The skin is the outermost layer, and comprises the epidermis and the very thin dermis. The skin of the eyelids is the thinnest in the body and has many elastic fibers that allow it to be stretched during dissection and retraction. It is loosely attached to the underlying muscle; therefore, in contrast to most areas of the face, relatively large quantities of fluid may accumulate subcutaneously in this loose connective tissue. The skin derives its blood supply from the underlying perforating blood vessels of the muscles (see subsequent text).

The orbicularis oculi muscle, the sphincter of the eyelids, lies subjacent and adherent to the skin (see Fig. 2.3). This muscle completely encircles the palpebral fissure and extends over the skeleton of the orbit. It can therefore be divided into orbital and palpebral portions (see Fig. 2.4). The palpebral portion can be further subdivided into the pretarsal portion (i.e., the muscle superficial to the tarsal plates) and the preseptal portion (i.e., the muscle superficial to the orbital septum). The palpebral portion of the orbicularis oculi muscle is very thin in cross section, especially at the junction of the pretarsal and preseptal portions. The orbital portion of the orbicularis oculi muscle originates medially from the bones of the medial orbital rim and the medial canthal tendon. The peripheral fibers sweep across the eyelid over the orbital margin in a series of concentric loops, the more central ones forming almost complete rings. In the lower eyelid, the orbital portion extends below the inferior orbital rim onto the cheek and covers the origins of the elevator muscles of the upper lip and nasal ala. The orbital portion of the orbicularis oculi muscle is responsible for tight closure of the eye.

The preseptal portion of the orbicularis oculi muscle originates from the medial canthal tendon and lacrimal diaphragm and passes across the eyelid as a series of half-ellipses, meeting at the lateral canthal tendon. The upper and lower pretarsal muscles contribute to the lateral canthal tendon which extends approximately 7 mm before inserting lateral orbital tubercle. Medially, they unite to form the medial canthal tendon, which inserts on the medial orbital margin, the anterior lacrimal crest, and the nasal bones. The palpebral portion of the orbicularis oculi muscle functions to close the eye without effort, as in blinking. It also functions to maintain contact between the lower eyelid and the ocular globe.

The orbicularis oculi muscle is innervated laterally from the branches of the facial nerve that enter the muscle on its deeper surface. The blood supply to the orbicularis oculi muscle is from the external facial artery tributaries arising from the deep branches of the ophthalmic artery. These arterial branches form a marginal arcade, traversing between the tarsal plate and the muscle and giving rise to branches that perforate the substance of the muscle, the orbital septum, and the tarsal plate.

The orbital septum is a fascial diaphragm between the contents of the orbit and the superficial face (Figs. 2.1 and 2.5). It is usually denser laterally than medially, but varies considerably in thickness from one individual to another, and weakens with age, allowing the orbital fat pads to bulge onto the face. The orbital septum is a fascial extension of the periosteum of the bones of the face and orbit. It originates along the orbital rim for most of its extent. Laterally and inferolaterally, however, it arises from the periosteum 1 to 2 mm beyond the rim of the orbit. Therefore, it is necessary to dissect a few millimeters lateral and/or inferior to the orbital rim before incising the periosteum to prevent incising through the orbital septum.

The orbital septum of the lower eyelid inserts into the inferior margin of the lower tarsus. The tarsal plate of the lower eyelid is a somewhat thin, pliable, fibrocartilaginous structure that gives form and support to the lower eyelid (see Fig. 2.6A and B). The edge of the tarsus that is adjacent to the free border of the eyelid is parallel to the palpebral fissure, whereas the deeper (inferior) border is curved such that the tarsus is somewhat semilunar in shape.
It is also, of course, curved to conform to the outer surface of the eyeball. The inferior tarsus at approximately 4 to 5 mm is half the height of the superior tarsus (approximately 10 mm). The tarsal glands, sandwiched between the layers of fibrocartilage in the lower eyelid, are smaller than their upper eyelid counterpart, and exit on the eyelid margin near the lash follicles. The lashes are supported by their roots, which are attached to fibrous tissue on the tarsal plate and not in the orbicularis oculi muscle anterior to the tarsal plate. Laterally, the tarsal plate becomes a fibrous band that adjoins the structural counterpart from the upper eyelid, forming the lateral canthal tendon. Medially, the tarsal plate also becomes fibrous and shelters the inferior lacrimal canaliculus behind, as it becomes the medial canthal tendon.

Embedded within the tarsal plates are large sebaceous glands called the tarsal or Meibomian glands, whose ducts may be seen along the eyelid margin. A grayish line or a slight groove, which is sometimes visible between the lashes and the openings of the tarsal glands, represents the junction of the two fundamental portions of the eyelid: the skin and muscle on one hand and the tarsus (the plate of closely packed tarsal glands) and conjunctiva on the other. This junction indicates a plane along which the eyelid may be split into anterior and posterior portions with minimal scarring.

The conjunctiva that lines the inner surface of the eyelids is called the palpebral conjunctiva (Fig. 2.2). It adheres firmly to the tarsal plate, and as it extends inferiorly toward the inferior conjunctival fornix, it becomes more loosely bound. At the inferior conjunctival fornix, the conjunctiva sweeps onto the ocular globe to become the bulbar conjunctiva.