Why does the clear front of your eye have no blood in it?
The window at the front of your eye is alive, yet not a single drop of blood reaches it.
The cornea is avascular - it has no blood vessels - because vessels would scatter light and blur your sight. Instead it takes most of its oxygen by diffusion straight from the air through your tear film, and gets nutrients from the fluid behind it.
When you wear contact lenses too long and your eyes feel dry and tired, you are partly suffocating the cornea by blocking the air it breathes from.
Clear vision costs the cornea its blood supply, so it breathes from the open air through your tears.
It explains why eye doctors warn against sleeping in regular contacts: you are cutting off the cornea's air all night.
No blood, no blur - the cornea breathes the air like a window that needs to stay see-through.
Learn the idea and practice English at the same time.