Parts Of The Eye
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 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.
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.
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.
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.
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.
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.
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.
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.
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.
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, including the interior and superior 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.
Vitreous body- 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 theretina. As the vitreous thins, it separates from the retina, often causing floaters.
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 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.
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.
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.
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.
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.
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.
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.
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.
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.
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, including the interior and superior 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.
Vitreous body- 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 theretina. As the vitreous thins, it separates from the retina, often causing floaters.
