The eyelids are thin structures comprised of skin, muscle, and fibrous tissue that serve to protect the eye. The great mobility of the lids is possible because the skin is among the thinnest of the body. Beneath the skin lies a very thin fibroadipose layer through which septa pass and closely adhere to the orbicularis oculi muscle. The orbicularis oculi muscle consists of striated muscle innervated on its deep surface by the facial nerve (cranial nerve VII). The muscle functions to close the lids and is divided into orbital, preseptal, and pretarsal divisions. The orbital portion, which functions primarily in forcible closure, is a circular muscle with no temporal insertion. The preseptal and pretarsal muscles are more involved in involuntary lid movements (blink). They have superficial and deep medial heads that participate in lacrimal pump function (see Lacrimal Apparatus).
The lid margins are supported by the tarsi, rigid fibrous plates connected to the orbital rim by the medial and lateral canthal tendons. The lateral canthus lies 1–2 mm higher than the medial. The orbital septum originates from the orbital rim and functions as an important barrier between the eyelids and the orbit. In the upper lid, the septum attaches to the levator aponeurosis which then joins the tarsus. Behind the septum lies the medial and the central or preaponeurotic fat pad, a helpful surgical landmark. In the lower lid, the septum joins the inferior border of the tarsus. The lower lid has three anatomically distinct fat pads beneath the orbital septum.
Deep to the fat lies the levator muscle complex—the principal retractor of the upper eyelid—and its equivalent, the capsulopalpebral fascia in the lower lid. The levator muscle originates in the apex of the orbit and is innervated by the oculomotor nerve (cranial nerve III). As it enters the eyelid, it forms an aponeurosis that attaches to the lower third of the superior tarsus. In the lower lid, the capsulopalpebral fascia originates from the inferior rectus muscle and inserts on the inferior border of the tarsus. It serves to retract the lower lid in downgaze. The superior (Müller's muscle) and inferior tarsal muscle form the next layer, which is adherent to the conjunctiva. These sympathetically innervated muscles are also lid retractors. Conjunctiva lines the inner surface of the lids. It is continuous with that of the eyeball and contains glands essential for lubrication of the ocular surface.
The upper lid is larger and more mobile than the lower. A deep crease usually present in the mid position of the upper lid in Caucasian populations represents an attachment of levator muscle fibers. The crease is much lower or is absent in the Asian eyelid. With age, the thin skin of the upper lid tends to hang over the lid crease and may touch the eyelashes. Aging also thins the orbital septum and reveals the underlying fat ...