Tears are secreted by the lacrimal glands, more than 95% of which are secreted by the main lacrimal gland, with a small percentage of tears coming from the secondary lacrimal gland. Tear fluid forms a tear film on the surface of the eye. Tear fluid is a weakly alkaline clear liquid, pH 7,2, composed of mucus, plasma and lipids. Water accounts for most of it, solid components include protein, sugar and inorganic salts, also includes lysozyme, many kinds of immunoglobulin, complement, lactoferrin and other complex components; lipid accounts for 1,4%, scattered on the surface of the plasma to form a lipid film, which can reflect light and reduce tear evaporation, etc. Tear secretion includes basic secretion and reflex secretion, the former secretion activity is not innervated, day and night; the latter is innervated by sympathetic and parasympathetic nerves, etc., but collectively by local or systemic material or mental stimulation, through the nerve reflex to produce tear secretion effect, a large number of tears, the formation of tear flow, eyelid closure pressure increases, when tears along the tear point, tear duct, tear sac, nasolacrimal duct pathway flow Although the secretion of tears is continuous, its basic secretion is small, less than 1μl per minute on average. It is only when there is a large amount of tear production or when the tear duct is obstructed that a large amount of tears can overflow from the eye surface. The functions of tears are: 1) to form a tear film on the eye surface to prevent foreign bodies such as dust, smoke and microorganisms from directly attacking the eye surface; 2) to mechanically flush and clean the eye surface and tear ducts to remove foreign bodies and metabolic products, shed cells, etc.; 3) to contain a variety of anti-microbial substances such as immunoglobulins, complement, lysozyme and lactoferrin, which perform non-specific and specific immune functions to kill pathogenic microorganisms and can defend the ocular surface from pathogenic microorganisms; 4, moisten the ocular surface, maintain the normal function of the cornea, moisten the eyelid and conjunctival interface for lid and ball movement; the tear film, especially its surface lipid layer, constitutes a smooth optical interface and is an important component of the eye’s visual function; nourish the corneal and conjunctival epithelial cell layers to provide them with oxygen and sugar and other nutrients.