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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 ( [link] ).

A cross section of the eye. The large spherical center is the vitreous humor. The layer surrounding this is the retina. A projection out of the back of the eye is the optic nerve. A region on the retina just above the optic nerve is the fovea. At the front of the eye is the lens. In front of this is a space labeled pupil. The colored region around the pupil is the iris. The cornea is the covering in front of the iris and pupil. The conjunctiva is a mucous membrane on the eye.
Some microbes live on the conjunctiva of the human eye, but the vitreous humor is sterile.

Infections of the eye

The 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 ( [link] ). 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 ( [link] ); 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.

a) photo of an eyelid being pulled back to show a red are. B) A photo of inflamed eyelids. C)A photo of an eye with a cloudy cornea.
(a) Conjunctivitis is inflammation of the conjunctiva. (b) Blepharitis is inflammation of the eyelids. (c) Keratitis is inflammation of the cornea. (credit a: modification of work by Lopez-Prats MJ, Sanz Marco E, Hidalgo-Mora JJ, Garcia-Delpech S, Diaz-Llopis M; credit b, c: modification of work by Centers for Disease Control and Prevention)

Infections leading to conjunctivitis, blepharitis, keratoconjunctivitis, or dacryocystitis may be caused by bacteria or viruses, but 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. [link] summarizes some common forms of conjunctivitis and blepharitis.

Types of Conjunctivities and Blepharitis
Condition Description Causative Agent(s)
Acute purulent conjunctivitis Conjunctivitis with purulent discharge Bacterial ( Haemophilus , Staphylococcus )
Acute hemorrhagic conjunctivitis Involves subconjunctival hemorrhages Viral ( Picornaviradae )
Acute ulcerative blepharitis Infection involving eyelids; pustules and ulcers may develop Bacterial ( Staphylococcal ) or viral (herpes simplex, varicella-zoster, etc.)
Follicular conjunctivitis Inflammation of the conjunctiva with nodules (dome-shaped structures that are red at the base and pale on top) Viral ( adenovirus and others); environmental irritants
Dacryocystitis Inflammation of the lacrimal sac often associated with a plugged nasolacrimal duct Bacterial ( Haemophilus, Staphylococcus , Streptococcus )
Keratitis Inflammation of cornea Bacterial, viral, or protozoal; environmental irritants
Keratoconjunctivitis Inflammation of cornea and conjunctiva Bacterial, viral (adenoviruses), or other causes (including dryness of the eye)
Nonulcerative blepharitis Inflammation, irritation, redness of the eyelids without ulceration Environmental irritants; allergens
Papillary conjunctivitis Inflammation of the conjunctiva; nodules and papillae with red tops develop Environmental irritants; allergens
  • How does the lacrimal apparatus help to prevent eye infections?

Key concepts and summary

  • Human skin consists of two main layers, the epidermis and dermis , which are situated on top of the hypodermis , a layer of connective tissue.
  • The skin is an effective physical barrier against microbial invasion.
  • The skin’s relatively dry environment and normal microbiota discourage colonization by transient microbes.
  • The skin’s normal microbiota varies from one region of the body to another.
  • The conjunctiva of the eye is a frequent site for microbial infection, but deeper eye infections are less common; multiple types of conjunctivitis exist.

Fill in the blank

The ________ is the outermost layer of the epidermis.

stratum corneum

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The mucous membrane that covers the surface of the eyeball and inner eyelid is called the ________.

conjunctiva

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Short answer

What is the role of keratin in the skin?

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What are two ways in which tears help to prevent microbial colonization?

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Which label indicates a sweat gland?

A diagram of skin. i – a vase shape in the center of skin with a long projection out the top. iv – the long projection from i. iii – a small vase shape attached to i. ii – a coiled structure in the center with a tube leading out.
(credit: modification of work by National Cancer Institute)
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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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