Crying@Everything2.com: "THE SCIENCE OF TEARS
Tears flow through tear ducts in glands located in the upper eyelids. There are three types of tears. All higher animals produce basal tears to regularly lubricate the eyeballs to guard against dust, and protect against infection. The tears we experience when we cut an onion are reflex tears, which are released at the detection of smoke, foreign objects or irritants entering the eye, such as the propenyl sulphuric acid contained in onions. The third type, emotional tears, are a little harder to explain. What is physiologically different about these tears is that they have a distinctive chemical make-up, containing more hormones and protein.
Crying occurs when the tear ducts produce too many tears to be taken back into the nose (where they usually end up) and they brim over and run down our cheeks. We also produce tears when we laugh and yawn, which is due to unnatural facial contortions squeezing our lacrimal glands.
Like many things, we take the ability to cry for granted. Owing to a disorder of the tear glands, people with dry eye usually caused by Sjogren's syndrome are unable to cry and have to use artificial tears regularly to keep the eyes lubricated. Brain damage to the frontal lobe can cause the opposite problem, pathological crying. It is a disorder of emotional expression in which the part of the brain that regulates the execution of emotional tears in response to a situational stimulus is damaged. The patient is no longer able to associate their feelings of happiness or sadness with the act of crying and will weep uncontrollably for hours at a time. In a bizarre case of crying disorder, a patient researchers call 'Eloise', developed a rare condition called alternating unilateral lachrymation. Each eye would produce tears separately. Reportedly, Eloise cried from one eye when she thought about her mother, "
