The lacrimal duct includes the upper and lower lacrimal dots, the upper and lower lacrimal ducts, the common lacrimal duct, the lacrimal sac, and the nasolacrimal duct. The lacrimal duct is located at the upper and lower lid margins of the medial canthus and is 0.2 to 0.3 mm in diameter, and is the opening for the lacrimal duct. There is a valve at the lower end of the nasolacrimal duct, called the Hasner valve, which is closed before birth and gradually opens after birth. Under normal conditions, tears are produced from the lacrimal glands, the paracolic glands on the conjunctiva and the cupped cells, which flush across the immediate surface and then converge to the medial corner of the eye, where they enter the lacrimal duct, the common lacrimal duct, the tear sac and the nasolacrimal duct in turn, under the suction of the lacrimal duct, and finally flow into the nasal cavity. Under normal circumstances, adults secrete 0.5-1.0 ml of tears every 16 hours, and these tears, in addition to moistening the eye and part of the evaporation, the remaining are led by the tear duct into the nasal cavity.