Eye

The eye is a sense organ involved in vision. It consists of the eyeball and the optic nerve. The eye gathers light, converts the light into neural activity, and sends this information to the brain, where it is processed and interpreted as an image.

Top panel: outer anatomy of the eye; bottom panel: inner anatomy of the eye, including cornea, lens, ciliary body, retina, choroid, optic nerve, and vitreous humor. Source: National Cancer Institute.

Other Names

Synonyms

• Oculus
• Special sense organ of vision
• Photosensory organ

Related terms

• Eyeball
• Bulbus oculi
• Orb
• Globe

Description

Anatomy of the eye. Source: National Eye Institute (NIH).

Eyeball

The structure of the eyeball is three concentric layers around the light-refracting media:

• The layers (coats or tunica) of the eyeball
  • The outermost layer is the fibrous layer of the eyeball (the corneosclera).
    • The cornea is the clear, dome-shaped surface that covers the front of the eye.
    • The sclera is the white part of the eye.
  • The middle layer is the vascular layer of the eyeball (the uvea).
    • The iris, the colored part of the eye, is a contractile diaphragm that determines the size of the pupil.
    • The ciliary body contains the ciliary muscle and produces aqueous humor.
    • The choroid (the area of the eye containing
  • The inner layer is the retina. Parts of the retina include the macula, fovea, and optic disc.
    • The nonvisual retina are the double-layered epithelial tissues that line the inner surface of the iris and ciliary body.
    • The optic part of the retina lines the inner surface of the choroid. It has two layers.
      • The retinal pigmented epithelium is the outer layer of the optic part of the retina.
      • The neural retina (often casually called "the retina") is the inner layer of the optic part of the retina. It contains rod and cone photoreceptors, which detect light, and neurons that transmit the light-induced signals to the brain for interpretation.
• Refracting media of the eye
  • The aqueous humor is present in both chambers of the anterior segment of the eyeball
  • The lens (the crystalline humor) is transparent, biconvex, elastic and avascular.
  • The vitreous humor is a clear gel within the hyaloid membrane and is located in the posterior segment of the eyeball.

Optic Nerve

The optic nerve (cranial nerve II) contains the axons of retinal ganglion cells that terminate in the diencephalon and brainstem. Although counted as a cranial nerve, the optic nerve is embryologically and histologically related to the forebrain.

Accessory Visual Structures

The eye's movements are controlled by several muscles in the orbit, called extraocular muscles.

Other accessory structures include the eyebrow, eyelids, conjunctiva and lacrimal apparatus.

Development of the Eye

Evaginations of the forebrain create optic vesicles that induce overlying ectoderm to form lens discs (optic placodes). The lens discs become internalized and form the lenses. The surface epithelium that covers the internalized developing lenses forms the anterior epithelium of the cornea and conjunctiva.
The optic vesicles indent to form two-layer optic cups, which develop into the two epithelial layers of the retina (the inner layer of the eyeball), including the neural retina. Mesenchyme surrounding the optic cup forms the uvea and corneosclera (the middle and outer layers of the eyeball). It is believed that most of this mesenchyme is of neural crest rather than mesodermal origin.

Transverse section of chick embryo head (48 hr) showing optic cups and invaginating lens discs. Gray's Anatomy (1918) figure 863 (originally from Duval). Source: Wikimedia Commons

Transverse section of chick embryo head (53 hr) showing optic cups and developing lenses. Gray's Anatomy (1918) figure 864 (originally from Duval). Source: Wikimedia Commons

Horizontal section of eye of 18-day-old rabbit embryo. Gray's Anatomy (1918) figure 866 (originally from Kölliker). Source: Wikimedia Commons

Sagittal section of human embryo eye (about 6 weeks). Gray's Anatomy (1918) figure 867 (originally from Kollmann). Source: Wikimedia Commons

Role of Eye in the Body

The eyes are a key component of the visual system. This system is one of five making up the sensory system.

How the Eyes Work

The eye and the brain work together to produce vision. Light enters the eye and strikes the cornea, which bends—or refracts—the incoming light onto the lens. The lens further refocuses that light onto the retina, a layer of light-sensing cells lining the back of the eye that starts the translation of light into vision. The retina converts the light rays into impulses that are sent through the optic nerve to the brain, which interprets them as images. The brain normally processes images from both eyes.

Cornea

Because the cornea is as smooth and clear as glass, but is also strong and durable, it helps the eye in two ways:

• It helps to shield the rest of the eye from germs, dust, and other harmful matter. The cornea shares this protective task with the eyelids, the eye socket, tears, and the sclera, or white part of the eye.
• The cornea acts as the eye's outermost lens. It functions like a window that controls and focuses the entry of light into the eye. The cornea contributes between 65%&endash;75% of the eye's total focusing power.

The refractive process is similar to the way a camera takes a picture. The cornea and lens in the eye act as the camera lens. The retina is similar to the film. If the image is not focused properly, the film (or retina) receives a blurry image.

The cornea also serves as a filter, screening out some of the most damaging ultraviolet (UV) wavelengths in sunlight. Without this protection, the lens and the retina would be highly susceptible to injury from UV radiation.

Lens

The lens is a clear part of the eye that helps to focus light, or an image, on the retina. The lens lies behind the iris and the pupil. It works much like a camera lens. It focuses light onto the retina at the back of the eye, where an image is recorded. The lens also adjusts the eye's focus, letting us see things clearly both up close and far away. The lens is made of mostly water and protein. The protein is arranged in a precise way that keeps the lens clear and lets light pass through it.

Retina

The retina is the light-sensitive layer of tissue that lines the inside of the eye and sends visual messages through the optic nerve to the brain. In a normal eye, light passes through the transparent lens to the retina. Once it reaches the retina, light is changed into nerve signals that are sent to the brain. The lens must be clear for the retina to receive a sharp image.

The eye sees by using the retina. The retina is a net of nerve cells located at the back of each eye that enable vision. The retina is made up of two main kinds of cells: rods and cones. Cones are responsible for color vision and work best in bright light. Rods work in low light, but are unable to distinguish color. The cones are located mainly in the center of the retina, while the rods are located around the edges. This is why our vision at night is less colorful. This is also why we can see better in very dim light when we look slightly away from what we are looking at. In complete darkness (an absolute absence of light), neither the cones nor the rods can function, and we see absolutely nothing. However, in dim light, we can see amazingly well, mainly because of the rods in the retina. A rod can register a single photon of light. This means that a human with perfect vision can see a lit match several miles away on a clear night.

Optic nerve

The optic nerve is a bundle of nerve fibers that connects the retina in the eye to the brain. The retina is the light-sensitive tissue at the back of the eye. Nerve impulses travel from the retina to the brain along the optic nerves.

Macula

The macula is located in the center of the eye's light-sensitive tissue called the retina. The macula provides the sharp, central vision we need for reading, driving, and seeing fine detail. Within the area of the macula is the fovea which, in the normal eye, has the highest concentration of cones, and thus provides the sharpest vision.

Vitreous

Most of the eye's interior is filled with vitreous, a gel-like substance that fills about 80% of the eye and helps it maintain a round shape. The vitreous contains millions of fine fibers that are attached to the surface of the retina.

Trabecular meshwork

The eye is constantly producing fluid that drains out little by little through a part of the eye called the trabecular meshwork.

Diseases of the Eye

Diseases occurring in the eyes include the following:

• Conditions primarily affecting vision:
  • Amblyopia
  • Blindness, which can result from a great many conditions
  • Hyperopia (Farsightedness)
  • Myopia (Nearsightedness)
  • Night blindness

• Other conditions
  • Age-related macular degeneration (AMD)
  • Anophthalmia and microphthalmia
  • Blepharitis
  • Blepharospasm
  • Cataract
  • Glaucoma
  • Macular hole
  • Macular pucker
  • Retinal detachment
  • Retinitis pigmentosa
  • Retinoblastoma
  • Retinopathy of prematurity

Diseases that occur throughout the body and may also have an effect on the eyes include:

• Diabetic retinopathy
• Histoplasmosis
• Usher syndrome
• Waardenburg syndrome
• Wegener's granulomatosis

Several infections can also affect the eye directly, such as:

• Herpes simplex virus
• Herpes zoster (shingles)
• Many different bacteria, viruses and parasites causing
  • Keratitis—infection of the cornea
  • Conjunctivitis— infection of the conjunctival tissues (this includes pinkeye)
  • Endophthalmitis—infection of the vitreous, and adjacent structures
  • Periorbital infections—infection of the structures around the eyes

Eye Trauma and injury

• Corneal abrasion
• Corneal foreign body
• Sub tarsal foreign body
• Chemical burn
• Penetrating trauma
• Arc eye
• Sub-conjunctival haematoma
• Traumatic mydriasis

Symptoms

Symptoms of eye disease vary, but may include the following:

• Blurred vision
• Dry eye
• Jaundice—yellowing of the white part of the eyes
• Eye pain
• Photophobia—sensitivity to light
• Red eye or conjunctivitis—redness or bloodshot appearance of the white of the eyes
• Clouding of the cornea

Related Professions

• An ophthalmologist is a licensed medical doctor (MD) who has completed advanced training in ophthalmology after medical school. Ophthalmologists diagnose and treat disorders of the visual system and perform surgery on the eye.
• An optometrist is a licensed doctor who has completed training and earned a degree as a doctor of optometry (OD). Optometrists diagnose and treat disorders of the visual system; they refer patients who need surgery to ophthalmologists.
• An optician fits people for glasses, contact lenses, low vision aids and ocular prosthetics. He or she works from a prescription from an ophthalmologist or optometrist.

Other eye care professionals include:

• Ophthalmic Medical Practitioners, a British term for an eye MD who has not completed an ophthalmology specialization) and whose practice is roughly similar to an American optometrist's.
• Ocularists, who create and fit artificial eyes and must learn their skill through apprenticeship, as there are no schools that teach it. There are only several hundred such professionals in the world.
• Orthoptists, eye-movement specialists who complete a two-year fellowship after their bachelor's degree
• Vision therapists, who are optometrists specializing in neurological training or rehabilitation for certain eye conditions. The profession is somewhat analogous to occupational therapy or physical therapy.
• Optometric assistants, also called paraoptometrists, who assist optometrists.

The word oculist is an older term that may refer to either an ophthalmologist or an optometrist.

History

Etymology

The term eye appeared in Old English as ēage and in Latin as oculus. It has been in use since before the 12th century.^[1]

Research

The National Eye Institute (NEI), part of the National Institute of Health, is working with the vision community to initiate the eyeGENE National Ophthalmic Disease Genotyping Network. A primary goal of this project is to facilitate research on the discovery of the genetic causes of ocular diseases.^[2]

References

1. ↑ Merriam-Webster Online. Eye.
2. ↑ National Eye Institute Web site. eyeGENE^TM - National Ophthalmic Disease Genotyping Network.

External Links

American professional societies:

• American Academy of Ophthalmology
• American Optometric Association
  • Optometric Assistant/Technician Training Programs
• Network: What is Vision Therapy?
• American Society of Ocularists
• American Association of Certified Orthoptists
• American Board of Opticianry

World Ophthalmology Societies, arranged alphabetically by country

National Eye Institute: Eye Health Organizations