The eye is located in a bony shell called the orbit. It is surrounded by soft fatty tissue. Six extraocular muscles inserted on the eye provide the movements of the eye.



CORNEA

(Word meaning: corn=horn, strong as a horn)
The cornea is the outermost glass-like part of the eye (Figure 1). This is the part that is irritated upon dust in the eye and where we place the contact lenses on. It is approximately half a millimeter (550 microns) thick and has a diameter of 12 mm. It is like a section of a sphere. This is the part that refracts the light rays the most. Therefore, the cornea is of utmost importance in the focusing ability of the eye. A minimal change in the structure of the cornea makes a big change in the focusing function. Therefore, refractive surgery, which has been developed to solve the refractive errors of the eye, treats these problems usually by changing the shape of the cornea.
The cornea consists of 3 main and 2 auxiliary layers (Figure 5). The main layers from the surface to the inner eye are epithelium, stroma and endothelium. Bowman membrane lies between the epithelium and the stroma and the Descemet layer between the stroma and the endothelium. The epithelium consists of cells, which have the ability to multiply, and is covered by the tear film. It is located on Bowman's layer and it has 5 layers of cells with a total thickness of 50 microns. The epithelium can regenerate itself. After a trauma, depending on the extent of the trauma epithelial cells multiply and cover the traumatized area within a few days. In approximately 1 week the whole epithelium can regenerate itself. Another feature of this layer is free nerve endings. This is the part of the cornea that feels irritation upon dust in the eye.
The stroma, the main layer of the cornea makes up %90 of the corneal thickness. The major constituent of the stroma is collagen fibers. These collagen fibers form lamellar sheets, like the sheets of a notebook. There are 200 to 300 transparent sheets each having a thickness of 1 to 2 microns (1 mm= 1000 micron). The regular packing and equal spacing of the collagen fibers provide the transparency of the cornea. Stromal cells (keratocytes), which produce these collagen fibers, don't have the ability to reproduce or to feel the pain. The main function of the stroma is to give the cornea its strength and its shape like that of a dome. That is why refractive surgery is generally performed on this layer.
The endothelium, which is the innermost layer of the cornea, looks like a transparent honeycomb under a microscope. The endothelium is just one layer of cells located on the Descemet membrane, but it has a vital function: to keep the cornea transparent. Normally there is equilibrium of liquid in the cornea and this is achieved by pumping excess liquid by means of the endothelium.
An amazing feature of the cornea is its avascular (having no blood vessels) structure. Although it has several multifunctional layers and cells, it does not contain any blood vessels. This is necessary for its transparency.
The cornea is of utmost importance for our ability to see clearly.