A drop of clear salty liquid secreted from the glands in a person's eye when they cry or when the eye is irritated.
Crying is as simple as blinking your eyes. It is a self-conscious act that we have barely any control over. We accept this unusual act into our lives as common place. But is it? Amongst any other species we are the only living organisms to be able to cry. Not cry as in wail, let out a yelp, but to actually produce liquid tears in vast amounts.
Why is this so you may ask. Well it is due to an adaptation lost somewhere far off in the past amongst our ancestors; Though this statement can raise some controversy. For example, was it an adaptation, or some kind of mutation within a cell, or a mutation in the amount of chemicals secreted by the brain causing us to have an emotional overdrive with a side effect of tears?
To us the brain is a very abstract and unknown realm. Certain things can be deduced such as the parts of the brains that are active during certain activities. For example when one is experiencing sadness the activity within the Amygdala increases in rate. Thus allowing us to deduce that the Amygdala has something to do with sadness, or is where the emotion is created. Though the Amygdala, a definite candidate for the base line of why we cry, is not the only candidate. The Hypothalamus also is active when one is experiencing an emotion (e.g. sadness).
This activity can be caused by chemical reactions within the brain. These chemicals such as serotonin, and norepinephrine act as neurotransmitters within the brain. The increased production of these chemicals could be what is causing the increased activity in the Amygdala and the Hypothalamus when one is experiencing sadness. When the production of these chemicals are low, less of it is transmitted between neurons causing us to enter a state of depression.
Now what does all this have to do with the chemical process of tear production and the process of crying, and WHY we cry?
…. I have no idea.