The Cornea:

The cornea is the front part of the eye and is a transparent tissue that lacks blood vessels and covers the iris and pupil. The cornea allows light rays to permeate and bends the light enough to allow for focusing of the eye.

The cornea contains five different layers:

• the epithelium
  This is the outermost layer of the cornea. The cells of the layer offer a smooth surface that serves the tear layer well.
  
• Bowman's membrane
  This is the main part of the cornea. It is a tough layer of transparent tissue that has no cells. It is made up of mostly collagen material.
  
• the stroma
  This layer has other layers in it of tissue called lamellae which go through the entire length of the cornea. Almost ninety per cent of the stroma is composed of stromal tissue. The stroma also has cells that are called keratocytes.
  
• Descernet's membrane
  This layer is simply secretory products from the endothedlium.
  
• the endothedlium
  The innermost layer of the eye is a single layer of cells that are lined up against Descemet's membrane. The quality and quantity of these cells decrease over time and cannot be replaced. When one cell dies, the rest of the cells enlarge to keep the same distance covered.

Picture from http://www.stlukeseye.com/Anatomy.asp.

Drawn by Mark Erickson

The Sclera:

The sclera is the white of the eye that is visible between the lids. It is made of tough material, and few diseases affect it. The episclera and conjunctiva cover it, and the eye muscles are connected to it.

The Conjuctiva:

The conjuctiva is, simply put, "a thin lining over the sclera." (www.pasadenaeye.com/faq/faq15/faq15_text.html) It is the outermost tissue of the eye and thus is exposed to a great deal of substances and microorganisms. Tears are produced by the conjunctival epithelium, and these help fight off the organisms that enter the eye.