There are many theories about the formation of monolids: 1. By comparing the ocular anatomy of Caucasian and Mongolian populations, scholars have found that the Caucasian levator muscle, which powers the eye opening movement, has many muscle bundles that reach into the various layers of the lower part of the eyelid, including the orbicularis oculi muscle and the skin, so that while opening the eye, the levator muscle drives the skin of this part of the eyelid upward, thus forming This is what we call a double eyelid. In people with single eyelids, this structure is missing. 2. The structure of our eyelids is complex. The reason why the eyelids do not deform is because we have a structure called the lid plate, which is hard and similar to cartilage and plays a role in supporting and stabilizing the eyelids. In our daily life, you may notice that it is difficult to turn your eyelid up when you order eye drops, but once it is turned up it is fixed in a certain position, and the half-moon shape is then the outline of the lid. In people with double eyelids, the skin in front of the lid is so tightly bonded to the lid that when the eye is opened, the skin above the lid folds back to form a fold, whereas in people with single eyelids, the skin in front of the lid is more loosely bonded to the lid. 3. The upper lid is more swollen in people with monolids, and this bloating is due to the richness of the fat in the orbital diaphragm. The orbital diaphragm is supposed to limit the protrusion of orbital fat into the eyelid. However, the level at which the orbital diaphragm protrudes into the eyelid varies from person to person. If it is too low in the eyelid, it will prevent the formation of double eyelids. 4. The levator muscle travels below the orbital septum and eventually fuses with the lowest point of the orbital septum, and the height of this fusion point determines the height of the double eyelid fold in Caucasians. If this fusion point is very low, then a very low double eyelid fold or monocular eyelid will be formed. By understanding these theories, you will also then understand the theoretical basis of eyelid surgery, the technical term for which is blepharoplasty. The purpose of blepharoplasty is: 1. to remove excess fat that protrudes into the upper eyelid; 2. and more importantly, to fix the muscles or skin in front of the lid in the right place to the levator muscle or its extension.