The outer most surface of the eye, or the clear outer structure is called the cornea. This is the only part of the eye that can be transplanted, that is, there is no such thing as an eye transplant. The cornea's primary function is to bend light rays coming into the eye so that it focuses them on the retina. Thus, light rays enter the eye, get partially bent, continue through the pupil (which is a black hole) and then enter the lens of the eye. The lens, a flat, clear structure (in youth) which may become cloudy later in life as a cataract whose primary function is to also continue to bend light rays to focus light on the retina. 

It should be stated that when you hear of procedures such as RK, cornea transplants (donated tissue from an eye bank), or cataract surgery with lens implants, these are references to structures in the front of the eye which are much different from the retina which is in the back of the eye. 
 

MORE ABOUT THE RETINA

As we stated earlier, the retina is the inner layer of the back of the eye and is much like a relay station. The first area touched by light rays are the rod and cone cells. Cone cells live primarily in the macula while rod cells live in the peripheral retina. Rods and cones are like rabbit ears on a TV set. They help catch the light rays that are focused on the retina. The disease of Macular Degeneration is being studied to determine if there is an abnormality in the rods or cone cells or if the primary problem is in a layer of cells below the rod and cone cells called the RPE cells. These cells called the retinal pigment epithelial cells is a layer of cells like a carpet underneath the rods and cones. The primary purpose of this layer is to hook up with the rods and cone cells to bring nourishment for healthy tissue.

There are also several layers beneath the RPE layer including Bruch's membrane, and a layer known as the choroid which contains the blood vessels of the eye. In order for vision to work properly, think of it as a TV set. The TV antenna must work properly with the right "hook up" as well as the cable to the TV set must also work. In the retina there are other "hook-ups" in different types of cells and cell layers - some called bi-polar cells, some called ganglion cells as well as the optic nerve. Thus, there are many difficult hurdles to overcome for "experimental surgery" or treatments for Macular Degeneration. It is understood that the retina connects like "links in a chain" and many areas hook up in the relay station so that one problem or link may be fixed, but there may also be a weaker link where the problem is not easily solved. 
 

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