"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 (figure 3.5). 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 dacryocyst 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 the 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.\n\nUse 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.\n\nThe watery material inside of the eyeball is called vitreous humor. Unlike the conjunctiva, it is protected from contact with the environment and is almost always sterile, with no normal microbiota (figure 3.6). Some microbes live on the conjunctiva of the human eye, but the vitreous humor is sterile.\n\nThe conjunctiva is a frequent site of infection of the eye; like other mucous membranes, it is also a common portal of entry for pathogens. Inflammation of the conjunctiva is called conjunctivitis, although it is commonly known as pinkeye because of the pink appearance in the eye. Infections of deeper structures, beneath the cornea, are less common (figure 3.7). Conjunctivitis occurs in multiple forms. It may be acute or chronic. Acute purulent conjunctivitis is associated with pus formation, while acute hemorrhagic conjunctivitis is associated with bleeding in the conjunctiva. The term blepharitis refers to an inflammation of the eyelids, while keratitis refers to an inflammation of the cornea; keratoconjunctivitis is an inflammation of both the cornea and the conjunctiva, and dacryocystitis is an inflammation of the lacrimal sac that can often occur when a nasolacrimal duct is blocked.\n\nInfections leading to conjunctivitis, blepharitis, keratoconjunctivitis, or dacryocystitis may be caused by bacteria or viruses. Allergens, pollutants, or chemicals can also irritate the eye and cause inflammation of various structures. Viral infection is a more likely cause of conjunctivitis in cases with symptoms such as fever and watery discharge that occurs with upper respiratory infection and itchy eyes.\n\nCondition: Acute purulent conjunctivitis. Description: conjunctivitis with purulent discharge. Causative agent(s): bacterial (Haemophilus, Staphylococcus).\n\nCondition: Acute hemorrhagic conjunctivitis. Description: involves subconjunctival hemorrhages. Causative agent(s): viral (Picornaviridae).\n\nCondition: Acute ulcerative blepharitis. Description: infection involving eyelids; pustules and ulcers may develop. Causative agent(s): bacterial (Staphylococcal) or viral (herpes simplex, varicella-zoster, etc.).\n\nCondition: Follicular conjunctivitis. Description: inflammation of the conjunctiva with nodules, which are dome-shaped structures that are red at the base and pale on top. Causative agent(s): viral (adenovirus and others); environmental irritants.\n\nCondition: Dacryocystitis. Description: inflammation of the lacrimal sac often associated with a plugged nasolacrimal duct. Causative agent(s): bacterial (Haemophilus, Staphylococcus, Streptococcus).\n\nCondition: Keratitis. Description: inflammation of cornea. Causative agent(s): bacterial, viral, or protozoal; environmental irritants.\n\nCondition: Keratoconjunctivitis. Description: inflammation of cornea and conjunctiva. Causative agent(s): bacterial, viral (adenoviruses), or other causes, including dryness of the eye.\n\nCondition: Nonulcerative blepharitis. Description: inflammation, irritation, and redness of the eyelids without ulceration. Causative agent(s): environmental irritants; allergens.\n\nCondition: Papillary conjunctivitis. Description: inflammation of the conjunctiva; nodules and papillae with red tops develop. Causative agent(s): environmental irritants; allergens."