The eye endows us with probably our most precious sense – the ability to see. It is a remarkable organ, just 25 mm in diameter yet composed of many intricate structures that help to make vision possible. Rays of light pass through these structures, to be converted into electrical impulses at the back of the eye, which pass along the optic nerve to the brain. Although the eye detects light intensity and colour, it is in the brain I that the image is interpreted and ‘seen’.
The front of the eye
The front one-tenth of each eyeball’s surface is protected by the upper and lower eyelids. These close automatically if we anticipate injury or if the delicate covering of the front part of the eyeball, the conjunctiva, is lightly touched. The eyelids also close regularly in blinking, to keep the eyeball moist and to wash away dust and bacteria by spreading watery secretions (tear fluid) across the eye. Eyelashes grow out from hair follicles along the rim of each eyelid. These help to keep airborne particles off the conjuctiva. If the root of an eyelash becomes infected, a stye forms. Close to the eyelashes is a row of tiny glands called meibomian glands which secrete a fluid to keep the lids moist. Sometimes these glands become blocked, giving rise toon the eyelid. Tears are produced by the lacrimal glands, which are located above and behind each eyeball, close to the nose. In addition to keeping the eyeball moist, tear fluid contains a substance called lysozyme that kills any bacteria on the eye’s surface. Tears can, of course, be produced in much larger amounts at times of sadness or pain.
Another line of defence against damage to the main structure of the eye is the conjunctiva, a transparent membrane that covers the front surface of the eye and is folded back on itself to cover the inside surface of each eyelid. Inflammation of this membrane, known as conjunctivitis, causes the eye to become red and sore and may be caused by infection or allergy.
Moving the eyes
Eye movement is controlled by six tiny muscles, fixed at one end to the bony wall of the eye socket and at the other end to the tough outer wall of the eyeball itself. Each muscle causes the eye to move in a different direction. For example, one rotates the eyeball outwards, another rotates it inwards; others swivel the eye to look up or down. Two of the muscles cause a combined movement: the superior oblique muscle rotates the eye in a downwards and outwards direction; the inferior oblique muscle rotates the eye upwards and outwards. Usually more than one muscle works at a time to produce smooth movement, and also both sets of eye muscles are closely co-ordinated so that both eyes look at the same object. If one eye has a weak muscle, or the nerve controlling a muscle is not working correctly, the result is a squint.
Inside the eye
The eyeball has several concentric outer layers of tissue enclosing a fluid interior. The outermost layer is the sclera, made of tough, fibrous material that gives shape and strength to the eyeball. It forms the ‘white’ of the eye. At the front of the eyeball it changes to become more dome-shaped and transparent, forming the cornea – the ‘window’ of the eye. Look at your eyes in a mirror and you can see through the cornea to the coloured irises (usually blue, brown or green) with the dark pupils at their centres. The iris is a disc-shaped structure at the front of the eye, extending from a ring of muscle called the ciliary body on the inside of the sclera. It is actually a series of muscle fibres: one set radiates outwards, like the spokes of a wheel; a second set is circular in form. The main function of the iris is to control how much light enters the eye through the pupil. It works rather like the diaphragm of a camera. In bright light its circular muscle fibres contract to make the pupil small, so that less light passes, whereas in dim conditions the radial muscle fibres contract to dilate the pupil and allow more light through. Pigmentgive the iris its colour. The eye hue depends on how many pigment are present – if there are few pigment cells the iris appears blue, whereas many cells give a brown coloration. Eye colour is inherited, and is often used as an example of how genes are passed from parents to offspring. The ciliary body also holds in place the lens of the eye, which is suspended from it by a series of suspensory ligaments. The lens is only about 10mm across but is fairly elastic and can change its shape in order to focus on near or distant objects. If the ring-shaped ciliary muscle contracts the lens becomes more spherical in shape for near vision; ciliary relaxation pulls the lens into a thinner shape for distant vision. Lack of elasticity in the lens causes blurred vision.
The space between the lens and the cornea is filled with a watery fluid called the aqueous humour. This fluid is produced continuously by the ciliary body, from where it passes slowly through the pupil and is absorbed into a network of fine canals at the junction of iris and cornea. If the fluid cannot drain away for some reason, pressure builds up as it accumulates and the result is a potentially very serious eye disease called glaucoma.
The largest part of the eyeball, behind the lens, is the eyeball cavity, which is filled with another fluid, the vitreous humour. This is transparent and viscous enough to maintain constant pressure within the eyeball – the intraocular pressure. As well as giving the eyeball its firmness and spherical shape, the vitreous humour helps to keep the light-sensitive layer of cells, the retina, fixed firmly in place.
The retina is the innermost layer of the eyeball, and its six square centimetres cover about three-quarters of the eyeball’s interior surface. Between it and the sclera is a layer called the choroid, which extends as far as the ciliary body. The choroid layer is well furnished with tinyvessels that supply oxygen and nutrients to the retina.
The retina contains special ‘photosensitive’ cells – cells sensitive to light. There are two types of cell, rods and cones (so called because of their shapes). Cone cells are especially sensitive to bright light and colours, whereas rod cells can detect dim light. These cells turn light energy into nerve impulses, which are sent to the brain for interpretation.
Besides being organs of vision, eyes are important for conveying mood and emotion. People give a ‘piercing stare’ or ‘show terror in their eyes’. And, most revealing of all, eye-to-eye contact can establish feelings of fondness, love and sexual desire.