Though small in surface area, the eyelid is complex in its structure and function. When the anatomy of the upper eyelid is described, it is helpful to divide it into distinct segments from the dermal surface inward. These segments include the following structures (Fig 1-25; see also Figs 1-26 through 1-34):
skin and subcutaneous connective tissue
muscles of protraction (orbicularis oculi muscle, the main protractor)
muscles of retraction (levator palpebrae superioris, Müller muscle, capsulopalpebral fascia, inferior tarsal muscle)
Figure 1-24 Landmarks of the external eye.
(Illustration by Christine Gralapp.)
Eyelid skin and subcutaneous connective tissue
The eyelid skin, the thinnest in the body, contains fine hairs, sebaceous glands, and sweat glands. A superior eyelid crease is present near the upper border of the tarsus, where the levator aponeurosis establishes its first insertional attachments. In many individuals of Asian descent, there are few attachments of the levator aponeurosis to the skin near the upper tarsal border, and the superior eyelid crease is minimal or absent. Figure 1-26 depicts the 2 major racial variations in eyelid anatomy.
The loose connective tissue of the eyelid contains no fat. Blood or other fluids can accumulate beneath the skin and cause rapid and dramatic swelling of the eyelids.
The eyelid margin contains several important landmarks (Fig 1-27). A small opening, the punctum of the canaliculus, presents medially at the summit of each lacrimal papilla. The superior punctum, normally hidden by slight internal rotation, is located more medially than the inferior punctum, which is usually apposed to the globe and is not normally visible without eversion.
Figure 1-25 Eyelid anatomy: schematic cross section of the upper and lower eyelid area.
(Modified from Stewart WB. Surgery of the Eyelid, Orbit, and Lacrimal System. Ophthalmology Monograph 8, vol 2. San Francisco: American Academy of Ophthalmology; 1994:23, 85. Illustration by Cyndie C.H. Wooley.)
Along the entire length of the free margin of the eyelid is the delicate gray line (or intermarginal sulcus), which corresponds histologically to the most superficial portion of the orbicularis oculi muscle, the muscle of Riolan, and to the avascular plane of the eyelid. Anterior to this line, the eyelashes (or cilia) arise, and behind this line are the openings of the meibomian (or tarsal) glands just anterior to the mucocutaneous junction.
The eyelashes are arranged in 2 or 3 irregular rows along the anterior dermal edge of the eyelid margin. They are usually longer and more numerous on the upper eyelid than on the lower one. The margins contain the glands of Zeis, which are modified sebaceous glands associated with the cilia, and the glands of Moll, which are apocrine sweat glands in the skin (see Fig 1-27; Table 1-2).
Figure 1-26 Racial variations in eyelid anatomy. Variant I (left): the orbital septum inserts onto the levator aponeurosis above the tarsus. Variant II (Asian, right): the orbital septum inserts onto the levator aponeurosis between the eyelid margin and the superior border of the tarsus, and there are fewer aponeurotic attachments to the skin.
(Modified with permission from Katowitz JA, ed. Pediatric Oculoplastic Surgery. Philadelphia: Springer-Verlag; 2002.)
Muscle of protraction: orbicularis oculi muscle
The orbicularis oculi muscle, the main protractor of the eyelid, is arranged in several concentric bands around the palpebral fissure and can be divided into orbital and palpebral (preseptal and pretarsal) parts (Fig 1-28). Innervation occurs by CN VII (the facial nerve). The orbital part inserts in a complex way into the medial canthal tendon and into other portions of the orbital margin and the corrugator supercilii muscle. The orbital part acts as a sphincter and functions solely during voluntary closure of the eye.
The palpebral orbicularis oculi muscle functions both voluntarily and involuntarily in spontaneous and reflex blinking. The preseptal and pretarsal portions unite along the superior palpebral furrow. The pretarsal orbicularis muscle adheres firmly to the tarsus; a portion of it attaches to the anterior lacrimal crest and the posterior lacrimal crest (sometimes called the Horner muscle) and plays a role in tear drainage. Orbicularis fibers extend to the eyelid margin, where there is the small bundle of striated muscle fibers called the muscle of Riolan (Fig 1-29; see also Fig 1-27B).
The orbital septum is a thin sheet of connective tissue that encircles the orbit as an extension of the periosteum of the roof and the floor of the orbit (Fig 1-30). Superiorly, the septum is attached firmly to the periosteum of the superior half of the orbital margin, at the arcus marginalis. It passes medially in front of the trochlea and continues along the medial margin of the orbit, along the margin of the frontal process of the maxillary bone, and onto the inferior margin of the orbit. Centrally, the orbital septum attaches to the aponeurosis of both the upper and lower eyelids. The septum delimits the anterior or posterior spread of edema, inflammation, or blood. Clinical examples include preseptal cellulitis, orbital cellulitis, and retrobulbar hemorrhage.
Figure 1-27 Anatomical landmarks of the lower eyelid margin. A, The gray line, or inter marginal sulcus, is visible between the bases of the cilia and the orifices of the meibomian glands. The lower eyelid has been slightly everted to clearly expose the inferior lacrimal punctum. B, Cross section of the lower eyelid margin.
(Illustrations by Christine Gralapp.)
Table 1-2 Glands of the Eye and Adnexa
Figure 1-28 The 3 parts of the orbicularis oculi muscle. A, Orbital, preseptal, and pretarsal. Note the relationship of the orbicularis oculi with the frontalis, depressor supercilii, and procerus muscles. B, The corrugator supercilii muscle with a segment of the orbital portion of the orbicularis oculi muscle removed.
(Modified with permission from Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy. 2nd ed. Philadelphia: Elsevier/Saunders; 2011, Figs 8-12, 8-13.)
Figure 1-29 Lacrimal drainage system. A, Superficial extensions of the orbicularis oculi muscle. B, Deep head of the orbicularis oculi muscle; superficial components are reflected.
(Part A reproduced with permission from Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy. Philadelphia: Saunders; 1994. Part B reproduced with permission from Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy. 2nd ed. Philadelphia: Elsevier/Saunders; 2011, Fig 9-3.)
Posterior to the septum lie the orbital (preaponeurotic) fat pads, 2 behind the superior septum and 3 behind the inferior septum (see Fig 1-30).
Figure 1-30 Orbital septum. A, The orbital septum arises from the periosteum of the bones of the orbital margin and inserts on the aponeurosis of the upper and lower eyelids. B, Preaponeurotic fat pads.
(Modified with permission from Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy. 2nd ed. Philadelphia: Elsevier/Saunders; 2011, Figs 8-8, 8-9.)
Muscles of retraction: upper eyelid
In the upper eyelid, the retractors are the levator palpebrae superioris muscle with its aponeurosis and the Müller muscle (superior tarsal muscle).
Levator palpebrae superioris muscle The levator palpebrae superioris muscle originates from the lesser wing of the sphenoid bone (see Fig 1-25). The body of the levator muscle overlies the superior rectus as it travels anteriorly toward the eyelid (Fig 1-31). The muscle itself, which is 40 mm long, is innervated by the superior division of CN III, and its action can lift the upper eyelid 15 mm.
Figure 1-31 Levator palpebrae superioris muscle. Note the levator muscle’s transition into the aponeurosis at the Whitnall ligament (A) and the aponeurosis passing through the pretarsal orbicularis muscle to the eyelid skin (B).
(Modified with permission from Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy. 2nd ed. Philadelphia: Elsevier/Saunders; 2011, Figs 7-11, 8-15.)
The Whitnall (superior transverse) ligament is formed by a condensation of tissue surrounding the levator muscle (Fig 1-32; see also Fig 1-31). It provides support for the upper eyelid and surrounding tissues. At the Whitnall ligament, the levator muscle transitions into the aponeurosis anteriorly and the Müller (superior tarsal) muscle posteriorly. The Whitnall ligament is also where the levator muscle’s anterior–posterior vector changes to superior–inferior, toward the aponeurosis.
The levator aponeurosis, the tendon of the levator muscle, is 14–20 mm in length and has many attachments to the eyelid and surrounding orbit (see Figs 1-31, 1-32). Anteriorly, it passes through the orbicularis oculi muscle and inserts subcutaneously to produce the superior eyelid crease (see Fig 1-26). Posteriorly, the levator aponeurosis inserts into the surface of the tarsus. The aponeurosis forms its firmest attachments on the anterior aspect of the tarsus, approximately 3 mm superior to the eyelid margin. The aponeurosis also inserts into the trochlea of the superior oblique muscle and into the fibrous tissue bridging the supraorbital foramen/notch. The lateral horn of the aponeurosis divides the lacrimal gland into orbital and palpebral lobes and inserts at the lateral orbital tubercle. The medial horn inserts at the posterior lacrimal crest. Aponeurotic attachments also exist with the conjunctiva of the upper fornix and the orbital septum.
Figure 1-32 Levator aponeurosis and the Whitnall ligament. Note the medial and lateral horns of the aponeurosis and the suspensory ligament of Lockwood.
(Modified with permission from Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy. Philadelphia: Saunders; 1994.)
Müller muscle The Müller (superior tarsal) muscle originates from the undersurface of the levator palpebrae superioris muscle in the upper eyelid. This smooth muscle is innervated by the sympathetic nervous system, and its action is responsible for 1–2 mm of upper eyelid lift. The Müller muscle attaches to the upper border of the upper tarsus and to the conjunctiva of the upper fornix (see Fig 1-31B).
Muscles of retraction: lower eyelid
In the lower eyelid, the retractors are the capsulopalpebral fascia, which is analogous to the levator aponeurosis in the upper eyelid, and the inferior tarsal muscle. The inferior tarsal muscle arises from the capsulopalpebral head of the inferior rectus muscle in the lower eyelid. Like the Müller muscle, the inferior tarsal muscle is smooth muscle, but it is much weaker. It attaches to the lower border of the lower tarsus.
The inferior equivalent to the Whitnall ligament is the suspensory ligament of Lockwood, a fusion of the sheath of the inferior rectus muscle, the inferior tarsal muscle, and the check ligaments of the medial and lateral rectus muscles (see Fig 1-32). This ligament provides support for the globe and the anteroinferior orbit.
Figure 1-33 Posterior view of the eyelids with the palpebral fissure nearly closed. Note the meibomian (tarsal) glands with their short ducts and orifices. The palpebral conjunctiva has been removed to show these glands in situ.
(Modified with permission from Snell RS, Lemp MA. Clinical Anatomy of the Eye. Boston: Blackwell; 1989.)
The tarsal plates consist of dense connective tissue, not cartilage. They are attached to the orbital margin by the medial and lateral canthal tendons (see Figs 1-25, 1-31B). Although the upper and lower tarsal plates are similar in width (29 mm) and thickness (1 mm), the height of the upper tarsus (11 mm) is almost 3 times greater than that of the lower tarsus (4 mm).
The meibomian glands (also called tarsal glands) are modified holocrine sebaceous glands that are oriented vertically in parallel rows through the tarsus (Fig 1-33; see also Figs 1-26, 1-27). Their distribution and number within the eyelid can be observed by infrared imaging of the eyelid (Fig 1-34). A single row of 30–40 meibomian orifices is present in the upper eyelid, but there are only 20–30 orifices in the lower eyelid. Oil (meibum) from meibomian orifices forms a reservoir on the skin of the eyelid margin and is spread onto the tear film with each blink. Alterations in meibomian gland lipid composition and secretion play a role in dry eye. Aging is associated with an alteration in the lipid profile of meibum and with meibomian gland loss.
Arita R, Itoh K, Inoue K, Amano S. Noncontact infrared meibography to document agerelated changes of the meibomian glands in a normal population. Ophthalmology. 2008;115(5):911–915.
Sullivan BD, Evans JE, Dana MR, Sullivan DA. Influence of aging on the polar and neutral lipid profiles in human meibomian gland secretions. Arch Ophthalmol. 2006;124(9):1286–1292.
The palpebral (tarsal) conjunctiva is a transparent vascularized membrane consisting of nonkeratinized stratified squamous epithelium that lines the inner surface of the eyelids. Continuous with the conjunctival fornices (cul-de-sacs), it merges with the bulbar conjunctiva (covering the anterior portion of the sclera) before terminating at the limbus (Fig 1-35). The conjunctiva is discussed further later in the chapter.
Figure 1-34 Infrared meibography image of the upper eyelid demonstrates normal meibomian gland architecture.
(Courtesy of Mina Massaro-Giordano, MD.)
Figure 1-35 The different parts of the conjunctiva are depicted in this photograph: limbus (Li), bulbar conjunctiva (BC), forniceal conjunctiva (FC), palpebral conjunctiva (PC), and marginal conjunctiva (MC). Additional structures shown are the caruncle (Ca) and the lacrimal punctum (LP).
(Courtesy of Vikram S. Brar, MD.)
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.