Crying has been one of the interesting riddles - what is the function of crying? What is the evolutionary perspective on crying? Is crying good for you or bad for you?
As it turns out, there are no easy answers. Darwin already was not completely sure what do with crying. Quote from Darwin (1872)
(p. 172) In considering how far this view is probable, we should bear in mind that the eyes of infants have been acted on in this double manner during numberless generations, whenever they have screamed; and on the principle of nerve-force readily passing along accustomed channels, even a moderate compression of the eyeballs and a moderate distension of the ocular vessels would ultimately come, through habit, to act on the glands. We have an analogous case in the orbicular muscles being almost always contracted in some slight degree, even during a gentle crying-fit, when there can be no
(p. 173) distension of the vessels and no uncomfortable sensation excited within the eyes.
Moreover, when complex actions or movements have long been performed in strict association together, and these are from any cause at first voluntarily and afterwards habitually checked, then if the proper exciting conditions occur, any part of the action or movement which is least under the control of the will, will often still be involuntarily performed. The secretion by a gland is remarkably free from the influence of the will; therefore, when with the advancing age of the individual, or with the advancing culture of the race, the habit of crying out or screaming is restrained, and there is consequently no distension of the blood-vessels of the eye, it may nevertheless well happen that tears should still be secreted. We may see, as lately remarked, the muscles round the eyes of a person who reads a pathetic story, twitching or trembling in so slight a degree as hardly to be detected. In this case there has been no screaming and no distension of the blood-vessels, yet through habit certain nerve-cells send a small amount of nerve-force to the cells commanding the muscles round the eyes; and they likewise send some to the cells commanding the lacrymal glands, for the eyes often become at the same time just moistened with tears. If the twitching of the muscles round the eyes and the secretion of tears had been completely prevented, nevertheless it is almost certain that there would have been some tendency to transmit nerve-force in these same directions; and as the lacrymal glands are remarkably free from the control of the will, they would be eminently liable still to act, thus betraying, though there were no other outward signs, the pathetic thoughts which were passing through the person's mind.
Since then many researchers have studied the mechanics and chemistry of crying and tears. It is still a complicated story though ... is it satisfying? Can it cope with sentimental crying? What about crying for joy?
One of the key researchers on crying is Ad Vingerhoets from Tilburg University. Here is a brief article from the Guardian that refers also to Ad's recent book Why only humans weep.
I have also tried my hand at the topic, but then - Ad's books are probably the best place to start ...
In the meanwhile - Cmdr Hadfield answers the question what happens to tears in space ...
Darwin, C. (1872). The expression of the emotions in man and animals. London, UK: Murray.
Kappas, A. (2009). Mysterious tears. The phenomenon of crying from the perspective of social neuroscience. In: Thorsten Fögen (ed.), Tears in the Graeco-Roman World (pp. 419-438) , Berlin & New York,: DeGruyter.