The Eye

  § Cornea

§ Sclera

§ Anterior chamber

§ Uveal tract

§ Pupil

§ Lens

§ Vitreous

§ Retina

§ Optic disc

  The cornea is the transparent anterior structure of the globe. Its transparency results from a highly ordered cell structure and low water content. The cornea loses its transparency if either of these factors is altered.

There are five layers of the corneal tissue, but the outermost and innermost layers are most important. The outermost layer is called the “Epithelium” which consists of cells that are constantly regenerating.

A superficial injury to this layer, by a contact lens or small foreign bodies, will usually heal within 24 to 48 hours. But infections with viruses, bacteria or fungi will require vigorous medical treatment. The innermost layer, the “Endothelium”, prevents the intraocular fluid from penetrating the cornea. If disease or trauma has damaged the endothelial cells, they may no longer be able to regulate the amount of water that reaches the internal tissue of the cornea. When this occurs, the cornea swells and becomes cloudy, this condition is called “Edema”.

Situated between the outermost epithelium and the innermost endothelium are three layers, called “Bowman’s Membrane”, the “Stroma” and “Descemet’s Membrane”. They each add rigidity to the cornea and provide additional barriers to infection.

The front surface of the cornea focuses light rays and contributes about two-thirds of the optical power of the eye. Because it must remain clear, the cornea contains no blood vessels. The cells are nourished by the aqueous humour and by the tear film, which also serves to maintain a smooth optical surface. The cornea is about 11.5 mm in diameter and 0.5 mm thick at its centre. Its junction with the sclera is called the “Limbus”. The limbus is also the location of the attachment of the bulbar conjunctiva to the globe. Tiny conjunctival capillaries at the limbus supply nutrients to the peripheral cornea.

Corneal inflammation is referred to as “Keratitis” and may result from a great variety of conditions such as a tear deficiency, toxic chemicals or infections. With any keratitis the patient usually experiences a foreign body sensation, eye pain aggravated by bright light, the condition called ”Photophobia”, blurred vision, redness and excessive tearing. In severe cases of keratitis, the anterior corneal tissue is destroyed and a corneal ulcer develops. Bacterial ulcers may be mild or severe, but the reaction to them is striking. The bulbar conjunctiva, which covers the white of the eye, becomes quite red, and there is often a copious discharge from the eye. 

The active ulcer produces an opaque area on the cornea. As it heals, it leaves a scar that may result in permanently decreased vision. The surface of the ulcer must be scraped and cultured in order to discover the specific causative organism, which is treated with the appropriate antibiotic. Fungi may also cause corneal ulcers, usually as a result of a corneal injury with some type of vegetable matter (thorns, branches, pieces of wood). Without adequate treatment, the ulcers may extend through the cornea and cause infection inside the globe.

Herpes simplex keratitis caused by the virus responsible for “cold sores” produces branch-like (“dendritic”) erosion of the corneal epithelium that may progress to form an ulcer. Symptoms of an early infection are often mild, but careful examination reveals that the eye is red and watery. This ulcer must be treated promptly with special anti-viral drugs to prevent spreading to deeper corneal tissues. Once the disease has spread beneath the epithelium, the scar that remains after the healing process may seriously impair vision and leave a total corneal opacity, which may require corneal transplant surgery.

Corneal dystrophies are hereditary defects in the structure of cornea due to which the cornea loses its clarity. Corneal dystrophies become apparent at all ages. One example is keratoconus, in which the central cornea becomes thin and cone-shaped. The increase in corneal curvature at the centre of the cone causes an increasing nearsightedness (myopia) and astigmatism. The process begins in the teenage years and is usually completed by the late twenties. The decreased vision may be corrected in early stages by Rigid Gas Permeable Contact Lenses, but corneal thinning may become severe enough to require a corneal transplant.

A common change in the cornea of older patients is an opaque ring, called “Arcus Senilis”, seen near the limbus. It represents deposition of fatty substances from the blood. While it causes no symptoms, it may be a signal of an abnormally high fat content of the blood.

The sclera, the white of the eye, is the rigid outer portion of the wall of the eye that protects and supports the delicate inner structures. The sclera meets the cornea at the limbus. Occasionally the sclera or overlaying episcleral tissue becomes inflamed (scleritis and episcleritis) and may require treatment. Patients with rheumatoid arthritis may develop areas of marked sclera thinning (scleromalacia) which allows the purple colour of the underlying choroid to show through.

The anterior chamber is the space between the back of the cornea and the front of the iris. It is filled with clear aqueous fluid humour produced by the ciliary body lying behind the iris.

The aqueous fluid is secreted by the processes of the ciliary body and flows through the pupil into the anterior chamber. It exits from the eye at the junction of the cornea and the iris, where it flows through a filter called the “Trabecular Meshwork” and into the canal of “Schlemm”. From here it empties into episcleral veins.

The balance between aqueous production and drainage normally maintains the intraocular pressure between 12 and 20 mm Hg (mercury). If aqueous drainage is impaired, the intraocular pressure rises and may cause optic nerve damage and loss of vision, this condition is called “Glaucoma”.

Inflammation or infection within the eye may cause a pool of pus (hypopyon) to layer out at the bottom of the anterior chamber. When blood is found in the anterior chamber, after an injury or in certain diseases, it is called “Hyphema”.

  The uvea or “uveal tract” is the middle layer of the wall of the eye and is composed of the iris, the ciliary body and the choroid. The iris is the coloured part of the eye that controls the size of the central opening, the “Pupil”. The front layers of the iris contain its blood supply, muscles and pigment. The dialator muscle, which extends radially from the iris root to the pupil margin, opens the pupil (mydriasis) when it contracts.

The sphincter muscle encircles the pupil margin and makes the pupil smaller (miosis) when it contracts. A blue iris contains a relatively small amount of pigment while brown iris contains a relatively large amount. Pigment is deposited in the iris as the nervous system is developing. Because some of this development occurs after birth, most newborn babies have light eyes. Since the appearance of the net of new blood vessels on the iris gives it reddish cast, it is called “Rubeosis. Rubeosis may be responsible for blood leakage into the anterior chamber or obstruction of the filtration angle. Several diseases (especially diabetes) may cause an abnormal growth of new blood vessels (neovascularisation) on the surface of the iris. The iris may also be the site of nevi (freckles), tumours, cysts or nodules. The ciliary body is a band-like structure of muscle and secretory tissue that encircles the inside of the eye from behind the root of the iris to the anterior edge of the retina (the ora serrata). Most of the function of the ciliary body is concentrated in the anterior part, a folded muscle mass called the “Pars Pilicata”, which is composed of finger like ciliary processes that are responsible for secretion of the aqueous fluid. The posterior portion of the ciliary body is flat and is called the “Pars Plana. This latter area is a frequent site for surgical entry into the back of the eye because it contains relatively few delicate structures. Some of the muscle fibers of the ciliary body are arranged in a circular fashion and these control the eye’s ability to focus on near objects (the phenomenon is called “accommodation”).                    

The ciliary body may become inflamed in diseases that affect the iris and choroid as well (uveitis), it may also give rise to tumours or cysts.

The choroid is the posterior portion of the uveal tract. It is largely composed of blood vessels and lies between the sclera and the retina. It provides the blood supply for the outer cells of the retina. Inflammation of the choroid (choroiditis, posterior uveitis), may result from infection or from unknown (idiopathic) causes. The choroid may be the site of tumours such as malignant melanoma or metastatic cancer. Malignant melanoma is recognised as an elevated pigmented lesion that may cause the retina to detach. It is important to diagnose malignant melanomas early, so they can be treated before they spread to the rest of the body and cause death.

The pupil is the central opening in the iris that regulates the amount of light reaching the retina. The size of the pupil is controlled by the sphincter and dilator muscles of the iris. Each of these muscles is governed by a different part of the autonomic nervous system. 

The sympathetic system controls the dilator muscle that opens the pupil and the parasympathetic system controls the sphincter muscle that closes the pupil. Ordinarily the pupils are about 4 mm in diameter in a dimly illuminated environment and equal in size in both eyes. It is not unusual for the pupil sizes, in an individual, to differ, from o.5 to 1.0 mm. When the two pupils of an individual differ in size by more than 1.0 mm (anisocoria), there is an abnormality of one pupil or other (or both). If the inequality is greater in dim illumination where the pupils usually larger, then the dilator muscle is not working in the eye with the smaller pupil. On the other hand, if the anisocoria is greater in bright light, when the pupils are usually smaller, then the sphincter is not working in the eye with the larger pupil. Once the weak muscle has been determined, the part of the nervous system that is affected can be identified.

  The crystalline lens is a normally transparent, biconvex structure that is located in the posterior chamber between the iris and the vitreous body. The lens is composed of an inner nucleus, a surrounding cortex and an enveloping capsule.

The lens is responsible for about 1/3rdof the total focussing power of the eye and accounts for the ability to change focus from distance to near objects. The lens is suspended just behind the pupil by fibers called “Zonules” that are attached to the ciliary processes. When the ciliary muscles muscle contract, it relaxes the tension on the zonuls and allows the lens to become more round. The increased curvature of the lens makes it a more powerful refracting surface. 

The process of ciliary muscle contraction and increased lens curvature is called accommodation. The ability of the lens to change its shape depends on its elasticity. As it ages, the lens tissue becomes increasingly more rigid and accommodation power is gradually lost (presbyopia). This is the reason that most people, as they age, have to wear corrective bifocal spectacles; the lower segment helps the eye focus on near objects.

When the lens proteins degenerate, the lens loses its transparency, this is called a “Cataract”. Thus synonyms for cataract are lens opacification. Cataracts may be present at birth, though this is rare. Most commonly, cataracts occur as part of the normal aging process (senile cataract). Lens opacities may also be the result of an injury to the eye or may be associated with ocular or systemic disease or with chronic use of certain drugs. When vision has become significantly impaired, the lens is surgically removed. An eye with a lens in place is called “Phakic”. Once the lens has been removed, the eye becomes “Aphakic”. When the eye’s own lens has been removed, its refractive power must be replaced with a powerful artificial lens, either as a spectacle, a contact lens or an intraocular lens (implant).

  The vitreous body is a clear, gelatinous mass that fills the intraocular cavity behind the lens and helps maintain the spherical shape of the globe. When small particles drift across the vitreous, patients report that they see floaters. Floaters are a normal occurrence and need not be investigated unless they suddenly increase in number, if so, they may represent condensations of a portion of vitreous that has become detached from the retina. This vitreous detachment occurs as a normal part of aging, but may lead, in rare cases, to a retinal detachment, a vision threatening emergency.

The vitreous material is an excellent culture medium for bacteria. When infectious agents are accidentally introduced into the eye, either by injury or surgery, their growth may be so rapid as to destroy an eye within days. Infection of the vitreous and the adjacent tissue is called “Endophthalmitis” and constitutes an emergency. The infection is treated with large doses of antibiotics, but may also require the surgical removal of the infected vitreous (vitrectomy).

Abnormal retinal blood vessels may produce a haemorrhage into the vitreous that obscures vision. Usually this blood is absorbed over time. However, if the blood or resulting fibrous tissue remains, the vitreous may have to be removed surgically.

  The retina is a transparent structure that lines the inner surface of the globe posteriorly, and is actually an extension of the brain. It converts light to electrical (nerve) impulses and transmits these impulses to the brain’s visual cortex where they are integrated into the sensation of sight. When viewed through an ophthalmoscope, the normal retina appears orange, the retina though transparent, appears orange due to the blood vessels of the underlying choroid. Lying on the inner surface of the retina are the retinal arterioles (bright red) and veins (dark red). 

The retina is composed of an inner sensory portion and an outer pigment epithelium. The most important part of the sensory retina is the photoreceptor layer. It contains two types of photoreceptor cells, the cones and the rods, each with a different function in the visual process. Cones provide sharp central vision (acuity) and colour vision and function best in daylight. Rods are largely responsible for peripheral vision and function even in dim illumination. The innermost part of the retina consists of ganglion cells. Fibres from the ganglion cells for m the nerve fibre layer of the retina that converges at the optic disc. A specialised area of the retina containing most of the cone cells is located lateral to the optic disc and is called the “Macula”. The centre of the macula is called the “Fovea”. Damage to the macula will greatly reduce visual acuity and the eye will be left with only peripheral vision. When any other area of the retina is damaged, blind spots occur in off-centre position of the visual field. During development of the embryo, a cleft forms between the sensory and pigment epithelial portions of the retina that, under certain circumstances, may lead to an actual separation of these layers. Such a separation is called a “Retinal Detachment”. Trauma inflammation, eye surgery or the natural aging process may result in a tear in the sensory retina. Vitreous fluid leaks through the tear and spreads under the sensory retina causing it to become detached from the pigment epithelium. Patients who have a retinal detachment notice floaters, light flashes, blurred vision and sometimes the sensation of a “curtain” obscuring a portion of their field of vision. The detachment must be repaired surgically as soon as possible to prevent irreparable damage to sight.

Of the many disorders that affect the retina, senile macular degeneration is probably the most common. It occurs in the elderly and produces a slow decline in visual acuity usually in both eyes. The cause appears to be poor choroidal blood supply. In this condition, abnormal blood vessels (neovascularisation) that form beneath the macula leak and deform this delicate structure. In some cases such neovascularisation may be treated with the laser beam to prevent further vision loss. 

A rare tumour called a “Retinoblastoma” can arise in the retina of very young children. Because retinoblastoma is life threatening, the eye containing this tumour is treated promptly, either by radiation or surgical removal (enucleation).

  After passing through the transparent inner layers of the retina, light is converted by the rods and cones to a nerve impulse that travels forward to the ganglion cell layer and then to the optic disc by way of the nerve fiber layer. The optic disc is situated in the nasal portion of the fundus, 15 degrees off-centre. Because no rods and cones are present in this area, it is sightless and termed the physiologic blind spot in the field of vision. Inflammation and stroke (blood vessel blockage) can damage the optic disc and produce defects in any portion of the field of vision. When the intracranial pressure is abnormally high, the disc tissue will swell (papilledema). There are other causes of optic disc swelling, including inflammations and strokes of the disc tissue itself. The optic disc tissue becomes excavated (cupped) in advanced glaucoma.

  The eye is only the first part of the visual pathway, which extends to the back of the brain. The pathway that conducts visual messages after they leave the eye is termed the retrobulbar visual pathway.

The nerve impulse triggered by light striking the retina exits from the eye through the optic nerve, travelling to the optic chiasma. Here only the nasal nerve fibres from each eye cross to the opposite side, while the temporal fibres continue along on the same side. After the chiasma, the temporal nerve fibres from one eye and the nasal fibres from the other eye travel together in the optic tract and end in the lateral geniculate body.

From there the impulse is carried via the optic radiations to the occipital lobe in the posterior part of the brain. The destination of these fibres within the occipital lobe is called the “Visual (calcarine) Cortex”, where the visual message is integrated with information derived from other parts of the brain in a process called “Perception”.

Disorders affecting other parts of the visual pathway produce characteristic changes in the field of vision. The most common disorders are tumours of the pituitary gland (which lies directly under the chiasma) and strokes, aneurysms, inflammations, infections and trauma.