The oxygen supply to the cornea is precarious
and it needs to respire across both its surfaces in order to maintain its
metabolism. For example, cutting off the access to air to the front surface
with a contact lens, will lead to swelling and clouding of the tissue.
To bring oxygen to the rear surface it is necessary
to continually stir the fluid in contact with it, the aqueous humor. With
the eye open this is assured by a thermal circulation resulting from the
cooling by the corneal surface by the air.
When the eye is closed during sleep, on the other
hand, this circulation is abolished and the cornea is at risk of suffocation.
This is avoided by episodes of REM (rapid eye movement) which stir the