Eye is an important organ and is well protected from external agents of threat like flying insects, dust, extreme light etc. Eyelids two in numbers, lower and upper one reflexly closes when an external dust, insect tries to enter the eye. Closure of the eye lid is quick enough to ward off the danger.
If anyhow, extraneous particle enters, lachrymal glands, washes it off by its Lachrymal secretions. They are also known as tear glands. They are very rich in lysosome hydrolytic enzymes which dissolves organic substances and wash off the debris. Lachrymal glands secrete the liquids which reach the lacrimalduct.
The trigeminal nerve carries impulses of touch, pain, thermal impulses to the brain from the eye.
It is worth mentioning how an eye function. The light falls on an object and illuminates it. The light rays reflected from the object falls on the transparent cornea and passes on through aqueous humor present in anterior chamber. Depending on the brightness of light, the pupil either dilates or contracts. The light fromthe object passes through the pupil, reaches the lens and then passes through vitreous humor, falls on the retina. The nerve cells of the retina are stimulated. These send nerve impulses through the optic nerve to the brain which recognizes the figure or object.
During the passage the light rays from the object travels through cornea, acqueous humor, pupil, lens, vitreous humour, and refract. The pupil plays an important role in admitting the enough light. The lens adjust itself so that the image of the object falls exactly on the retina. Then the neural impulses reach the brain and the colour or object is recognized.
With the advancement of age the lens looses its elasticity. Hence the lens cannot become either thin or thick in which case the image falls far before or beyond the retina. To compensate the loss of accommodative power one will be advised to wear convex/concave lenses.