All About the Human Cornea: Structure and FunctionFitness Equipment
Vision is one of the five human senses, and is a truly remarkable, complex, function involving the eyeball (orb), the surrounding muscles, optic nerve, and of course the brain. One such structure of the eye is the cornea, which is the clear refractive surface covering the iris (colored part of the eye) and pupil. The cornea makes up a relatively small percentage of the eye, but is responsible for so much! Read on to learn all of the amazing facts about our corneas.
STRUCTURE AND FUNCTION OF THE CORNEA
It is somewhat difficult to refer separately to the structure and function of the cornea, as they are very inter-related. However, the primary function of the cornea is to refract light. The cornea is actually a powerful lens. In fact, it is the cornea which is responsible for most (about two-thirds) of the refraction that takes place; the lens inside your eye accounts for the remaining 1/3 refraction!
In the illustrations above, you will see that the cornea appears as a convex layer at the surface of the eyeball. It is actually made up of five layers, measured in thickness by micrometers (µm) - a µm is one one-thousandth of a millimeter (1/1000 mm). The five layers (which are all transparent) each have their own function and unique physical properties. Beginning with the outer-most layer, these five corneal layers are as follows:
- Corneal Epithelium. This layer acts as a barrier to germs (with the help of tears) and as a protective shield to the underlying layers. The epithelium provides a shiny refractive surface which is very important for the path of light to penetrate and bend. The thickness of the epithelium is a mere 50 µm - 5 to 7 cells thick - but if injured heals quickly without leaving a scar. Superficial abrasions and small foreign bodies (insects, dust, etc.) usually have no long-term effect on this layer of the cornea. Any injury to the layers below the epithelium will usually result in scarring, which can obstruct vision.
- Bowman's Membrane. This is an acellular structure, meaning it is not comprised of cells, but of collagen fibrils, and is only 10 µm thick. Interestingly, it is unknown what the function of this membrane is. Once damaged, however, the Bowman's Membrane cannot regenerate.
- Stroma. This is the thickest layer by far, making up 90% of the corneal thickness. Stroma is made up primarily of water (78%), as well as Type I collagen. The term stroma is actually used in other parts of the body, also, as it refers to connective and supporting tissue of an organ. Incidentally, the peripheral thickness (950 µm) of the stroma is about twice as thick as the central area (450-550 µm).
- Descemet's Membrane. The thinnest of all layers (at birth), this membrane is only 3 µm thick in a newborn infant. The purpose of this membrane is similar to a foundation of a building; it supports the structures which lie on top of it. It is made of type III collagen and is very elastic. In an elderly person, Descemet's Membrane will have grown to a thickness of 10-12 µm.
- Corneal Endothelium. This is a monolayer of 500,000 cells. This means that all the cells are identical, but profuse. This layer regulates the fluid in the stromal layer, acting as a pump to reduce excess water accumulation, if necessary.
It is also interesting to note that the cornea is completely avascular, meaning there are no blood vessels or capillaries of any kind to feed and nourish the cornea. The cornea relies on three sources for its nutrients and regeneration: 1) the tear film; 2) the aqueous humor (the fluid in the anterior chamber, directly behind the cornea); 3) a network of fine capillaries at the limbus (which is the juncture between the cornea-covered iris and the conjunctiva-covered white of the eye [sclera]).
In summary, the cornea provides refraction, protection, and regulation and support for the other parts of the eye. In an upcoming article, common conditions and diseases of the cornea will be discussed, as well as how to protect yourself against them. Hopefully, this article has served to teach you and to clarify how the cornea functions as an integral component of giving us our vision.
© Sharla Smith, July 2011
The Ophthalmic Assistant; A Text for Allied and Associated Ophthalmic Personnel, 8th Edition; Stein, Stein, and Freeman, 2006
Dictionary of Eye Terminology, Fifth Edition; Barbara Cassin, Melvin Rubin, MD; 2006
Illustrations courtesy Wikimedia, as follows: