Jaundice (including hemolytic jaundice, hepatic jaundice, and obstructive jaundice) is the yellowing of the skin, mucous membranes, and sclera due to elevated levels of bilirubin in the serum. Normal bilirubin does not exceed 17.1 umol/L. When the bilirubin level is between 17.1 and 34.2 umol/L, it is not easily detectable clinically and is called invisible jaundice, but when it exceeds 34.2 umol/L, jaundice is visible to the naked eye, manifesting as yellowing of the skin, mucous membranes and sclera, but the first to appear is But the first to appear yellowish is the sclera of both eyes, why? This starts with the connective tissue. There are three types of fibers in connective tissue, the most numerous of which are called collagen fibers, which are called white fibers because of their white color; followed by elastic fibers, which are also called yellow fibers because their elastin binds strongly to bilirubin, and reticular fibers, which are also called silver-loving fibers because they can be stained with silver. The sclera of the human eye is mainly collagen fibers, so the sclera not only has a certain degree of toughness, but also is white. In addition, the sclera also contains a certain number of elastic fibers, and thus has a certain degree of elasticity. Under normal circumstances, the sclera is white and yellowish in appearance because the elastin in the elastic fibers can bind a small amount of bilirubin. When bilirubin metabolism disorder occurs, the level of bilirubin in the blood increases significantly, and the excessive bilirubin binds with the elastin in the sclera, turning it from white to yellow. Under normal circumstances, the sclera is white, but when jaundice suddenly turns yellow, the color contrast is very obvious and easy to notice. In fact, when jaundice occurs, the skin and oral mucosa may also turn yellow, but the color contrast with normal is not strong enough to attract attention.