The cornea is clear like glass, but it is full of living cells. Flat, quiet cells called keratocytes help keep it that way.
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Most of the cornea's thickness is its middle layer. It is made of many thin sheets of collagen fibers, stacked like pages in a book.
Keratocytes live between the sheets. They are flat, with thin arms that reach out to touch their neighbors. Most of the time they are very quiet.
Keratocytes make and care for the collagen fibers. The fibers in each sheet run in neat, straight rows, and they are all spaced about the same. That neatness lets light pass straight through.
If the cornea gets hurt, some keratocytes wake up and help with repairs.
Press Shine a light on the 3D model and watch light pass through the neat layers.
The white of your eye is made of collagen too. But there the fibers are jumbled, so it looks white instead of clear!