The cornea is the convex, highly transparent membrane at the very front of the eye. It covers the iris, pupil and anterior chamber and provides most of the refractive power for the eye. Together with the refractive power of the lens, light can be accurately focused on the retina to form an image. The cornea has very sensitive nerve endings, and if an external object comes in contact with the cornea, the eyelids will involuntarily close to protect the eye. The cornea is completely transparent, located at the front of the eye, and is transversely oval in shape. The cornea, which makes up 1/6 of the outer wall of the eye, and the sclera together form the outer wall of the eye. The mean horizontal diameter of the cornea is 11.04 mm in adult males and 10.05 mm in females, and the mean vertical diameter is 10.13 mm in males and 10.08 mm in females. The corneal diameter of children over 3 years of age is close to that of adults. Histologically, the cornea can be divided into 5 layers from anterior to posterior, namely the epithelium, the anterior elastic layer, the stroma, the posterior elastic layer and the endothelium. The cornea is transparent and non-vascular, but has rich sensory nerve tissue, with a certain shape, size, thickness and radius of curvature, and also has a certain refractive power, which plays an important role in the refractive system of the eye. In addition, together with the scleral tissue, it provides special protection to the delicate ocular contents.