The eyelid skin is the thinnest in the body, averaging about 0.5 mm in thickness. It consists of the same basic layers as in other parts of the body. The epidermis is composed of four layers. The outermost layer, the stratum corneum, is a thin layer of dead cells filled with keratin that flake off the surface (Figure 3.3). The stratum granulosum is the first layer of living cells with a granular appearance and produce keratin. The stratum spinosum is a thick layer with a spiny appearance from fine filaments that join the cells together. The stratum basalis is the lowermost layer of the epidermis where cell production occurs and is characterized by mitoses.

Beneath the epidermis is the dermis composed of a papillary and reticular layer.² The dermis contains dense connective tissue, collagen, and elastic fibers, melanocytes, nerves, and blood vessels. Several dermal appendages lined with epithelium lie within the dermis. These include hair follicles associated with an arrector pili muscle attached to the dermal-epidermal junction. Apocrine sweat glands of Moll are coiled glands in the deep dermis that empty via a long ductule into the uppermost portion of the hair follicle. Apocrine glands secrete by cellular decapitation with the apical portion of the secretory cell mixed with sialomucin producing a more viscous secretion with cellular debris. They are concentrated along the eyelid margins. Eccrine sweat glands are also present in the dermis but they are not associated with the hair follicle. They open directly onto the epidermal surface via a long straight ductule. Eccrine glands secrete a clear fluid composed of water, salts, glycogen, and sialomucin.

Sebaceous glands contain epithelium that is an outgrowth of the external root sheath of the hair follicle. These are holocrine glands that shed the entire epithelial cell along with secretory products of complex oils, fatty acids, wax, and cholesterol esters called sebum. Usually two sebaceous glands, termed glands of Zeis, are associated with each follicle and empty their secretions directly into the follicle through short ducts. The hair follicle along with the sebaceous and Moll glands forms the pilosebaceous unit.

Beneath the dermis is a subcutaneous layer of fat and connective tissue. Subcutaneous fat is very sparse beneath the preseptal portion of the eyelid skin, and absent from the more distal pretarsal portions. In the skin of the eyelid are also found other structures that can be the focus of disease processes. On the subconjunctival side of the eyelid, these structures include the accessory lacrimal glands of Krause and Wolfring beneath the conjunctiva (Figure 3.4) and the meibomian glands which are modified sebaceous glands within the tarsal plates (Figure 3.5).²

The eyelashes protect the eye from particulate matter and wind from impacting the corneal surface. In the adult, there are about 70 to 80 lashes on the lower eyelid margin and 90 to 160 in the upper eyelid. The growth phase is short so that they fall and are replaced without attaining a length greater than 10 to 12 mm. Unlike hairs on the body, eyelashes are not associated with arrector pili muscles.² Many disorders can affect the eyelashes, including trichiasis, distichiasis, hypertrichosis, and poliosis, as well as several neoplastic diseases such as trichoepithelioma and trichofolliculoma.

The orbicularis oculi is a complex striated muscle sheet that lies just below the skin (Figures 3.5, 3.6, 3.7). It is divided anatomically into three contiguous parts—the orbital, preseptal, and pretarsal portions (Figure 3.8). The orbital portion overlies the bony orbital rims. It arises from insertions on the frontal process of the maxillary bone, the orbital process of the frontal bone, and the common medial canthal tendon. Its fibers pass around the orbital rim to form a continuous ellipse without
interruption at the lateral palpebral commissure and insert just below their points of origin.

The palpebral portion of the orbicularis muscle overlies the mobile eyelid from the orbital rims to the eyelid margins. The muscle fibers sweep circumferentially around each lid as a half ellipse, fixed medially and laterally at the canthal tendons. Although this portion forms a single anatomic unit in each eyelid, it is customarily further divided topographically into two parts, the preseptal and pretarsal orbicularis.²

The preseptal portion of the muscle is positioned over the orbital septum in both upper and lower eyelids, and its fibers originate perpendicularly along the upper and lower borders of the medial canthal tendon. Fibers arc around the eyelids and insert along the lateral horizontal raphé. The pretarsal portion of the muscle overlies the tarsal plates. Its fibers originate from the medial canthal tendon via separate superficial and deep heads, arch around the lids, and insert onto the lateral canthal tendon and raphé. Contraction of these fibers aids in the lacrimal pump mechanism.² Medially the deep heads of the pretarsal fibers fuse to form a prominent bundle of fibers, Horner muscle, that runs just behind the posterior limb of the canthal tendon. It inserts onto the posterior lacrimal crest. Horner muscle helps maintain the posterior position of the canthal angle, tightens the eyelids against the globe during eyelid closure, and may aid in the lacrimal pump mechanism.³

The orbital septum is a thin fibrous multilayered membrane anatomically beginning at the arcus marginalis along the orbital rim and represents a continuation of the orbital fascial system (Figure 3.9). The orbital septum originates from the arcus marginalis of the frontal bone and consists of two layers. The outer (superficial) layer descends just posterior to the orbicularis muscle to interdigitate with the levator aponeurosis via loose connective tissue. The inner (deep) layer follows the superficial one initially, but at the levator aponeurosis, it changes direction and continues posteriorly with the levator sheath.⁴,⁵ The point of insertion is usually about 3 to 5 mm above the tarsal plate but can be as much as 10 to 15 mm.⁶ In the lower eyelid the septum fuses with the capsulopalpebral fascia several millimeters below tarsus, and the common fascial sheet inserts onto the inferior tarsal edge.⁷,⁸