A scratch to the surface of your eye heals quite rapidly, often in a matter of minutes. But a surface scratch on your elbow takes days rather than minutes to heal.
The reason for these different rates of healing is directly linked to the kinds of cells that exist on the outer-most layers of your skin and your eyeball.
The Elbow Vs. The Eyeball
When your elbow or your eyeball is scratched, a layer of cells is actually scraped away from the surface. The healing of a scratch happens when new cells take the place of the ones lost to the minor injury.
When the outer layer of cells on an eyeball is scratched, cellular replacement happens quickly because the surface of an eyeball consists of living cells. These living cells have the ability to rearrange themselves and migrate to where the cells are missing.
The outer layers of skin, however, consist of non-living cells, sometimes layered three or four cells deep. These dead
cells create a surface that protects the underlying, more tender, living cells. Because they are dead, the cells that make up the outer layer of your skin can't move like the living cells of an eyeball.
In order for new skin cells to replace old ones--in other words, in order for healing to occur--live cells, several layers below the surface, must work their way up to the outer-most layer of the skin.
When compared to the quick healing that the surface cells of an eyeball can accomplish, the healing of an elbow, which must begin several cell layers below the surface, can take a long time.