Corneal problems including dry eyes
For you to be able to see clearly there are many structures of the eye with specific evolutionary roles that need to function effectively. As with any other structure of the body, things do go wrong.
The cornea is the transparent front part of the eye that covers the iris and pupil and allows viewing, when clear, of the back of the eye.
When functioning normally, the eyelids and the glands associated with eyelids are designed to protect the cornea and other surfaces of the eyeball from drying out or suffering trauma. The eyelids have several glands that assist lubrication. Meibomian glands secrete a oily substance which covers the water layer produced by the lacrimation gland. The conjunctiva, the transparent skin of the eye, produces a mucus layer allowing the water layer to attach to the corneal epithelium. Together the inner mucus layer, middle water layer and outer oily layer form a structure called the tear film.
Front of eye problems
The eyeball itself is approximately 24mm in size and is in contact with the outside world through the transparent surfaces of the eyeball.
The cornea, commonly know as the window of the eye is a clear transparent structure that allows light in from the front to then project to the photographic film of the eye at the back, called the retina. The cornea is the structure that is covered by contact lenses if you wear them.
Over the cornea, there is a lubricating film, called the tear film, which keeps the eye moist, maintaining clarity and fights infection. Surrounding the cornea, is a white surface called the sclera which forms a hard surface to the eyeball. Overlying the white of the eye is a layer of transparent skin called the conjunctiva, which contains the blood vessels.
As with any part of the body, any single structure can have problems.