The human eye as an optical instrument may be compared to a camera on a tripod. The eyeball is the camera, and a bony socket in the skull, the orbit, is the tripod. The eyeball is moved from side to side and up and down and obliquely by six voluntary muscles. Each of these muscles is attached at one end to the eyeball and at the other to the edge of the orbit. The visual centers of the brain are so trained that these muscles move both eyes together so that synchronous vision is obtained.
The optic nerve passes from the back of the eyeball through a hole in the back of the orbit into the visual center of the brain. This center has many connections with other centers controlling bodily movements, and the close association of the visual and motor centers allows us to direct motions accurately. When vision is lost, when you are in a dark room, you can still move your muscles, but you make the hesitating uncontrolled motions of a blind person.
The eyeball consists of three coats. The outer sclerotic coat is simply a tough membrane to hold and protect the contents. The choroid coat is pigmented, and like the black paint put on a dark room to serve as a background for the visual functions. In the choroid also run the blood vessels which nourish the eye. The inner coat of the eyeball is the nervous layer, the retina, which records the visual image and transmits it to the optic nerve, to be carried to the brain.
In front the sclera thins out and becoming transparent, forms the cornea. The limits of the cornea are marked by the edges of the colored part of the eye, the iris. The iris is a circular, pigmented muscle which automatically varies the size of the pupil, according to the intensity of the light and to the distance of the object to be seen.
Behind the iris is the crystalline lens, the focusing part of the eye. It also acts automatically, flattening or thickening in accommodation to the distance of the object to be seen.
The eyeball is protected in front by the eyelids. Inside the lids and going over the front of the eyeball is a layer of mucous membrane, the conjunctiva. The motions of the lids are lubricated by the tears. These are secreted by the lachrymal glands and an excess is carried off by the lachrymal duct, which runs from the inside part of the lower lid into the inside of the nose.
Every part of this complicated mechanism must be in perfect working order to insure perfect vision. All the diseases which affect the rest of the body affect the eyes. A complete examination of the eyes, therefore, can be conducted only by someone well acquainted with all these complicated functions.