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TodAy's VeTerinAry PrAcTice | July/August 2015 | tvpjournal.com obserVATions in oPhThAlmology Peer reviewed 72 FIGURE 2. Anatomy of the eye: Orbicularis oculi muscle (a), cilium (B), cornea (C), sclera (D), bulbar conjunctiva (e), palpebral conjunctiva (F), meibomian gland (G), levator palpebral tendon (h). Courtesy Dr. Lisa Wirth FIGURE 3. Anatomy of the eye: Gland of Moll ( a), gland of Zeiss ( B), orifce of meibomian gland (C), meibomian gland ( D), cartilage of third eyelid (e), gland of third eyelid (F), goblet cells ( G), conjunctival fornix (h). Courtesy Dr. Lisa Wirth The eyelids can be divided into 4 histologic layers (Figure 2): 1 1. outermost layer contiguous with the skin 2. orbicularis oculi muscle layer 3. Tarsal plate with stromal layer 4. innermost palpebral conjunctival layer. near the margins of both eyelids are meibomian glands, which form parallel rows of lobules containing duct openings that are visible at the eyelid margin. These ducts—gland orifces—are lined by keratinized stratifed squamous epithelium. The levator palpebrae superioris muscle, innervated by the oculomotor nerve, is the main muscle responsible for elevation of the superior eyelid. Tear Film The precorneal tear flm is classically depicted with 3 layers: 1-3 1. Outer lipid layer: An oily substance (polar and nonpolar lipids) produced by the meibomian glands that prevents evaporation of aqueous tears; the meibomian glands are arranged linearly within the eyelid and secrete compounds (esters, hydrocarbons, free esterols, and fatty acids) that are fluid at body temperature. 2. Middle aqueous layer: Produced by the lacrimal gland and gland of the nictitans, and functions as lubrication and nutrition for the avascular cornea; it also provides a fushing mechanism for the corneal surface and has antibacterial properties, containing substances such as secretory igA, lysozymes, lactoferrin, lipocalin, and interleukins, that are necessary for ocular immunity. 3. Inner mucin layer: Produced by conjunctival goblet cells (Figure 3), which are apocrine secretory cells found in highest density at the level of the conjunctival fornices, and composed of immunoglobulins, glycoproteins, salts, enzymes, and leukocytes; helps provide a smooth refractive surface over the cornea and anchors the aqueous tear flm to the corneal epithelium to prevent desiccation.