Cornea

The cornea is the transparent, dome-shaped surface which covers the front of the eye, that is to say the pupil and the iris, and is a continuation of the sclera. The cornea refracts light into the crystalline lens, which then focus the image on the retina. While the cornea has a fixed focus, which means it only refracts light, the lens can be adjusted, changing its shape, to tune the focus depending on the object's distance.

In contrast to other tissues in the body, the cornea contains no blood vessels to nourish or protect it against infection. Instead, the cornea obtains its nourishment from the aqueous humor that fills the anterior chamber behind it. The cornea must remain transparent to refract light properly, and the presence of even the tiniest blood vessels can interfere with this process. All layers of the cornea must be free of any cloudy or opaque areas.

The cornea is made up of highly organized group of cells and proteins which are arranged in five layers; 1) the corneal ephithelium, which is a multicellular tissue layer of fast-growing and easily-regenerated cells; 2) Bowman's layer, a tough layer which protects the corneal stroma, consisting of irregularly-arranged collagen fibers; 3) the corneal stroma, which is a thick, transparent middle layer, consisting of regularly-arranged collagen fibers along with sparsely populated keratocytes; 4) Descemet's membrane, which is a thin but strong sheet of tissue that serves as a protective barrier against infection and injuries; 5) endothelium, the extremely thin, innermost layer of the cornea, consisting of endothelial cells that are essential in keeping the cornea clear.