Each of the upper and lower folds of SKIN which cover the EYE when closed.

Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye (Figure 14.13). The eyelids, with lashes at their leading edges, help to protect the eye from abrasions by blocking particles that may land on the surface of the eye. The inner surface of each lid is a thin membrane known as the palpebral conjunctiva. The conjunctiva extends over the white areas of the eye (the sclera), connecting the eyelids to the eyeball. Tears are produced by the lacrimal gland, located just inside the orbit, superior and lateral to the eyeball. Tears produced by this gland flow through the lacrimal duct to the medial corner of the eye, where the tears flow over the conjunctiva, washing away foreign particles.

Figure 14.13 The Eye in the Orbit The eye is located within the orbit and surrounded by soft tissues that protect and support its function. The orbit is surrounded by cranial bones of the skull.

Movement of the eye within the orbit is accomplished by the contraction of six extraocular muscles that originate from the bones of the orbit and insert into the surface of the eyeball (Figure 14.14). Four of the muscles are arranged at the cardinal points around the eye and are named for those locations. They are the superior rectus, medial rectus, inferior rectus, and lateral rectus. When each of these muscles contract, the eye moves toward the contracting muscle. For example, when the superior rectus contracts, the eye rotates to look up. The superior oblique originates at the posterior orbit, near the origin of the four rectus muscles. However, the tendon of the oblique muscles threads through a pulley-like piece of cartilage known as the trochlea. The tendon inserts obliquely into the superior surface of the eye. The angle of the tendon through the trochlea means that contraction of the superior oblique rotates the eye laterally. The inferior oblique muscle originates from the floor of the orbit and inserts into the inferolateral surface of the eye. When it contracts, it laterally rotates the eye, in opposition to the superior oblique. Rotation of the eye by the two oblique muscles is necessary because the eye is not perfectly aligned on the sagittal plane. When the eye looks up or down, the eye must also rotate slightly to compensate for the superior rectus pulling at approximately a 20-degree angle, rather than straight up. The same is true for the inferior rectus, which is compensated by contraction of the inferior oblique. A seventh muscle in the orbit is the levator palpebrae superioris, which is responsible for elevating and retracting the upper eyelid, a movement that usually occurs in concert with elevation of the eye by the superior rectus (see Figure 14.13).

The extraocular muscles are innervated by three cranial nerves. The lateral rectus, which causes abduction of the eye, is innervated by the abducens nerve. The superior oblique is innervated by the trochlear nerve. All of the other muscles are innervated by the oculomotor nerve, as is the levator palpebrae superioris. The motor nuclei of these cranial nerves connect to the brain stem, which coordinates eye movements.

Figure 14.14 Extraocular Muscles The extraocular muscles move the eye within the orbit.

Although the eye and skin have distinct anatomy, they are both in direct contact with the external environment. An important component of the eye is the nasolacrimal drainage system, which serves as a conduit for the fluid of the eye, called tears. Tears flow from the external eye to the nasal cavity by the lacrimal apparatus, which is composed of the structures involved in tear production. The lacrimal gland, above the eye, secretes tears to keep the eye moist. There are two small openings, one on the inside edge of the upper eyelid and one on the inside edge of the lower eyelid, near the nose. Each of these openings is called a lacrimal punctum. Together, these lacrimal puncta collect tears from the eye that are then conveyed through lacrimal ducts to a reservoir for tears called the lacrimal sac, also known as the dacrocyst or tear sac.

From the sac, tear fluid flows via a nasolacrimal duct to the inner nose. Each nasolacrimal duct is located underneath the skin and passes through the bones of the face into the nose. Chemicals in tears, such as defensins, lactoferrin, and lysozyme, help to prevent colonization by pathogens. In addition, mucins facilitate removal of microbes from the surface of the eye.

The surfaces of the eyeball and inner eyelid are mucous membranes called conjunctiva. The normal conjunctival microbiota has not been well characterized, but does exist. One small study (part of the Ocular Microbiome project) found twelve genera that were consistently present in the conjunctiva. These microbes are thought to help defend the membranes against pathogens. However, it is still unclear which microbes may be transient and which may form a stable microbiota.

Use of contact lenses can cause changes in the normal microbiota of the conjunctiva by introducing another surface into the natural anatomy of the eye. Research is currently underway to better understand how contact lenses may impact the normal microbiota and contribute to eye disease.

The watery material inside of the eyeball is called the vitreous humor. Unlike the conjunctiva, it is protected from contact with the environment and is almost always sterile, with no normal microbiota.

The healthy human eyelid is a remarkable but all too frequently neglected structure. Its anatomy and physiology are adapted to a number of specific functions, collectively crucial to the health of the eye, that include protection of the vulnerable ocular surface from physical insult and providing a lubricated and hydrated environment for movement of the eyelid and a smooth ocular surface for optimal visual acuity. The International Dry Eye Workshop in 2007 defined the “lacrimal functional unit” as an integrated structure comprising the lacrimal glands, ocular surface (cornea, conjunctiva, and meibomian glands), eyelids, and the sensory and motor nerves that connect them. This concept gives this functional system the prominence it deserves, given its important role in maintaining the health of the exterior optical surface, and thereby vision.

The cornea is the most fragile external structure of the body and relies entirely on the eyelid and adjacent structures to maintain its patency; a cornea directly and permanently exposed to the environment will rapidly succumb to epithelial defects, scarring, vascularization, and infection, and is experienced by the patient as irritation, pain, loss of visual acuity and, eventually, loss of sight. As with many ophthalmic disorders, even small degrees of dysfunction can have very significant impacts on quality of life and the ability to carry out normal daily tasks.

The healthy eyelid comprises a lamellar structure with fine skin on the outer surface and conjunctiva on the inner surface. Between these layers lie a number of muscle groups that control the movement of the eyelid, and in particular the blink reflex, as well as the tarsal plate that comprises the meibomian glands.

The tear film can be considered a substructure of the eyelid and forms a highly important layer between the eyelid and the surface of the eye and between the exterior environment and the eye surface. The tear film is predominantly aqueous in nature and is formed from the secretions of the lacrimal glands. However, although minor in quantity, the lipids in the tear film formed from the sebaceous secretions of the meibomian glands are crucial to its function. The tear film provides protective, lubricant, nutritional, and antimicrobial functions, as well as playing an important role in visual acuity. The physiology of the tear film is relatively complex, comprising a very thin (less than 100 nM) outer lipid layer that overlies an aqueous layer enriched with water-soluble proteins, electrolytes, carbohydrates, and other materials; the innermost layer is also aqueous and contains mucins. The aqueous layers are considerably thicker than the lipidic layer (around 4 μm). The meibomian glands are responsible for production and secretion of the lipid and protein components of the tear film, the function of which is to stabilize and, most importantly, to prevent evaporation of the tear film. The meibomian glands are adapted sebaceous glands located on the edge of each eyelid although, unlike sebaceous glands in other parts of the body, each is not specifically associated with a hair (or eyelash) follicle. Each meibomian gland is formed from a long central duct with chains of secretory acini arranged around it in a radial pattern. The glands are arranged in a single row extending the width of the eyelid. The proteinaceous lipidic material produced, meibum, is secreted from a terminal duct onto the posterior lid margin and expressed on the ocular surface during eyelid movements. During sleep and periods of reduced blinking (eg, during visual concentration), meibum accumulates in the ducts of the gland and can be expressed in quantity by forced blinking. Production of meibum is modulated by a very large number of hormonal and neural influences, including androgens, progestin, estrogen, corticotrophin-releasing hormone, and substance P, as well as by the autonomic nervous system. The relative importance of these systems in the physiology of the meibomian gland is not well understood, but clearly offers the opportunity for a rich control system.

Your eyelids help protect your eyes. When you blink, your eyelids spread moisture over your eyes. Blinking also helps move dirt or other particles off the surface of the eye. You close your eyelids when you see something coming toward your eyes. This can help protect against injuries.

Like most other parts of your body, your eyelids can get infected, inflamed, or even develop cancer. There are also specific eyelid problems, including

• Eyelids that turn in or out
• Eyelids that droop
• Abnormal blinking or twitching

Treatment of eyelid problems depends on the cause.