The eyes are the two organs of sight. They are located in the front upper part of the skull and consist of structures that focus an image onto the retina at the back of the eye which is a network of nerves that convert this image into electrical impulses to be recorded in a region of the brain. 

The eyeball lies in pads of fat within the orbit, a bony socket that provides protection from injury. Each eyeball is moved by six delicate muscles which are activated and coordinated by nerves in the brain stem. The eyeball has a tough, outer coat called the "sclera," or white part of the eye. The front, circular part is the "cornea" and is transparent. 

The cornea is the main lens of the eye and performs most of the focusing. Behind the cornea is a shallow chamber full of watery fluid, at the back of which is the "iris" (colored part) with the "pupil" (center). The pupil is black and its diameter is changed by light intensity to control the amount of light which enters the eye. Immediately behind the iris, and in contact with it is the crystalline lens, which contracts to alter its shape and allow focusing power. 

Behind the lens is the main cavity of the eye, filled with a clear gel. On the inside of the back of the eye is the retina, a structure of nerve tissue on which the image formed by the cornea and the crystalline lens forms. The retina needs a constant supply of oxygen and sugar, and the need is supplied by a thin network of branching blood vessels which lie just under it called the choroid plexus. 

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 and contains many tiny tear-secreting and mucus-forming glands that protect the eyes from damage due to dryness. A "black eye" is really not black at all. A "shiner" is a mixture of purple, red and yellow hues caused by hundreds of tiny, broken blood vessels.