Nociceptors. These repond to stimuli which could signal harm to the body.

Photoreceptors. These are only found in the eyes and respond to light.

Thermoreceptors. These respond to temperature change.

The lens is an elastic solid body enclosed in a transparent capsule. The lens focuses light onto the retina. The curvature, and thus the focal length of the lens may be altered by the ciliary muscles.

The iris is a ring of pigmented cells and muscle fibres. When the muscles contract, the iris constricts, reducing the central aperture, or pupil.

The iris controls the amount of light reaching the retina by opening in dim light, and closing in bright light.

Retina (inner layer)

The retina lines the interior of the eye, except for the section from the ciliary body forwards.

The retina is composed mainly of nerve and fibre cells, with light sensitive cells on the inner face.

These cells are divided into rods which sense black to white shades, and cones which sense colour.

The eye contains approximately 125 million rods and 7 million cones.

The central area of the retina, known as the macula lutea, contains only cones. The remainder of the retina contains a mixture of rods and cones.

The optic nerve connects to the eye at a point on the nasal side of the posterior centre. This point has no rods or cones, and is known as the blind spot.

Interior of the eye

The main body of the eye is filled with a clear gel called vitreous humour.

The anterior part of the eye is filled with a clear fluid called aqueous humour.

After bathing the eyes tears normally drain via naso-lacrimal ducts, leading from the medial edge of each eye socket, into the nasal cavity.

Vestibule. This is the section nearest the middle ear. Sound waves enter the vestibule from the stapes bone, via an orifice known as the oval window.

Cochlea. This resembles a snail’s shell, being a concentric spiral. Its origin is continuous with the vestibule.

The cochlea within the membranous labyrinth contains the spiral organ (organ of Corti). This holds the nerve endings of the cochlear nerve which provide the sense of hearing.

Semicircular canals. (3) These are tubes, arranged such that there is one in each of the three planes of space. They are continuous with the vestibule.

They contain the sensory nerve endings for balance.

Tooth decay may also cause pain in the ear.

Hearing loss

Hearing loss, or deafness, may occur for a large number of reasons, including genetic and congenital abnormalities, infection and disease, direct injury to the ear, injury to the surrounding bones, the effects of some poisons, long-term exposure to excessive sound levels, and age-related degeneration.

Conductive loss. This occurs through mechanical means, such as a simple blockage of the external acoustic meatus by wax or other foreign substance, or damage to the tympanic membrane or ear mechanisms.

Nerve loss. This occurs when the auditory nerve is damaged, either by injury or through disease.

Labyrinthitis

Labyrinthitis is an inflammation of the inner ear, and is a common cause of vertigo. It often follows on from infections such as the common cold or influenza.

Labyrinthitis may follow more severe infections such as mumps, and may then lead to permanent damage and hearing loss.

Ménière’s disease

Ménière’s disease leads to excessive endolymph, and dilation of the labyrinth, which distorts the sensation of balance causing attacks of extreme vertigo, as well as causing deafness.

The condition takes the form of repeated attacks, with remission in between, although tinnitus may occur during these times. Eventually, the organ of Corti is damaged by the attacks and permanent hearing loss results.

Tinnitus

Tinnitus is the sensing of noise – often a ‘ringing’ sound – when no actual sound is present.

It is believed to occur through misinterpretation of nerve impulses from the ear or from other areas of the auditory nerve pathway.

The underlying cause is often unknown, but many factors, including alcohol, medications, poisons, infection, injury, a build-up of wax or other contaminants may contribute, as may hearing loss allowing otherwise masked noises to be audible.

The condition usually causes a characteristic redness of the affected eye.

Floaters

Floaters are small particles of matter which drift inside the eye. They cause slight visual disturbances, but are normally benign and temporary. They may, however, be an indication of retinal detachment – a section of the retina splitting or breaking free from the choroid.

Glaucoma

Glaucoma is a condition which results from increased pressure inside the eye. This occurs when the aqueous humour fails to drain properly to balance its production.

The increased pressure compresses the junction between the retina and the optic nerve, reducing its blood supply, and causing individual neurones to die.

The condition first shows as ‘blind spots’ in the peripheral vision, but if not treated, it will eventually lead to complete blindness.

Macular degeneration

Macular degeneration occurs when the layer of epithelium which forms a filter between the retina and its blood supply breaks down. The retina is then exposed to substances which begin to destroy the macula lutea.

As the condition progresses, central vision is lost, although peripheral fields remain.

Retinitis pigmentosa

Retinitis pigmentosa is a progressive degeneration of the retina. Usually, rods are most affected, reducing night vision and peripheral vision. The condition is largely hereditary, and tends to develop during late childhood, although it rarely causes complete blindness.