Eye

The organ of sight. The eye consists of structures that focus an image on to the retina at the back of the eye and nerve cells that convert this image into electrical impulses. These impulses are carried by the optic nerve to the visual cortex (an area at the back of the brain concerned with vision) for interpretation. The eyes work in conjunction with each other, under the control of the brain, aligning themselves on an object so that a clear image is formed on each retina. If necessary, the eyes sharpen images by altering focus in an automatic process called accommodation. The eyeballs lie within the bony orbits. Each eyeball is moved by six delicate muscles. The eye has a tough outer coat, the sclera. At the front of the sclera, the transparent cornea serves as the main “lens” of the eye and does most of the focusing. Behind the cornea is a chamber of watery fluid, at the back of which is the iris with its pupil, which appears black. Tiny muscles alter the size of the pupil in response to changes in light intensity to control the amount of light entering the eye. Immediately behind the iris is the lens, suspended by fibres from a circular muscle ring called the ciliary body. Contraction of the ciliary body changes the shape of the lens, enabling fine focusing. Behind the lens is the main cavity of the eye, containing a clear gel, the vitreous humour. On the inside of the back of the eye is the retina, a complex structure of nerve tissue. The retina requires a constant supply of oxygen and glucose, and a network of blood vessels, the choroid, surrounds it. The eyeball is sealed off from the outside by a flexible membrane called the conjunctiva, which is attached to the skin at the corners of the eye and forms the inner lining of the lids. The conjunctiva contains tear- and mucus-secreting glands. They, along with an oily secretion from the meibomian glands in the lids, provide the tear film that protects the cornea and conjunctiva. The blink reflex is protective and helps to spread the tear film evenly over the cornea to enable clear vision.