The eye is a delicate and complicated biological camera. Everything we see in the world about us reflects light into the eye, the light is focused onto the back of the eye, and the pattern of that light relayed to the brain where the message is interpreted to create vision. The eye is protected by the skull and orbit, and by the eyelids, conjunctiva and tear film; eye movements are generated by contraction of muscles attached to the bony walls of the orbit.
This consists of skin, semi-rigid fibrous tarsal plate, glandular tissue and eyelashes, and lined internally by conjunctiva. This protects and lubricates the surface of the eye. The main tear gland (lacrimal gland) sits under the outer part of the upper lid.
This is a fine translucent membrane that covers the surface of the eye and is reflected back onto the eyelids (so that foreign bodies such as grit or contact lenses on the front of the eye cannot move back behind the eye). It is very sensitive, and is important in protection of the eye against infection.
The cornea is the transparent front portion of the eye. It needs to be clear and have a normal curved shape to focus light accurately and provide good vision. The cornea owes its transparency to the presence of a regular lattice structure of collagen fibres. Anything which affects this regularity results in loss of the transparency which is essential for good corneal function and health.
The sclera is the relatively inert, tough fibrous tissue that forms the outer coat of the eye, and provides most of its strength.
The iris is a flattened doughnut of muscle that acts as an expanding and contracting diaphragm, controlling the amount of light entering the inside of the eye. It contracts in bright light and expands in dim light.
This is a narrow strip of tissue in the angle between the outer part of the iris and the cornea. It is involved in drainage of fluid from the inside of the eye, failure of which may lead to glaucoma.
The ciliary body forms the base of the iris and has two functions. It produces the fluid (aqueous humour) that circulates within the eye and supports the suspensory ligament that in turn supports the lens.
The choroid is the extension of the iris and ciliary body around the back of the eye. It is very vascular and contributes to the nutrition of the other parts of the eye.
The lens (crystalline lens) fills the pupil, sitting just behind the iris. It contributes approximately one third of the eyes focussing power. It is loss of the ability of the lens to alter its shape that leads to the need for reading glasses from 45-50 years of age onwards. Cataract occurs when the lens loses clarity, again usually associated with increased age, and may require cataract extraction with intraocular lens implantation.
The Vitreous fills the inside of the eye behind the lens, and is important in development of the eye. In adult life the vitreous is passive, allowing light through to the retina, but may become involved in diseases of the retina.
The retina is like the film in a camera. It consists of millions of nerve cells that respond to light, sending electrical signals along fine nerves that then travel to the brain in the optic nerve.
The optic nerve is in fact a bundle of around a million nerve fibres, 1.5mm across where it leaves the eye, and surrounded by protective coats to form a 3mm diameter nerve that passes from the back of the eye, through a gap at the back of the orbit and into the brain.