FRANCISCO J. MORALES OLIVAS answers:
The conjunctiva (a mucosa that covers the eyelids and the anterior part of the eye) is constantly lubricated by tears, whose functions are to ease the sliding of the eyelids and prevent dryness in the part of the eye that is exposed to air. Tears are composed of water (98%), salt, and small amounts of protein and fat. They also contain lysozyme, an antibacterial agent discovered by Fleming.
Tears are generated in the tear glands and eliminated through the inner corner of the eye towards the nostrils. When tears are excessive, it is not possible to eliminate all of them, which causes the action we know as crying. The tears that moisten the eyes constantly are basal tears. When intense light, a foreign body, a liquid or an irritating gas reaches the ocular mucosa, a greater production of tears called reflex tears occurs. Their function is to wash and protect the eye. This reflex secretion may also be caused by stimuli in the mouth or nose.
When the reason for tearing is emotional, it is usually accompanied by sobbing and can be caused by pleasant or unpleasant situations. These so-called psychological or emotional tears are produced by a different mechanism, since they do not respond to a local reaction, but to a reaction originating from the limbic system (a part of the CNS related to emotion control) and driven to the tear gland by parasympathetic nerves. The composition of these emotional tears is also different, since they contain small amounts of hormones such as prolactin, encephalin or adrenocorticotropin, related to a sensation of well-being.
The function of crying is not well known and Darwin is credited with the opinion that it only lubricates the eye. Today it is accepted that it has an intrapersonal and an interpersonal function. Intrapersonal, because after the discomfort produced by an emotion, crying produces a sensation of well-being, and interpersonal because it favours the psychological connection with other people, both as empathy and as a defense when manifesting weakness.
Some neurological diseases produce an increase in crying due to an unknown mechanism, but it improves with low doses of antidepressants. There are also people who are unable to cry, sometimes following trauma or mental illness. A recent study shows that people with difficulty with crying are less empathetic and have avoidance behaviors more frequently, but their feelings of well-being are no different from those who usually cry. It is important to note that, although all mammals produce tears, only humans cry, which would prove its psychological component and probably its social function.
Francisco J. Morales Olivas, president of the Valencian Medical Institute