Everyone starts life with that first cry, thats a fact. As you grow up, you still produce about five to ten ounces of tears a day, even if you never or rarely cry. Tears are produced mostly by glands located just above the outer corner of each eye; blinking spreads the tears across the eye surface. Tears drain into the tiny openings in the eyelids, and then through ducts to the nasal cavity, where you either swallow them or they become part of nasal fluid—which is why you also get stuffy when you cry. If production of tears out paces drainage, the excess will spill from the lower eyelids, as when you cry.
Besides lubricating the eye and flushing away debris, tears serve the same purpose in the eye that blood does elsewhere in the body, delivering oxygen and nutrients and removing waste products. Tears also improve the image that forms on the retina by smoothing any irregularities on the surface of the eyeball—that’s why people with dry eyes may have blurry vision. Tears are a defence mechanism as well: their antibodies, enzymes and other immune components help protect eyes against micro-organisms.
There are three types of tears. Continuous tears are produced for basic eye function, such as lubrication. Reflex tears occur when the eye is exposed to excessive light, cold, wind, a foreign body or irritating gas (as from cut onions). Psychogenic tears are shed for emotional reasons.
When cut, onions release an enzyme that causes the formation of an airborne chemical that irritates the eyes. New Zealand researchers have produced a tearless onion by inhibiting the gene that makes this enzyme.