaging to the eyeball. The iritis of early poker-spine is not local but internal and responds only to anti-arthritic and anti-inflammatory agents such as Guaiacum. Alternatives. Dilation of pupil by a mydriatic administered by a medical practitioner. Alternatives (internal):–

Black Cohosh: Dose: Liquid Extract: 5-15 drops. Tincture: 10-30 drops. Every two hours, acute cases, otherwise thrice daily.

Pulsatilla. Dose: Liquid Extract: 5-10 drops. Tincture: 10-20 drops. Acute cases: every two hours, otherwise thrice daily.

Formula. Tinctures, Eyebright and Goldenseal, equal parts. Dose: 10-20 drops in water or cup of German Chamomile tea.

Topical. Elderflower tea or lotion eye douche, morning and evening. Aloe Vera, gel or pulp from fresh plant leaves.

Supplements. Vitamins A, C, D, E. Zinc. ... iritis

The 6th cranial nerve.

It supplies the lateral rectus muscle of each eye, which is responsible for moving the eyeball outwards.

The nerve originates in the pons (part of the brainstem) and passes along the base of the brain, entering the back of the eye socket through a gap between the skull bones.... abducent nerve

The small, oval-shaped area on the retina of the eye where the optic nerve leaves the eyeball.

The area is not sensitive to light because it has no light receptors (nerve endings responsive to light).

The blind spot can also be used to describe the part of the visual field in which objects cannot be detected.... blind spot

Also known as diplopia, the seeing of 2 instead of 1 visual image of a single object. It is usually a symptom of a squint, especially of paralytic squint, in which paralysis of 1 or more of the eye muscles impairs eye movement. Other causes include a tumour in the eyelid or a tumour or blood clot behind the eye. Double vision can also occur in exophthalmos, when the eyeballs protrude because of an underlying hormonal disorder. A child with squint needs treatment to prevent amblyopia (lazy eye). In adults double vision needs immediate investigation.... double vision

Any material on the surface of the eye or under the lid, or an object that penetrates the eyeball.

A foreign body may cause irritation, redness, increased tear production, and blepharospasm. In some cases, a foreign body left in the eye may cause a reaction that results in permanent loss of sight in both eyes.

Foreign bodies on or in the conjunctiva can usually be flushed out with water.

However, medical attention is needed if the object has penetrated the eyeball.

Dropping the dye fluorescein into the eye reveals corneal abrasions or sites of penetration.

Ultrasound scanning or an X-ray of the eye may also be performed.

Local anaesthetic eye-drops may be applied and a spatula used to remove an object from the cornea.

The eye may then be covered with a patch.

Antibiotic drugs may also be prescribed.... eye, foreign body in

The pressure within the eye that helps to maintain the shape of the eyeball, due to the balance between the rate of production and removal of aqueous humour. Aqueous humour is continually produced from the ciliary body and exits from the drainage angle (a network of tissue between the iris and cornea). If drainage is impeded, intraocular pressure builds up (a condition known as glaucoma). If the ciliary body is damaged (as a result of prolonged inflammation), ultrasound scanning may be performed to assess the problem. The underlying cause is treated, if possible. If the baby’s growth is slowing, induction of labour or a caesarean section may be necessary. Most babies whose growth was retarded in the uterus gain weight rapidly after delivery. However, if an intrauterine infection or genetic disorder was the cause, poor growth may continue.

less fluid is produced and the eye becomes soft.... intraocular pressure

n. poor sight, not due to any detectable disease of the eyeball or visual system, known colloquially as lazy eye. In practice this strict definition is not always obeyed. For example, in toxic amblyopia, caused by tobacco, alcohol, certain other drugs, and vitamin deficiency, there is a disorder of the *optic nerve. The commonest type is amblyopia ex anopsia, in which factors such as squint (see strabismus), cataract, and other abnormalities of the optics of the eye (see refraction) impair its normal use in early childhood by preventing the formation of a clear image on the retina. This in turn leads to a cortical visual impairment.... amblyopia

adj. 1. relating to or affecting the medulla oblongata. 2. relating to a bulb. 3. relating to the eyeball.... bulbar

n. the layer of the eyeball between the retina and the sclera (see also Bruch’s membrane). It contains blood vessels and a pigment that absorbs excess light and so prevents blurring of vision. See eye. —choroidal adj.... choroid

the separation of the *choroid from the *sclera of the eye as a result of leakage of fluid from the vessels of the choroid. It occurs when pressure inside the eyeball is very low, usually after trauma or intraocular surgery.... choroidal detachment

n. thinning of the *stroma of the cornea to such an extent that *Descemet’s membrane is all that is maintaining the integrity of the eyeball. It occurs in severe ulceration of the cornea.... descemetocele

n. 1. turning out of the eyelid, away from the eyeball. The commonest type is senile ectropion, in which the lower eyelid droops because of laxity of the eyelid in old age. If the muscle that closes the eye (orbicularis oculi) is paralysed the lower lid also droops. Ectropion may also occur if scarring causes contraction of the surrounding facial skin. 2. (cervical ectopy) the replacement of the stratified squamous epithelium that normally lines the vaginal part of the cervix (ectocervix) by columnar epithelium, which has migrated from the *endocervix. As a result, the junction of squamous and columnar epithelia does not coincide with the opening of the cervix into the uterus. Cervical ectropion is caused by exposure to higher levels of oestrogen, which occurs normally at certain times (e.g. at puberty or in pregnancy) or with *anovulation. It usually causes no symptoms but in a few cases may be associated with postcoital bleeding and a mucoid vaginal discharge.... ectropion

n. inturning of the eyelid towards the eyeball. The lashes may rub against the eye and cause irritation (see trichiasis). The commonest type is spastic entropion of the lower eyelid, due to spasm of the muscle that closes the eye (orbicularis oculi). Entropion may also be caused by scarring of the lining membrane (conjunctiva) of the lid.... entropion

n. the complete removal of an organ, tumour, or cyst leaving surrounding structures intact. In ophthalmology it is an operation in which the eyeball is removed but the other structures in the socket (e.g. eye muscles) are left in place. Commonly a plastic ball is buried in the socket to give a better cosmetic result when fitting an artificial eye.... enucleation

1. n. live tissue transferred from the body (or any organism) to a suitable artificial medium for culture. The tissue grows in the artificial medium and can be studied for diagnostic or experimental purposes. Tumour growths are sometimes examined in this way. 2. n. silicone rubber material sutured to the outside of the eyeball over a retinal tear or hole (see plombage). The resulting indent allows the retina to reattach. 3. vb. to transfer live tissue for culture outside the body. —explantation n.... explant

a muscle, such as any of those controlling movements of the eyeball, that has its origin some distance from the part it acts on. See also eye.... extrinsic muscle

Medicines are drugs made stable, palatable and acceptable for administration. In Britain, the Medicines Act 1968 controls the making, advertising and selling of substances used for ‘medicinal purposes’, which means diagnosing, preventing or treating disease, or altering a function of the body. Permission to market a medicine has to be obtained from the government through the MEDICINES CONTROL AGENCY, or from the European Commission through the European Medicines Evaluation Agency. It takes the form of a Marketing Authorisation (formerly called a Product Licence), and the uses to which the medicine can be put are laid out in the Summary of Product Characteristics (which used to be called the Product Data Sheet).

There are three main categories of licensed medicinal product. Drugs in small quantities can, if they are perceived to be safe, be licensed for general sale (GSL – general sales list), and may then be sold in any retail shop. P (pharmacy-only) medicines can be sold from a registered pharmacy by or under the supervision of a pharmacist (see PHARMACISTS); no prescription is needed. P and GSL medicines are together known as OTCs – that is, ‘over-thecounter medicines’. POM (prescription-only medicines) can only be obtained from a registered pharmacy on the prescription of a doctor or dentist. As more information is gathered on the safety of drugs, and more emphasis put on individual responsibility for health, there is a trend towards allowing drugs that were once POM to be more widely available as P medicines. Examples include HYDROCORTISONE 1 per cent cream for skin rashes, CIMETIDINE for indigestion, and ACICLOVIR for cold sores. Care is needed to avoid taking a P medicine that might alter the actions of another medicine taken with it, or that might be unsuitable for other reasons. Patients should read the patient-information lea?et, and seek the pharmacist’s advice if they have any doubt about the information. They should tell their pharmacist or doctor if the medicine results in any unexpected effects.

Potentially dangerous drugs are preparations referred to under the Misuse of Drugs Act 1971 and subsequent regulations approved in 1985. Described as CONTROLLED DRUGS, these include such preparations as COCAINE, MORPHINE, DIAMORPHINE, LSD (see LYSERGIC ACID

DIETHYLAMIDE (LSD)), PETHIDINE HYDROCHLORIDE, AMPHETAMINES, BARBITURATES and most BENZODIAZEPINES.

Naming of drugs A European Community Directive (92/27/EEC) requires the use of the Recommended International Non-proprietary Name (rINN) for medicinal substances. For most of these the British Approved Name (BAN) and rINN were identical; where the two were di?erent, the BAN has been modi?ed in line with the rINN. Doctors and other authorised subscribers are advised to write titles of drugs and preparations in full because uno?cial abbreviations may be misinterpreted. Where a drug or preparation has a non-proprietary (generic) title, this should be used in prescribing unless there is a genuine problem over the bioavailability properties of a proprietary drug and its generic equivalent.

Where proprietary – commercially registered

– names exist, they may in general be used only for products supplied by the trademark owners. Countries outside the European Union have their own regulations for the naming of medicines.

Methods of administration The ways in which drugs are given are increasingly ingenious. Most are still given by mouth; some oral preparations (‘slow release’ or ‘controlled release’ preparations) are designed to release their contents slowly into the gut, to maintain the action of the drug.

Buccal preparations are allowed to dissolve in the mouth, and sublingual ones are dissolved under the tongue. The other end of the gastrointestinal tract can also absorb drugs: suppositories inserted in the rectum can be used for their local actions – for example, as laxatives – or to allow absorption when taking the drug by mouth is di?cult or impossible – for example, during a convulsion, or when vomiting.

Small amounts of drug can be absorbed through the intact skin, and for very potent drugs like OESTROGENS (female sex hormones) or the anti-anginal drug GLYCERYL TRINITRATE, a drug-releasing ‘patch’ can be used. Drugs can be inhaled into the lungs as a ?ne powder to treat or prevent ASTHMA attacks. They can also be dispersed (‘nebulised’) as a ?ne mist which can be administered with compressed air or oxygen. Spraying a drug into the nostril, so that it can be absorbed through the lining of the nose into the bloodstream, can avoid destruction of the drug in the stomach. This route is used for a small number of drugs like antidiuretic hormone (see VASOPRESSIN).

Injection remains an important route of administering drugs both locally (for example, into joints or into the eyeball), and into the bloodstream. For this latter purpose, drugs can be given under the skin – that is, subcutaneously (s.c. – also called hypodermic injection); into muscle – intramuscularly (i.m.); or into a vein – intravenously (i.v.). Oily or crystalline preparations of drugs injected subcutaneously form a ‘depot’ from which they are absorbed only slowly into the blood. The action of drugs such as TESTOSTERONE and INSULIN can be prolonged by using such preparations, which also allow contraceptive ‘implants’ that work for some months (see CONTRACEPTION).... medicines

This is formed by a crossing-over of the two optic nerves (see EYE) which run from the back of the eyeballs to meet in the mid line beneath the brain. Nerve ?bres from the nasal part of the retina cross to link up with ?bres from the outer part of the retina of the opposite eye. The linked nerves form two separate optic tracts which travel back to the occipital lobes of the brain.... optic chiasma

A hereditary disease due to an inherited abnormality of COLLAGEN. It is characterised by extreme fragility of the skeleton, resulting in fractures and deformities. It may be accompanied by blue sclera (the outermost, normally white coat of the eyeball), transparent teeth, hypermobility (excessive range of movement) of the joints, deafness, and dwar?sm (shortness of stature). The exact cause is not known, although there is some evidence that it may be associated with collagen formation. Parents of affected children can obtain help and advice from the Brittle Bone Society.... osteogenesis imperfecta

An alkaloid (see ALKALOIDS) derived from the leaves of Pilocarpus microphyllus (jaborandi). It produces the same effects as stimulation of the PARASYMPATHETIC NERVOUS SYSTEM: i.e. it has exactly the opposite e?ect to ATROPINE, but cannot be used in the treatment of atropine poisoning as it does not antagonise the action of poisonous doses of atropine on the brain. Its main use today is in the form of eye drops to decrease the pressure inside the eyeball in GLAUCOMA.... pilocarpine

The largest gland in the body. Situated on the right side under the dome of the diaphragm. At times, it may hold as much as a quarter of the body’s blood supply. Blood from the spleen, stomach and intestines passes to the liver via the portal vein from which it issues much changed. The liver works in close association with the pancreas, bitter remedies being beneficial to both. It detoxicates offending bacteria and drugs which may enter through the intestines.

One vast laboratory, the liver secretes bile, cholesterol and lecithin; breaks down old red cells; and its anti-anaemic factor (Vitamin B12) is necessary by the bone marrow for production of red blood cells for protection against pernicious anaemia. It aids the digestion of fats, and ensures the storage of carbohydrates in the form of glycogen, together with Vitamins D and K.

A faint yellow tinge of the skin and eyeballs may be the first indication of liver disturbance. The liver has great powers of recovery, herbal agents powerfully influencing regeneration of cells.

In all liver disorders the liver is less taxed on a low-fat or fat-free diet. Most effective remedies are Dandelion and Burdock. Treatment will depend upon the particular disturbance. Dandelion relieves portal vein congestion.

Simple test to spot liver disease: check that stools are the right colour and that the urine does not stain. ... liver

A group of drugs, also known as beta-adrenergic blocking agents, prescribed principally to treat heart and circulatory disorders such as angina and hypertension. Beta-blockers block the effects of the sympathetic nervous system, which releases adrenaline (epinephrine) and noradrenaline (norepinephrine) at nerve endings that are known as beta receptors.

There are 2 types of beta receptor: beta 1 and beta 2. Beta 1 receptors are present in the heart and blood vessels, and beta 2 in the lungs. Some betablockers (such as acebutolol, atenolol, and metoprolol) are termed cardioselective and, because they act mostly on beta 1 receptors, are used mainly to treat heart disease such as angina, hypertension, and cardiac arrhythmia. The drugs are sometimes given after a myocardial infarction (heart attack) to reduce the likelihood of further damage to the heart muscle.

Other types of beta-blocker, such as oxprenolol, propranolol, and timolol, may be given to prevent migraine attacks by acting on blood vessels in the head; reduce the physical symptoms of anxiety; or control the symptoms of thyrotoxicosis. Beta-blocker drugs such as timolol are sometimes given in the

form of eye drops to treat glaucoma and work by lowering the fluid pressure in the eyeball.

Beta-blockers may reduce an individual’s capacity for strenuous exercise. The drugs may worsen the symptoms of asthma, bronchitis, or other forms of lung disease. They may also reduce the flow of blood to the limbs, causing cold hands and feet. In addition, sleep disturbance and depression can be side effects of beta-blockers.... beta-blocker drugs

Inability to see. Definitions of blindness and partial sight vary. In the , blindness is defined as a corrected visual acuity of 3/60 or less in the better eye, or a visual field of no more than 20 degrees in the better eye. Blindness may result from injury to, or disease or degeneration of, the eyeball; the optic nerve or nerve pathways connecting the eye to the brain; or the brain itself. Clouding of the cornea may result from Sjögren’s syndrome, vitamin A deficiency, chemical damage, infections, and injury. Corneal ulcers can cause blindness due to scarring of the cornea. Uveitis and cataracts are other common causes of blindness. Diabetes mellitus, hypertension, or injury can all cause bleeding into the cavity of the eyeball and subsequent loss of vision. Bleeding into the fluid in front of the lens (hyphaema) or behind the lens (vitreous haemorrhage) can also result in loss of vision. Other conditions that may cause blindness include glaucoma; retinal artery occlusion or retinal vein occlusion; age-related macular degeneration; retinopathy; retinal detachment; tumours such as retinoblastoma and malignant melanoma of the eye; and retinal haemorrhage.

Loss of vision may be due to nerve conduction problems. These problems may be the result of pressure caused by a tumour; reduced blood supply to the optic nerve; optic neuritis; or toxic or nutritional deficiencies. Blindness can result if there is pressure on the visual cortex from a brain tumour or brain haemorrhage, or if the blood supply to the cortex is reduced following a stroke.

Treatment depends on the underlying cause. If the loss of vision cannot be corrected, the patient may then be registered as legally blind or partially sighted. (See also eye; vision, loss of.)... blindness

Serious eye injuries may be caused either by penetration of the eye by a foreign body (see eye, foreign body in) or by a blow to the eye.

A blow to the eye may cause tearing of the iris or the sclera, with collapse of the eyeball and possible blindness. Lesser injuries may lead to a vitreous haemorrhage, hyphaema, retinal detachment, or injury to the trabeculum (the channel through which fluid drains from inside the eye), which can lead to glaucoma. Injuries to the centre of the cornea impair vision by causing scarring. Damage to the lens may cause a cataract to form.... eye injuries

The socket in the skull containing the eyeball, protective fat, blood vessels, muscles, and nerves. The optic nerve passes into the brain through an opening in the back of the orbit.

A severe blow to the face may fracture the orbit, but the eyeball is often undamaged as it can move back into the socket.

Fractures often heal without treatment, but some cause deformity and require corrective surgery.

Rarely, bacterial infection spreads from a sinus or the face to cause orbital cellulitis.... orbit

A form of optic neuritis in which the optic nerve becomes inflamed behind the eyeball.... retrobulbar neuritis

Blockage of a venous sinus (a channel for venous blood deep in the skull behind an eye socket) by a thrombus (abnormal blood clot). The condition is usually a complication of a bacterial infection in an area drained by the veins entering the sinus. Such infections include cellulitis of the face, infections of the mouth, eye, or middle ear, sinusitis, and septicaemia.

Symptoms include severe headache, high fever, pain and loss of sensation in and above the affected eye, and proptosis (protrusion of the eyeball). Vision may become blurred and eye movements paralysed due to pressure on the optic nerve and other cranial nerves. Treatment with antibiotic drugs and anticoagulant drugs can save vision. Left untreated, blindness results, and the infection may prove fatal.... cavernous sinus thrombosis

A group of small protein molecules produced in the brain and by nerve endings elsewhere in the body. Enkephalins have an analgesic effect and are also thought to affect mood. Enkephalins are similar to endorphins but have a slightly different chemical composition and are released by different nerve endings.enophthalmos A sinking inwards of the eyeball. Enophthalmos is most often caused by fracture of the eye socket or shrinkage of the eye due to the formation of scar tissue following injury.

enteric-coated tablet A tablet whose surface is covered with a substance that is resistant to the action of stomach juices. Enteric-coated tablets pass undissolved through the stomach into the small intestine, where the covering dissolves and the contents are absorbed. Such tablets are used either when the drug might harm the stomach lining or when the stomach juices may affect the efficacy of the drug.... enkephalins

Many eye disorders are minor, but some can cause loss of vision unless treated. (See also cornea, disorders of; retinal detachment.)

Squint is sometimes present at birth. Rarely, babies are born with microphthalmos. Other congenital disorders that affect the eye are nystagmus, albinism, and developmental abnormalities of the cornea and retina.

Conjunctivitis is the most common eye infection and rarely affects vision. Trachoma or severe bacterial conjunctivitis can impair vision. Corneal infections can lead to blurred vision or corneal perforation if not treated early. Endophthalmitis (infection within the eye) can occur as a result of eye injury or infection elsewhere in the body.

Narrowing, blockage or inflammation of the blood vessels of the retina may cause partial or total loss of vision.

Malignant melanoma of the choroid is the most common cancerous tumour of the eye. Retinoblastoma is a cancerous tumour of the retina that most commonly affects children.

Various vitamin deficiencies (particularly of vitamin A) can affect the eye. This may lead to xerophthalmia, night blindness, or, ultimately, keratomalacia.

Uveitis may be caused by infection or an autoimmune disorder such as ankylosing spondylitis and sarcoidosis.

Macular degeneration of the retina is common in the elderly, as is cataract.

Glaucoma, in which the pressure inside the eyeball becomes raised, can lead to permanent loss of vision. In retinal detachment, the retina lifts away from the underlying layer of the eye.Ametropia is a general term for any focusing error, such as astigmatism, myopia, or hypermetropia. Presbyopia is the progressive loss with age of the ability to focus at close range. Amblyopia is often due to squint.... eye, disorders of

the sixth *cranial nerve (VI), which supplies the lateral rectus muscle of each eyeball, responsible for turning the eye outwards.... abducens nerve

n. a *carbonic anhydrase inhibitor used mainly in the treatment of glaucoma to reduce the pressure inside the eyeball and also as a preventative for epileptic seizures and altitude sickness. Side-effects include drowsiness and numbness and tingling of the hands and feet.... acetazolamide

n. the action of closing and opening the eyelids, which wipes the front of the eyeball and helps to spread the *tears. Reflex blinking may be caused by suddenly bringing an object near to the eye: the eyelids close involuntarily in order to protect the eye.... blinking

n. (pl. fornices) an arched or vaultlike structure, especially the fornix cerebri, a triangular structure of white matter in the brain, situated between the hippocampus and hypothalamus. The fornix of the vagina is any of three vaulted spaces at the top of the vagina, around the cervix of the uterus. The superior (upper) and inferior (lower) fornices of the conjunctiva are the loose folds of conjunctiva reflected between the posterior aspect of the eyelid and the eyeball.... fornix

n. 1. the base of a hollow organ: the part farthest from the opening; e.g. the fundus of the stomach, bladder, or uterus. 2. the interior concavity forming the back of the eyeball, opposite the pupil. Fundus flavimaculatus is a hereditary disease of the retina in which white material is deposited in the fundus at the level of the retinal pigment epithelium (see retina). It usually causes loss of central vision, but good vision may persist into adulthood. Fundus albipunctatus is a hereditary disease in which the fundus shows widespread distribution of uniform-sized white dots, resulting in poor *dark adaptation.... fundus

adj. of or relating to the area within the eyeball. An intraocular lens implant is a plastic lens placed inside the eye after *cataract extraction to replace the natural lens.... intraocular

n. (pl. lamellae) 1. a thin layer, membrane, scale, or plate-like tissue or part. In *bone tissue, lamellae are thin bands of calcified matrix arranged concentrically around a Haversian canal. 2. a thin gelatinous medicated disc used to apply drugs to the eye. The disc is placed on the eyeball; the gelatinous material dissolves and the drug is absorbed. —lamellar adj.... lamella

n. a disease, occurring in West and Central Africa, caused by the eye worm *Loa loa. The adult worms live and migrate within the skin tissues, causing the appearance of transitory calabar swellings. These are probably an allergic reaction to the worms’ waste products, and they sometimes lead to fever and itching. Worms often migrate across the eyeball just beneath the conjunctiva, where they cause irritation and congestion. Loiasis is treated with *diethylcarbamazine, which kills both the adults and larval forms.... loiasis

n. an abnormally large eyeball.... megophthalmia

n. measurement of the pressure inside the eyeball. A rising or above-normal pressure is an important indication of the presence of *glaucoma.... oculoplethysmography

(ophthalmo-) combining form denoting the eye or eyeball. Examples: ophthalmectomy (surgical removal of); ophthalmorrhexis (rupture of); ophthalmotomy (incision into).... ophthalm

the smallest of the three branches of the *trigeminal nerve. It supplies sensory fibres to the eyeball, conjunctiva, and lacrimal gland, to a small region of the nasal mucous membrane, and to the skin of the nose, brows, and scalp.... ophthalmic nerve

n. measurement of the blood pressure in the vessels of the retina of the eye. A small instrument is pressed against the eye until the vessels are seen (through an *ophthalmoscope) to collapse. The pressure recorded by the instrument reflects the pressure within the vessels of the retina. In certain disorders of the blood circulation to the eye, the pressure in the vessels is reduced and the vessels can be made to collapse by a lower than normal pressure on the eyeball.... ophthalmodynamometry

(optic commissure) the X-shaped structure formed by the two optic nerves, which pass backwards from the eyeballs to meet in the midline beneath the brain, near the pituitary gland (see illustration). Nerve fibres from the nasal side of the retina of each eye cross over to join fibres from the lateral side of the retina of the opposite eye. The optic tracts resulting from the junction pass backwards to the occipital lobes.... optic chiasm

the second *cranial nerve (II), which is responsible for vision. Each nerve contains about one million fibres that receive information from the rod and cone cells of the retina. It passes into the skull behind the eyeball to reach the *optic chiasm, after which the visual (or optic) pathway continues to the cortex of the occipital lobe of the brain on each side (see illustration).... optic nerve

n. inflammation of the first part of the optic nerve (the optic disc or optic papilla), i.e. where the nerve leaves the eyeball.... papillitis

adj. (in ophthalmology) denoting the area around the eyeball.... peribulbar

adj. adjacent to the eyeball.... periocular

n. 1. any disease resulting in wasting of tissues. Phthisis bulbi is a shrunken eyeball that has lost its function due to disease or damage. 2. a former name for pulmonary *tuberculosis.... phthisis

n. 1. a technique used in surgery for the correction of retinal detachment. A small piece of silicone plastic is sewn on the outside of the eyeball to produce an indentation over the retinal hole or tear to allow the retina to reattach. 2. the insertion of plastic balls into the pleural cavity to cause collapse of the lung. This was done in the days before effective antituberculous drugs to help tuberculosis to heal.... plombage

n. 1. the change in direction of light rays when they pass obliquely from one transparent medium to another, of a different density. Refraction occurs as light enters the eye, when it passes from air to the media of the eye, i.e. cornea, aqueous humour, lens, and vitreous humour, to come to a focus on the retina. Errors of refraction, in which light rays do not come to a focus on the retina due to defects in the refracting media or shape of the eyeball, include astigmatism and long- and short-sightedness. 2. determination of the power of refraction of the eye. This gives the degree to which the eye differs from normal, which will determine whether or not the patient needs glasses and, if so, how strong they should be.... refraction

rapid eye movement: describing a stage of *sleep during which the muscles of the eyeballs are in constant motion behind the eyelids. People woken up during this stage of sleep generally report that they were dreaming at the time.... rem

n. the light-sensitive layer that lines the interior of the eye. The outer part of the retina (retinal pigment epithelium; RPE), next to the *choroid, is pigmented to prevent the passage of light. The inner part, next to the cavity of the eyeball, contains *rods and *cones (light-sensitive cells) and their associated nerve fibres (see illustration). A large number of cones is concentrated in a depression in the retina at the back of the eyeball called the *fovea. —retinal adj.... retina

(detached retina) separation of the inner nervous layer of the *retina from the outer pigmented layer (retinal pigment epithelium, RPE). It commonly occurs when a break (hole or tear) allows fluid from the vitreous cavity of the eyeball to accumulate under the retina (rhegmatogenous) but can also occur when fluid accumulates by leakage from the RPE (nonrhegmatogenous). Vision is lost in the affected part of the retina. The retina can be reattached by surgical means, such as external *plombage or internal *vitrectomy, or by creating patches of scar tissue between the retina and the choroid by application of extreme cold (see cryosurgery) or heat (see photocoagulation).... retinal detachment

prefix denoting at the back or behind. Examples: retrobulbar (at the back of the eyeball); retroperitoneal (behind the peritoneum).... retro

(sclerotic coat) n. the white fibrous outer layer of the eyeball. At the front of the eye it becomes the cornea. See eye. —scleral adj.... sclera

n. an operation in which a portion of the sclera (the thick white layer of the eyeball) is removed.... sclerectomy

n. abnormal bulging of the cornea or sclera (white) of the eye. Anterior staphyloma is a bulging scar in the cornea to which a part of the iris is attached. It is usually the site of a healed corneal ulcer that has penetrated right through the cornea; the iris blocks the hole and prevents the further leakage of fluid from the front chamber of the eye. In ciliary staphyloma the sclera bulges over the ciliary body as a result of high pressure inside the eyeball. A bulging of the sclera at the back of the eye (posterior staphyloma) occurs in some severe cases of short-sightedness.... staphyloma

apparent widening of the distance between the upper and lower eyelids (the palpebral fissure) due to retraction of the upper lid and protrusion of the eyeball. It is a sign of exophthalmic *goitre. [C. Stellwag von Carion (1823–1904), Austrian ophthalmologist]... stellwag’s sign

n. a condition in which the eyelid adheres to the eyeball. It is usually the result of chemical (especially alkali) burns to the conjunctiva lining the eyelid and eyeball.... symblepharon

pl. n. the fluid secreted by the lacrimal glands (see lacrimal apparatus) to keep the front of the eyeballs moist and clean. Tears contain *lysozyme, an enzyme that destroys bacteria. Irritation of the eye, and sometimes emotion, cause excessive production of tears. See also blinking.... tears

the fibrous tissue that lines the orbit and surrounds the eyeball. [J. R. Tenon (1724–1816), French surgeon]... tenon’s capsule

n. the measurement and recording of intraocular pressure while the eyeball is subjected to pressure over a period of several minutes. It is used to assess aqueous outflow and diagnose glaucoma. See tonometer.... tonography

the fourth *cranial nerve (IV), which supplies the superior oblique muscle, one of the muscles responsible for movement of the eyeball in its socket. The action of the trochlear nerve is coordinated with that of the *oculomotor and the *abducens nerves.... trochlear nerve