1. STRUCTURE OF THE MODERN HUMAN EYE

The eye is a sphere with a slight bulge in front and a stalk behind, a little to the inner side of the mid-line. It has a firm shell, the sclera, with a transparent bulge in front (cornea). The stalk at the back carries the optic nerve, transmitting visual impulses to the brain.

The sclera is lined first with a pigmented layer of fine blood vessels (choroid) and second with a mesh of nerve fibres and their light-sensitive endings (retina).

Behind the cornea is a pigmented diaphragm supported by muscle (iris), and behind the iris, again supported by muscle, is the lens. The pupil is a hole in the middle of the iris.

The space between the cornea and the lens is filled with watery fluid (aqueous humour), and the rest of the eye is filled with a clear jelly (vitreous humour).

Light passes freely through cornea, aqueous humour, pupil, lens, and vitreous humour to the sensitive retina. The pigmented choroid behind the retina prevents reflection of light and so the capture of light photons is effective.

The orbit is a socket in the skull considerably larger than the eye. The dead space is filled with loose fat, leaving the eye free to move Six small muscles attach the sclera to the orbit.

A thin membrane, the conjunctiva, lines the eyelids and covers the front of the eye. The conjunctiva is constantly lubricated and washed with team The tears are secreted by the lacrymal gland under cover of the outer part of the upper lid, and drain into the nose through a small opening at the inner corner of the eye.

2. FUNCTION

The optical system of the eye, that is, the arrangement for focusing a sharp image on the retina, is simple. The 'lens' is only a minor component of the effective lens, for most of the work is done by the curved cornea, which on its own (with its backing of water) is a powerful fixed focus lens. The named lens behind it is flexible; its curvature is adjusted by its surrounding muscle. It is supplementary to the cornea, and the means of varying the focus.

The iris contracts or relaxes to adapt the size of the pupil to the amount of light. It incidentally constricts the pupil when the eye is focused for near vision.

The sensitive nerve-endings in the retina are of two kinds: cones, which function in good light and register colour, and rods, which need very much less light but are colour-blind. As in colour photography, all colours are recorded as different mixtures of three primary colours (red, green, blue), perceived by three kinds of cone. Again as in colour photography, vision depends on chemical reactions promoted by light on the nerves; the rods and cones are sensitive to the products of the reactions. The cones are most sensitive to yellow, which therefore seems brighter than other colours of the same intensity. The rods are sensitive only to blue and green: hence the apparent brightness of grass by moonlight.

Rhodopsin (visual purple) is the light sensitive chemical of the rods. It is inactivated by bright light, and thus the rods are switched off when the light is good enough for the less sensitive cones (otherwise the glare would be intolerable). Adaptation to night-vision depends on the slow resynthesis of rhodopsin. The process requires vitamin A, and nightblindness is an early symptom of vitamin A deficiency.

Impulses from the retina are carried back to the brain in the optic nerve. A cross-over system between the two optic nerves carries impulses from the left halves of both eyes (representing objects to the observer's right) to the left side of the back of the brain, and from the right halves to the right side. The brain takes several years to learn to interpret the slightly different images from the two eyes.

There are several relay stations in the nerve-path from the eyes to the back of the brain, concerned with reflex activity such as focusing and adjusting the pupils, and also remoter functions such as maintaining the posture and balance of the body. As an organ of balance the eyes are secondary to reflexes governed by sensation from the muscles and joints of the limbs.

3. DISORDERS
Optical defects (astigmatism, myopia etc.) are considered separately. Working from front to back disorders of the eye are as follows: the conjunctiva is protected from injury by the eyelids and the very rapid blink reflex, and from infection by the flow of tears. Infection, leading to conjunctivitis, is nevertheless very common. Even very slight injuries are troublesome if they involve the cornea. Small foreign bodies in the eye - and unskilled attempts to remove them - may cause ulcers on the cornea. These small wounds heal only slowly because the cornea has no blood supply, and if they are at all deep they may leave opaque scars that interfere with vision. An eye can be blinded by severe scarring of the cornea. This is the type of blindness that can be cured by grafting a cornea from someone who has bequeathed his eyes to a hospital. The aqueous humour is secreted from the tissues behind the iris and kept at a pressure between 10 and 24 mm of mercury by compensatory leakage in the angle between the outer rim of the iris and the back of the cornea. The flow is through the pupil from behind. Disturbances of this mechanism may cause the pressure to rise: this is glaucoma. The iris, its supporting tissue, and the choroid form a single layer, the uveal
tract. Inflammation from various causes (and often from no known cause) may involve the whole tract (uveitis) or one part of it (iritis, choroiditis). If possible, the cause is treated, but in most cases treatment is directed to preventing complications such as glaucoma or damage
to the retina, while the inflammation settles in its own time. Corticosteroid drugs
have become the standard treatment for most of these cases. Drugs such as atropine are used to keep the iris dilated and prevent it from sticking to the lens.

The one common disorder of the lens is cataract or opacity, which can be due to congenital defect, injury, or ageing. Cataract is treated by removing the useless lens and compensating for its loss with glasses or contact lenses, or by replacing it with an artificial implant.

The retina cannot function unless it is evenly applied to the back of the eye. Detachment of the retina may follow injury or may occur spontaneously; it occasionally complicates other conditions such as choroiditis. Vision is lost in the affected area. Detachment is treated by making minute injuries to stick the retina in place, by heat, cold, or other means. Inflammation of the retina - retinitis - may complicate inflammation elsewhere in the eye; other cases are ascribed to various more general ailments including diseases of blood vessels, chronic alcoholism, and nutritional deficiencies. Much the commonest retinal disorders are due to diabetes and high blood pressure. In all these cases it is the underlying disease that is treated.

The optic nerve arises in an area to the inner side of the mid-point of the retina; there are no rods or cones in this area,(optic disc) and it is therefore a blind spot. (Since the image on the retina is reversed, the blind spot corresponds with a small area in the field of vision to the outer side of centre.) The nerve is liable to damage by increased pressure within the eye (glaucoma) or inside the skull, by by the same conditions as cause retinitis and by multiple sclerosis, a disease of various parts of the nervous system.

Squint is a failure to balance the muscles controlling movement of the eyes. Since the eyes do not point in the same direction the images do not correspond. With the common squints of childhood, the brain resolves the conflict between disparate images by ignoring one. If the squint is neglected, the habit of ignoring impulses from one eye becomes fixed and the eye - although perfectly healthy - is in effect blind. It is for this reason, and not for the sake of appearance, that childhood squints need to be treated early. The much less common squints that come on in adult life, after the brain has learned to resolve the two images, cause double vision.