Nature's way of protecting eyes
The eyes have their own cleaning equipment. The lacrimal glands produce a steady stream of moisture -- tears, to flush away dust and other foreign material. The eyelids act just like the windshield wipers of a car. With each blink, it sweeps away the dust to the corner of the eyes. Usually one blinks three to six times a minute, but more when tired. This keeps the cornea moist and clean. The tears also contain a potent microbe-killer called lysozyme, which guards against infective bacteria.
The eye muscles are among the body's strongest ones and in an average day they work about 100,000 times to bring objects into sharp focus. Still, the eyes try to rest as much as possible. Each blink is a tiny resting period. Also the pair of eyes complement each other at times. While one carries 90 percent of the workload, the other rests. And then there is a reversal. (Try doing a piece of work with only one eye open. How does it feel?)
The eyes themselves are placed in a bony cave with protruding cheekbones and forehead to act as shock absorbers for direct blows. Also the supersensitive eye nerves are quick to shield the eyes. The slightest chance of anything falling into it, the eyelids snap shut.