The lids are thin structures comprised of skin, muscle, and fibrous tissue that serve to protect the eye (see Figure 1–22). 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 [CN] VII). The muscle functions to close the lids and is divided into orbital, preseptal, and pretarsal divisions. The orbital portion is a circular muscle with no temporal insertion and is thought to function primarily in forcible closure. The preseptal and pretarsal muscles are believed to be involved in involuntary blink. They have superficial and deep medial heads that participate in lacrimal pump function (see Section 4.3 Lacrimal Apparatus).
The lids are supported by the tarsi, rigid collagenous plates that are attached to the orbital rim via 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 lids 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 in the upper lid lies the levator palpebrae superioris (LPS)—the principal retractor of the upper lid—and its equivalent, the capsulopalpebral fascia in the lower lid. The LPS is a striated muscle that originates in the apex of the orbit and is innervated by the oculomotor nerve (CN III). As it enters the lid, it forms an aponeurosis that attaches to the lower third of the superior tarsal plate. A crease usually present in the mid position of the upper lid in Caucasians represents an attachment of levator aponeurosis fibers to the more superficial layers. The crease is much lower or is absent in the Asian lid. 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) and inferior tarsal muscle form the next layer, which is adherent to the conjunctiva. These sympathetically innervated smooth muscles are also lid retractors. Conjunctiva lines the inner surface of the lids and forms the blind cul-de-sacs of the upper and lower fornices as it reflects onto the eye. The conjunctiva contains glands essential for lubrication of the ocular surface.