Tear ducts are found between the eyes and the nose, and serve the purpose of tear drainage and express extreme emotion in most species.
Artificial tear ducts were included in Soong-type androids. (Star Trek Generations)
The tears of female Elasians caused men's hearts to be enslaved if they came in contact with their flesh. (TOS: "Elaan of Troyius")
Spock once claimed that Klingons lacked tear ducts, though Kahless the Unforgettable was said to have filled an ocean with his tears, and at least one Klingon, Kurn, has produced tears. (Star Trek VI: The Undiscovered Country; TNG: "Birthright, Part II"; DS9: "Sons of Mogh")