The eye is shielded at the front by the eyelids, upper and lower. At their outer margins these have short hairs, the eyelashes, which are a protective device. They are not there purely for adornment, as the advertisements would have you believe. The lashes are very important in helping to keep foreign particles away from the sensitive eyeball. Likewise, the eyebrows serve a similar function, and help keep fluid, sweat and moisture from draining into the eye.
The eye is covered by a fine membrane called the conjunctiva. This is reflected above and below so as to cover completely the inner surfaces of the upper and lower lids. It is very sensitive, as anyone who has had a foreign object, no matter how small, lodge here, knows.
In an effort to keep the surface of the eyes clean and clear at all times, tears are produced by the lachrymal gland, and wash across the front surface of the eyeball.
Excessive amounts spill over the lower lid margins and onto the face, as in the act of crying. Smaller quantities are removed from the surface by the nasolacrimal duct that commences at the inner, lower corner of the eye. It carries fluid into the nasal passageways. This is why with a watery eye, or after crying, the nasal passages seem to be filled with fluid. In fact they are – the fluid being the tears. Any foreign material on the eye surface will similarly cause fluid to flow, and this is an automatic process designed to remove foreign invaders and keep the surface clean and intact.
The movement of the eyes is governed by a series of small muscles attached to its outer part, and which are under nervous system control.
There is more than one reason why two eyes are present. The most important is that if one is damaged or destroyed, there is still one left to assist the body.