The transparent front part of the eye that covers the iris and pupil is called Cornea. Cornea contributes a lot to the focusing power of the eye. That means, light has to pass through it without obstruction. To do that it has to remain completely clear. Consequently, to remain transparent, it can’t have any impurities nor can it have any blood vessels – that would have made it less transparent. Every organ needs oxygen to run the cell processes with the energy that comes by oxidizing nutrients contained in the cells. To receive oxygen, they need to have access to blood. Since Cornea does not have any blood vessels, it cannot receive oxygen from blood.
So what does it do to stay alive?
It absorbs oxygen directly from the air through diffusion. Oxygen gets dissolved in the tears and then diffuses across the cornea. However, the amount of diffused oxygen is so less that it is just enough for only the cornea cells. This can’t be supplied to other parts of the body. And this is exactly the reason you would get yourself killed if you plug your nose and your mouth, expecting your eye would keep you alive by breathing in oxygen. In a sense, you could call it breathing.
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