Your eyeballs are wet because they are covered with living cells, and your cells will die if they are not kept moist. In the same way, your tongue and the other soft parts of the inside of your mouth are alive and wet. You have a complicated system that keeps your eyeballs wet with tears, and if it fails the result is ‘dry eye’ which can be painful. Glands just above your eyes make tears. Other glands at the edges of your eyelids produce oils that float on top of the watery tears and slow down evaporation, so your eyes stay wet even in the desert. There are drain holes near the inner corners of your eyes, where ‘used’ tears flow into the nose. When you cry these drains overflow.
The eye seems unusual because the rest of the outside of your body, the visible part of you, is covered with several layers of dead cells which are relatively dry. Only if you fall and graze your knee (or other parts), rubbing off the outer dry layers, do you see the wet inner living layers of your skin. A graze or other injury hurts; damage sensors trigger pain, which tells you to be more careful next time. If you are unfortunate enough to scratch your eyeball, particularly the central transparent part, the cornea, you will find this hurts even more. Damage there is very dangerous, for while a scar on your knee might spoil your beauty, a scar on the cornea could give you blurred vision from that eye for the rest of your life. However, if the damage is not so serious, then the living surface can regenerate and repair to be as good as new.
Not all eyes are wet; insect eyes are dry on the outside. The outer surface of insect eyes is a set of lenses made from transparent cuticle. Insects spend time cleaning their eyes by brushing them with a front leg, and can have eye cleaning reflexes like human blinking when anything gets on or near their eyes. Snakes have no eyelids to blink with, but have a transparent scale covering each eye, called a spectacle. Next time you see a snake, you can tell people that it had a pair of spectacles on its eyes. It is not so terrible if this scale gets scratched, because it is replaced every time the snake sheds its skin.
Ph.D. Oxford, UK
Structure of polymers (nylon) and biopolymers (silk, feathers)
Wife Sally, son Michael, daughter Jennifer
Scottish country dancing, science fiction, and sailing
Groton Elementary School
Girl Scouts, gymnastics, playing with friends