45-55% of them are corneas. The osmotic pressure of tears is 302-306mosm/kg. The basic tear secretion flow was 1.2 ul/min, and the turnover rate was 16%/min. After stimulation, the tear flow increased, generally 50- 100 times more than the original, reaching 40-50ul.
In the past 50 years, people have put forward many views on the structure of tears, but the structure of tear film is still controversial, and the understanding of it is deepening and changing. Among them, the three-layer structure of tears proposed by Wolff is the most classic, that is, the mucilage layer on the epithelial surface (confirmed by electron microscope scanning), the thicker water layer containing protein and water-soluble molecules in the middle, and the lipid layer on the surface. Generally speaking, the total thickness of tears is 7mm, the mucus layer is 0.02 to 0.05mm, the middle water layer is 4-7mm, and the outer lipid layer is 0.01mm. In fact, there is no clear boundary between the three layers of tears, and they are all gradual.
In addition to keeping the eyeball moist, tear film also plays a very important role in improving the refractive system of the eye. The cornea is one of the most important parts of the refractive system of the eyeball, but the cornea without tear film is not smooth, and then things will become blurred. When there is a tear film on the cornea, tears can fill the cornea, making it smooth and seeing things more clearly. The tear film on the eyeball is unevenly distributed, but it is thickest at the cornea and thinnest near the eyelid margin, so a theoretical "concave-convex lens" is formed in front of the cornea to improve the refractive performance of the eye. When the upper and lower eyelids are closed, the tear film gradually thickens. When the upper and lower eyelids are closed to 2mm, the tear film forms a slender "concave-convex cylindrical lens", through which things can become clearer. In addition, the tear film also provides necessary nutrition for the cornea.