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Anatomy of the Human Eye  

Human Eye image

 

 

Aqueous Humor
Choroid
Ciliary Body
Conjunctiva
Cornea
Fovea
Iris

  Lens
Macula
Optic Disc
Optic Nerve
Pupil
Retina
Sclera
Vitreous Body

 

Sclera

 

Sclera

The sclera is known as the white of the eye. Rugged and robust, the sclera protects the inner, more sensitive parts of the eye like the retina and choroid.

Six muscles control the movement of each eye. Four of the six eye muscles on each eye are known as "straight" eye muscles. The sclera is about 0.03 inch (0.762 mm) thick except for where the four "straight" eye muscles attach. There, the depth is thinner, no more than 0.01 of an inch (0.254 mm).

The episclera, located just outside the sclera, contains blood vessels that nourish the sclera with oxygen and nutrients. These vessels may be visible on the surface of the sclera (white of the eye).

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Choroid

 

Choroid

The choroid layer, located just inside the sclera, consists of a network of vessels that nourish the retina with oxygen and nutrients. The macula and the anterior part of the optic nerve are dependent upon blood supply from the choroid.

Macular degeneration (specifically the "wet" type) is an eye condition that is directly caused by abnormal blood vessel growth which starts in the choroid and goes through the Bruch's Membrane to the retina (known as choroidal neovascularization). Due to a weaker structure, these vessels have a tendency to rupture or to bleed, thus causing edemas (swelling due to the accumulation of fluid).

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Retina

 

Retina

The retina, the light sensitive tissue inside the back of the eyeball, absorbs light that is sent as visual signals to the brain. The retina contains two kinds of light receptors, cones and the rods. Cones are located in the macula and fovea in the center of the retina where they sense color and absorb bright light. Rods are more sensitive to light than cones, but they cannot distinguish colors. Instead, rods located peripheral to the fovea, detect light as shades of gray.

A serious eye condition related to the retina is "amotio retinae" or retinal detachment. Common symptoms are sensations of light flashes in the field of vision or partial to full vision loss that, if left untreated, can lead to permanent vision loss.
Note: If any of these symptoms occur, immediate medical care is required.

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Vitreous body

 

Vitreous Body

The vitreous body, also called the vitreous or the vitreous humor, is a gel-like mass within the eyeball, in the space between the lens and the retina. As people age, the vitreous body shrinks in volume.

When the vitreous body decreases in size it can detach from the retina. This vitreous body detachment may cause floaters and flashes or more serious retinal detachment. Retinal detachment occurs when the shrinking vitreous body causes the retina to tear away from the underlying choroid. Untreated, retinal detachment can lead to permanent vision loss.

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Macula

 

Macula

The macula is the area around the fovea. Closely-packed visual cells (cones and rods) in the macula result in high image resolution, or high visual acuity. Around the macular area, visual cells are not as closely packed as in the macula itself, but they are good enough for our peripheral vision to work properly.

A common eye disease among the elderly is age-related macular degeneration (AMD). This condition can be divided into two sub-categories: "dry" or "wet". Dry AMD is an incurable eye disease and the most common form of AMD. Wet AMD, which accounts for 10 percent of all AMD, is responsible for 80 percent of the associated vision loss. Wet AMD often leads to more serious cases of vision loss than dry AMD. Unlike dry AMD, wet AMD may be treated with FDA-approved drugs..

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Fovea

 

Fovea

The fovea is the most central part of the macula. The visual cells located in the fovea (cones) are packed tightest, resulting in optimal sharpness of vision, perfect for reading or watching TV. Rods are packed just outside the fovea, and are active in low light conditions. The fovea covers about 5 degrees of the vision field.

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Optic Disc

 

Optic Disc

Also known as the optic nerve head or the blind spot, the optic disc is where the optic nerve attaches to the eye. All visual cell nerve threads, and some blood vessels, have their entrance to the eyeball here. The absence of visual cells in the optic disc causes the appearance of a blind spot in your field of vision.

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Optic nerve

 

Optic nerve

The optic nerve sends signals from the eye to the different parts of the brain where the signals are interpreted into images. The optic nerve consists of about a million nerve threads. The optic nerves from both eyes are reconnected behind the eyes so that everything that is seen in the right field of vision is sent to the left cerebral hemisphere and vice versa.

Optic neuritis is inflammation of the optic nerve. It may cause sudden, reduced vision in the affected eye.

Glaucoma, a leading cause of blindness in the world, damages the optic nerve. This usually happens when the fluid pressure inside the eyes slowly rises, damaging the optic nerve, but glaucoma can also occur in eyes with normal pressure. Often there are no early symptoms but a comprehensive eye exam can detect glaucoma in its early stages so that treatment can prevent or reduce further damage.

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Lens

 

Lens

The lens is flexible and changes shape as we look at objects at different distances. An example is when we change focus from a TV screen across the living room to a newspaper in our hand. The lens is attached to a mass of threads called zonula threads that are attached to the ciliary body. We can compare this optic correlation to a bicycle wheel where the lens is the hub, the threads the spokes and the ciliary body the rim. When we focus on a nearby object, the ciliary muscle contracts. This causes the zonula threads to loosen, allowing the lens to contract in diameter and thicken, increasing its acuity.

As a person ages, the lens loses flexibility and develops cataracts. By the age of 45 the lens usually stiffens so much that normal reading vision is not possible without corrective glasses or lenses. Cataracts are an age-related clouding of the lens. The most common symptom associated with cataracts is blurry vision. Cataracts are treated by removal and replacement of the lens.

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Pupil

 

Pupil

The pupil is essentially a hole in the iris. It looks black because the layer of pigment inside the eye absorbs most of the light, resulting in a dark or black color. When pupils appear red in photos, the red is the color of the retina being reflected through the pupil.

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Iris

 

Iris

The iris regulates the amount of light that enters the eye. With strong light, the iris sphincter muscles contract the pupil. In darkness, the iris opens (expands) the pupil using the dilator muscles. When focusing on near objects, the pupil automatically decreases in diameter. The pupil automatically increases in diameter (opens) when the eye focuses on distant objects. This automatic expansion and contraction of the pupil opening by the iris is known as the Accommodation Reflex. A smaller pupil provides better focal depth.

Eye colors differ because of differing amounts and types of pigments in the iris. The most common color is brown, while the least common is green.

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Aqueous fluid

 

Aqueous humor

Aqueous humor (or aqueous fluid) fills the front part of the eye between the cornea and the lens. This fluid is produced at the back of the ciliary body, then seeps through the pupil, into the anterior chamber and ultimately is drained through the trabecular meshwork. The aqueous fluid's main function is to supply the cornea and the lens with nutrients and oxygen. The anterior chamber is the space located between the iris and the cornea. If an imbalance occurs between fluid production and outflow, the amount of aqueous fluid increases and the pressure in the eye inevitably increases. This is one of the contributing factors of glaucoma, one of the leading causes of blindness worldwide.

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Cornea

 

Cornea

The cornea is the transparent structure located in the front of the eye, covering the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts (bends) light accounting for about 2/3 of the eye's optical power. Unlike the lens, the shape of the cornea is fixed. It is supplied with oxygen and nutrients through tear fluid and not through blood vessels. This explains why the cornea is so clear. Overusing contact lenses can lead to oxygen deficiency, ultimately causing blood vessels to appear in the cornea. If the blood vessels grow too close to the middle, it can cause partial vision loss.

The shape of the cornea may cause refractive disorders such as myopia, hyperopia, and astigmatism. Vision correction requires glasses, contact lenses, or surgically reshaping the cornea.

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Ciliary body

 

Ciliary Body

The ciliary body contains the ciliary muscle, the muscle that controls the vision accommodation reflex. (See iris.) The ciliary body also produces aqueous humor (aqueous fluid) and is a point of attachment for the zonula threads that holds the lens.

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Conjunctiva

Credits: Eye drawings: Lensshopper.com  
Human_eye photo: Deniz_Keskin

Conjunctiva

The conjunctiva is a mucous membrane that covers the sclera and the inside of the eyelids. Many of the glands that play a part in the production of tear-film are located in the conjunctiva.

When the conjunctiva is irritated, often due to an allergy or infection, the membrane swells, becomes uneven and causes blood vessels to expand. "Pinkeye" refers to a contagious viral infection of the conjunctiva.

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Page posted: 15 September 2009

 
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