On an adult, lower and upper eyelids have the length of about 30 mms and take the shape of a bow from inner corners to outer corners. On the inner and outer corners lower and upper eyelids come together with about 30–40-degree angle. The coming together point of the outer side is slightly higher than the inner. While the upper eyelids cover the black structure called cornea with a 1 mm distance, lower eyelids forms a border with the cornea.
Eyelid skin is the thinnest skin on our bodies and it has no fatty tissue underneath. Instead, there is a muscle called orbicularis which helps us close our eyelids and make some mimics. Orbicularis is attached to the skin over it. So, as we open and close our eyes and make movements orbicularis stretches and loosens the skin over it. This causes wrinkles to form on the eyelid skin overtime.
The eyelashes on the free side of the eyelid are again positioned from inner side to outer side. This position of the eyelashes crucial in terms of not contacting the structures in the eye that are very limpid and sensitive. The inner area of the eyelid is covered with a thin, watery tissue called conjunctiva while the outer area is covered with skin. Conjunctiva and skin connects on a line right behind the eyelashes which is called green line. On some diseases that are seen on the eyelids if this structure becomes unfunctional, eyelashes starts to stink inside the eye and harms the sensitive tissues of the eye.
Our eyelids have a skeleton just like our bodies. The elements that make up that skeleton are a cartilaginous structure called tars and the tendons that attach tars to the eye socket bone from inner and outer sides. There are also various muscle, tendon and fat tissues on our eyelids. These tissues works in a balanced way and makes sure that our eyelids preserves its functionality. If any of these tissues we talked about becomes instable, the structure of the eyelid would become damaged.