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Know Your Eye

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A Guide to the Many Parts of the Human Eye and How they Function.

The ability to see is dependent on the actions of several structures in and around the eyeball.  The graphic below lists many of the essential components of the eye's optical system.

optical system

When you look at an object, light rays are reflected from the object to the cornea, which is where the miracle begins.  The light rays are bent, refracted and focused by the cornea, lens, and vitreous. The lens' job is to make sure the rays come to a sharp focus on the retina. The resulting image on the retina is upside-down.   Here at the retina, the light rays are converted to electrical impulses which are then transmitted through the optic nerve, to the brain, where the image is translated and perceived in an upright position!

retina

Think of the eye as a camera.  A camera needs a lens and a film to produce an image.  In the same way, the eyeball needs a lens (cornea, crystalline lens, vitreous) to refract, or focus the light and a film (retina) on which to focus the rays.  If any one or more of these components is not functioning correctly, the result is a poor picture.  The retina represents the film in our camera.  It captures the image and sends it to the brain to be developed. The macula is the highly sensitive area of the retina.  The macula is responsible for our critical focusing vision.  It is the part of the retina most used.  We use our macula to read or to stare intently at an object.

macula


Human Eye


The eye works much like a camera. Light enters the eye through the cornea via the pupil at the center of the iris, the colored part of the eye. The pupil controls the amount of light admitted while the cornea does most of the focusing. Light then passes through the lens, where it is finely focused, and travels back through the eye to the retina. The retina interprets images and then sends the information to the brain via the optic nerve.

Human Eye


Cornea

The cornea is sometimes referred to as the "window of the eye". It is composed of 5 layers of tissue. Its outer layer (the epithelium) provides protection for the eye. The epithelium is made up of highly regenerative cells that have the ability to grow back within 3 days, allowing for fast healing of superficial injuries. Most of the inner layers of the cornea provide strength to the eye.

Pupil

This is the black circle in the middle of the eye. The primary function of the pupil is to control the amount of light entering the eye. When you're in a bright environment, the pupil becomes smaller to allow less light through. When it's dark, the pupil expands to allow more light to reach the back of the eye.

Iris

This is the colored part of the eye surrounding the pupil. The primary function of the iris is to control the size of the pupil. This is acheived through contraction or expansion of the muscles of the iris.

Sclera

The sclera is the white part surrounding the iris. The sclera's purpose is to provide structure, strength, and protection to the eye.

Lens

The lens is the clear structure located behind the pupil. Its primary function is to provide fine-tuning for focusing and reading. The lens performs this function by altering its shape. At about the age of 45, the lens becomes less flexible. At about the age of 65, the lens becomes cloudy and hard, preventing light from entering the eye.

Vitreous Body

This is the clear gel-like substance located inside the eye's cavity. Its purpose is to provide a spherical shape to the eye. The vitreous body may develop small clumps known as 'floaters', which are especially common in nearsighted people.

Retina

The retina consists of fine nerve tissue which lines the inside wall of the eyes and acts like the film in a camera. Its primary function is to transmit images to the brain.

Optic Nerve

This nerve carries images from the retina to the brain.



Anatomy focus

Angle Structures

Angle Structures

The area in the anterior chamber where the cornea and iris join is known as the angle.  This is comprised of several structures that make up the eye's drainage system.  The angle structures include:  the outermost part of the iris, the front of the ciliary body, the trabecular meshwork, and the Canal of Schlemm. 

Aqueous is formed in the ciliary body behind the iris.  It flows through the pupillary space into the anterior chamber.  From there, the fluid travels into the angle structures and drains from the eye.  

As the aqueous fluid leaves the angle, it passes through a filter called the trabecular meshwork.  After leaving the trabecular meshwork, the aqueous travels through a tiny channel in the sclera called the Canal of Schlemm.  The aqueous flows into other tiny channels and eventually into the eye's blood vessels.

The production and drainage of aqueous fluid determines the eye's intraocular pressure (IOP).


Aqueous Humor

The aqueous is the thin, watery fluid that fills the space between the cornea and the iris (anterior chamber).  It is continually produced by the ciliary body, the part of the eye that lies just behind the iris.  This fluid nourishes the cornea and the lens and gives the front of the eye its form and shape


Choroid

Choroid
The choroid lies between the retina and sclera.  It is composed of layers of blood vessels that nourish the back of the eye.  The choroid connects with the ciliary body toward the front of the eye and is attached to edges of the optic nerve at the back of the eye.


Ciliary Body


Ciliary Body

The ciliary body lies just behind the iris.  Attached to the ciliary body are tiny fiber "guy wires" called zonules.  The crystalline lens is suspended inside the eye by the zonular fibers.  Nourishment for the ciliary body comes from blood vessels which also supply the iris.  

One function of the ciliary body is the production of aqueous humor, the clear fluid that fills the front of the eye.  It also controls accommodation by changing the shape of the crystalline lens.  When the ciliary body contracts, the zonules relax.  This allows the lens to thicken, increasing the eye's ability to focus up close.  When looking at a distant object, the ciliary body relaxes, causing the zonules to contract.  The lens becomes thinner, adjusting the eye's focus for distance vision.  
With age, everyone develops a condition known as presbyopia.  This occurs as the ciliary body muscle and lens gradually lose elasticity, causing difficulty reading.


Conjunctiva

Conjunctiva

The conjunctiva is the thin, transparent tissue that covers the outer surface of the eye.  It begins at the outer edge of the cornea, covering the visible part of the sclera, and lining the inside of the eyelids.  It is nourished by tiny blood vessels that are nearly invisible to the naked eye.  

The conjunctiva also secretes oils and mucous that moisten and lubricate the eye.

Cornea

The cornea is the transparent, dome-shaped window covering the front of the eye.  It is a powerful refracting surface, providing 2/3 of the eye's focusing power.  Like the crystal on a watch, it gives us a clear window to look through.

Cornea

Because there are no blood vessels in the cornea, it is normally clear and has a shiny surface.  The cornea is extremely sensitive - there are more nerve endings in the cornea than anywhere else in the body.  

The adult cornea is only about 1/2 millimeter thick and is comprised of 5 layers:  epithelium, Bowman's membrane, stroma, Descemet's membrane and the endothelium.  


The layers of the cornea

The epithelium is layer of cells that cover the surface of the cornea.  It is only about 5-6 cell layers thick and quickly regenerates when the cornea is injured.  If the injury penetrates more deeply into the cornea, it may leave a scar.  Scars leave opaque areas, causing the corneal to lose its clarity and luster.  

Boman's membrane lies just beneath the epithelium.  Because this layer is very tough and difficult to penetrate, it protects the cornea from injury.  

The stroma is the thickest layer and lies just beneath Bowman's.  It is composed of tiny collagen fibrils that run parallel to each other.  This special formation of the collagen fibrils gives the cornea its clarity.

Descemet's membrane lies between the stroma and the endothelium.  The endothelium is just underneath Descemet's and is only one cell layer thick.  This layer pumps water from the cornea, keeping it clear.  If damaged or disease, these cells will not regenerate.

Tiny vessels at the outermost edge of the cornea provide nourishment, along with the aqueous and tear film.

Extraocular Muscles

The six tiny muscles that surround the eye and control its movements are known as the extraocular muscles (EOMs).  The primary function of the four rectus muscles is to control the eye's movements from left to right and up and down.  The two oblique muscles move the eye rotate the eyes inward and outward.
All six muscles work in unison to move the eye.  As one contracts, the opposing muscle relaxes, creating smooth movements.  In addition to the muscles of one eye working together in a coordinated effort, the muscles of both eyes work in unison so that the eyes are always aligned.

Eyelids

The eyelids protect the eyes from the environment, injury and light.  They also maintain a smooth corneal surface by spreading tears evenly over the eye.  The lids are composed of an outer layer of skin, a middle layer of muscle and tissue that gives them form, and an inner layer of moist conjunctival tissue.

Several muscles work together to control the  actions of the lids.  Located in the middle layer of the eyelid is the orbicularis oculi, a circular muscle that closes the lids.  The levator muscle is attached inside the upper lid and elevates it.  A smooth muscle called Mueller's gives the lids tone and helps maintain elasticity.

Tiny oil-producing meibomian glands line the inner edge of the lids.  These glands produce oil that lubricates the eye.  Rows of lashes protect the eyes from the elements and debris. 

Not only do the eyelids provide protection and moisture, they display expression and emotions that are an important part of our individuality.


Iris


The colored part of the eye is called the iris.  It controls light levels inside the eye similar to the aperture on a camera.  The round opening in the center of the iris is called the pupil.  The iris is embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size.  

The sphincter muscle lies around the very edge of the pupil.  In bright light, the sphincter contracts, causing the pupil to constrict.  The dilator muscle runs radially through the iris, like spokes on a wheel.  This muscle dilates the eye in dim lighting.  

The iris is flat and divides the front of the eye (anterior chamber) from the back of the eye (posterior chamber).  Its color comes from microscopic pigment cells called melanin.  The color, texture, and patterns of each person's iris are as unique as a fingerprint.

Iris Lens



Lens

The crystalline lens is located just behind the iris.  Its purpose is to focus light onto the retina.  The nucleus, the innermost part of the lens, is surrounded by softer material called the cortex.  The lens is encased in a capsular-like bag and suspended within the eye by tiny "guy wires" called zonules.  
In young people, the lens changes shape to adjust for close or distance vision.  This is called accommodation.  With age, the lens gradually hardens, diminishing the ability to accommodate.


Macula

Macula

he macula is located roughly in the center of the retina, temporal to the optic nerve.  It is a small and highly sensitive part of the retina responsible for detailed central vision.  The fovea is the very center of the macula.  The macula allows us to appreciate detail and perform tasks that require central vision such reading. 

 

Optic Nerve

Optic Nerve

The optic nerve transmits electrical impulses from the retina to the brain.  It connects to the back of the eye near the macula.  When examining the back of the eye, a portion of the optic nerve called the optic disc can be seen.  
The retina's sensory receptor cells of retina are absent from the optic nerve.  Because of this, everyone has a normal blind spot.  This is not normally noticeable because the vision of both eyes overlaps.


Pupil

 

The pupil is the opening in the center of the iris.  The size of the pupil determines the amount of light that enters the eye.  The pupil size is controlled by the dilator and sphincter muscles of the iris.  Doctors often evaluate the reaction of pupils to light to determine a person's neurological function.

Retina

The retina is a multi-layered sensory tissue that lines the back of the eye.  It contains millions of photoreceptors that capture light rays and convert them into electrical impulses.  These impulses travel along the optic nerve to the brain where they are turned into images. 

There are two types of photoreceptors in the retina:  rods and cones.  The retina contains approximately 6 million cones.  The cones are contained in the macula, the portion of the retina responsible for central vision.  They are most densely packed within the fovea, the very center portion of the macula. Cones function best in bright light and allow us to appreciate color.

There are approximately 125 million rods.  They are spread throughout the peripheral retina and function best in dim lighting.  The rods are responsible for peripheral and night vision.

This photograph shows a normal retina with blood vessels that branch from the optic nerve, cascading toward the macula.

Sclera

The sclera is commonly known as "the white of the eye."  It is the tough, opaque tissue that serves as the eye's protective outer coat.  Six tiny muscles connect to it around the eye and control the eye's movements.  The optic nerve is attached to the sclera at the very back of the eye.

In children, the sclera is thinner and more translucent, allowing the underlying tissue to show through and giving it a bluish cast.  As we age, the sclera tends to become more yellow.

Tear Film

Tears are formed by tiny glands that surround the eye.  The tear film is comprised of three layers:  oil, water, and mucous.  The lower mucous layer serves as an anchor for the tear film and helps it adhere to the eye.  The middle layer is comprised of water.  The upper oil layer seals the tear film and prevents evaporation.    
The tear film serves several purposes:  it keeps the eye moist, creates a smooth surface for light to pass through the eye, nourishes the front of the eye, and provides protection from injury and infection.

Tear Production System

The eye's tears are composed of three layers:  oil, water and mucous.  The outermost oily layer is produced by the meibomian glands which line the edge of the eyelids.  The watery portion of the tear film is produced by the lacrimal gland.  This gland lies underneath the outer orbital rim bone, just below the eyebrow.  The mucous layer comes from microscopic goblet cells in the conjunctiva.  
With each blink, the eyelids sweep across the eye, spreading the tear film evenly across the surface.  The blinking motion of the eyelids forces the tears into tiny drains found at the inner corners of the upper and lower eyelids.  These drains are called puncta (plural for punctum).  
The tear film travels from the puncta into the upper and lower canaliculus, which empty into the lacrimal sac.  The lacrimal sac drains into the nasolacrimal duct which connects to the nasal passage.  This connection between the tear production system and the nose is the reason your nose runs when you cry.  Some patients can actually taste eye drops as they drain from the nasal passage into the throat.

Vitreous illustrationVitreous

The vitreous is a thick, transparent substance that fills the center of the eye. It is composed mainly of water and comprises about 2/3 of the eye's volume, giving it form and shape. The viscous properties of the vitreous allow the eye to return to its normal shape if compressed. 
In children, the vitreous has a consistency similar to an egg white. With age it gradually thins and becomes more liquid. The vitreous is firmly attached to certain areas of the retina. As the vitreous thins, it separates from the retina, often causing floaters.

   
     
     
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