In the sagittal section, the lower eyelid consists of at least four distinct layers - the skin and subcutaneous tissue, the orbicularis oculi muscle, the tarsus (upper 4 to 5 mm) or orbital septum, and the conjunctiva (Fig. 2-1).
Skin. The outermost layer is the skin, comprising the epidermis and the very thin dermis. The skin of the eyelids is the thinnest skin 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; hence, in contrast to the situation in 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 muscle vessel (see subsequent text).
Muscle. Subjacent and adherent to the skin lies the orbicularis oculi muscle, the sphincter of the eyelids (Fig. 2-2). This muscle completely encircles the palpebral fissure and extends over the skeleton of the orbit. In can thus be divided into orbital and palpebral portions (Fig. 2-3). The palpebral portion can be further subdivided into the pretarsal portion (the muscle superficial to the tarsal plates) and the preseptal portion (the muscle superficial to the orbital septum). The palpebral portions of the orbicularis oculi muscle are very thin ion cross section, especially at the junction of the pretarsal and preseptal portion. The orbital portion of the orbicularis oculi muscle takes its origin 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 muscle of the upper lip and nasal ala. The orbital portion of the orbicuralis oculi muscle is responsible for the 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 lid as a series of half-ellipses, meeting at the lateral canthal tendon. The upper and lower pretarsal muscles contribute to a common lateral canthal tendon about 7 mm from the lateral orbital tubercle, where it inserts. Medially, they unite to form the medial canthal tendon, which inserts on the medial orbital margin, the anterior crest, and nasal bones. The palpebral portions of the orbicularis oculi muscle function to close the eye without effort, as in blinking. It also functions to maintain contact between the lower lid and the ocular globe.
The orbicularis oculi muscle receives innervation from the lateral side, from branches of the facial nerve that enter the muscle on its deep surface. The blood supply to the orbicularis oculi muscle is from the external facial artery tributaries that come from deep branches of the ophthalmic artery. These arterial branches form a marginal arcade, traversing between the tarsal plate and muscle and giving rise to branches perforating the substance of the muscle, orbital septum, and tarsal plate.
Orbital Septum/Tarsus. The orbital septum is a fascial structure that forms a diaphragm between the contents of the orbit and the superficial face (Figs. 2-1 and 2-4). It is usually more dense laterally than medially, but varies considerably in thickness from one individual to another. The orbital septum is a fascial extension of the periosteum of the bones of the face and orbit. It takes its origin 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. Thus, it is necessary to dissect a few mm lateral and/or inferior to the orbital rim before incising the periosteum to prevent incising through the orbital septum.
The orbital septum in the lower eyelid inserts onto 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 (Figs. 2-5 and 2-6). The edge of the tarsus adjacent to the free border of the lid parallels the palpebral fissure, while the deeper (inferior) border is curved so
surface of the eyeball. The inferior tarsus is considerably smaller than the superior one, the greatest height of the inferior being about 4 to 5 mm and that of the superior about 10 mm. The tarsal glands sandwiched between the layer of fibrocartilage in the lower eyelid are smaller than their upper eyelid counterpart, and exit on the lid margin near the lash follicles. The lashes are supported by their roots, attached to fibrous tissue on the tarsal plate, 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 lid, 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 lid margin. A grayish line or a slight groove 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 indicates a plane along which the lid may be split into anterior and posterior portion with minimal scarring.
Palpebral Conjunctiva. The conjunctiva that lines the inner surface of the eyelids is called the palpebral conjunctiva (see Fig. 2-1). 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.
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