Monday, March 10, 2008/10:04 AM
internal structures of the human eye
To begin from where I left off previously, our eye comprises of two other layers - the choroid and retina, besides the sclera. Many structures are found within these two layers. Each layer and structure performs distinct functions to facilitate sight.
Choroid
- It is pigmented black to prevent internal reflection of light.
- It contains blood vessels which transport oxygen and nutrients to the eyeball and remove metabolic waste products.
Retina
- It is a light-sensitive layer on which images are formed.
- Light-sensitive cells called photoreceptors can be found here.
Photoreceptors
- They are light-sensitive cells which consist of cones and rods.
- They are connected to the nerve-endings from the optic nerve.
Cones
- There are about 7 million cone cells in our eye and they can be found in greatest concentration in the small, central part of the retina called the macula.
- They enable us to see colours in bright light, and do not work well in dim light.
- The three types of cones - red, blue and green cones, contain different pigments which absorb light of different wavelengths.
- All the cones allow us to see a wide variety of colours.
Rods
- There are about 100 million rod cells and they are found in the peripheral retina, away from the macula.
- Compared to cones, they are move sensitive to light and enable us to see in black and white in dim light as they contain a pigment called visual purple.
- When our eye is exposed to bright light, all the visual purple is bleached. Thus, when we enter a dimly-lit room, the visual purple in the rods has to be re-formed before we can see in the dark.
- Vitamin A helps in the formation of visual purple. A lack of vitamin A results in a person suffering from night-blindness.
Macula
- It is a specialized part of the retina.
- It is a patch of densely packed light-sensitive cells which is essential to our central vision and allows us to have a detailed vision.
Optic nerve
- It is a bundle of more than 1 million nerve fibers that exits the back of the eye.
- When the photoreceptors in the retina are stimulated, the optic nerve transmits nerve impulses to the brain where they are interpreted in the primary visual cortex.
Blind spot
- It is the region where the optic nerve leaves the eye.
- It contains neither cones nor rods, hence it is not sensitive to light.
Fovea
- Fovea, or otherwise known as the yellow spot, is the middle part of the macula.
- It is a small yellow depression in the retina and is located directly behind the lens.
- It contains the greatest concentration of cones, but no rods.
- It allows us to have a detailed colour vision in bright light.
Ciliary body
- It is a thickened region at the front end of the choroid.
- It contains ciliary muscles which control the curvature or thickness of the lens.
- It produces aqueous humor.
Lens
- It is a transparent, circular, biconvex structure, about 10mm in diameter.
- It is elastic, thus it is able to change its shape or thickness so as to refract light onto the retina.
- The change in lens shape allows our eyes to focus on objects at varying distances.
Suspensory ligament
- It is a tissue that attaches the edge of the lens to the cilary body.
Aqueous chamber
- It is the space between the lens and the cornea.
- It is filled with a transparent, watery fluid called aqueous humor.
Aqueous humor
- It is constantly being produced by the ciliary body and drained away.
- It circulates throughout the front of the eye, thus maintaining a constant pressure in the eye.
- It keeps the front of the eyeball firm and refracts light into the pupil. It also nourishes the lens and cornea.
Vitreous chamber
- It is the space behind the lens.
- It is filled with a transparent, jelly-like substance called vitreous humor.
Vitreous humor
- It keeps the eyeball firm and refracts light onto the retina.
Processes which occur that enable us to see: [conclusion]
Choroid
- It is pigmented black to prevent internal reflection of light.
- It contains blood vessels which transport oxygen and nutrients to the eyeball and remove metabolic waste products.
Retina
- It is a light-sensitive layer on which images are formed.
- Light-sensitive cells called photoreceptors can be found here.
Photoreceptors
- They are light-sensitive cells which consist of cones and rods.
- They are connected to the nerve-endings from the optic nerve.
Cones
- There are about 7 million cone cells in our eye and they can be found in greatest concentration in the small, central part of the retina called the macula.
- They enable us to see colours in bright light, and do not work well in dim light.
- The three types of cones - red, blue and green cones, contain different pigments which absorb light of different wavelengths.
- All the cones allow us to see a wide variety of colours.
Rods
- There are about 100 million rod cells and they are found in the peripheral retina, away from the macula.
- Compared to cones, they are move sensitive to light and enable us to see in black and white in dim light as they contain a pigment called visual purple.
- When our eye is exposed to bright light, all the visual purple is bleached. Thus, when we enter a dimly-lit room, the visual purple in the rods has to be re-formed before we can see in the dark.
- Vitamin A helps in the formation of visual purple. A lack of vitamin A results in a person suffering from night-blindness.
Macula
- It is a specialized part of the retina.
- It is a patch of densely packed light-sensitive cells which is essential to our central vision and allows us to have a detailed vision.
Optic nerve
- It is a bundle of more than 1 million nerve fibers that exits the back of the eye.
- When the photoreceptors in the retina are stimulated, the optic nerve transmits nerve impulses to the brain where they are interpreted in the primary visual cortex.
Blind spot
- It is the region where the optic nerve leaves the eye.
- It contains neither cones nor rods, hence it is not sensitive to light.
Fovea
- Fovea, or otherwise known as the yellow spot, is the middle part of the macula.
- It is a small yellow depression in the retina and is located directly behind the lens.
- It contains the greatest concentration of cones, but no rods.
- It allows us to have a detailed colour vision in bright light.
Ciliary body- It is a thickened region at the front end of the choroid.
- It contains ciliary muscles which control the curvature or thickness of the lens.
- It produces aqueous humor.
Lens
- It is a transparent, circular, biconvex structure, about 10mm in diameter.
- It is elastic, thus it is able to change its shape or thickness so as to refract light onto the retina.
- The change in lens shape allows our eyes to focus on objects at varying distances.
Suspensory ligament
- It is a tissue that attaches the edge of the lens to the cilary body.
Aqueous chamber
- It is the space between the lens and the cornea.
- It is filled with a transparent, watery fluid called aqueous humor.
Aqueous humor
- It is constantly being produced by the ciliary body and drained away.
- It circulates throughout the front of the eye, thus maintaining a constant pressure in the eye.
- It keeps the front of the eyeball firm and refracts light into the pupil. It also nourishes the lens and cornea.
Vitreous chamber
- It is the space behind the lens.
- It is filled with a transparent, jelly-like substance called vitreous humor.
Vitreous humor
- It keeps the eyeball firm and refracts light onto the retina.
Processes which occur that enable us to see: [conclusion]
- Light first passes through the cornea and then through the aqueous humor.
- Next, light passes through the pupil, which is the round opening in the center of the iris.
- Light then penetrates through the lens and passes through the vitreous humor.
- Following this, light reaches the retina where the cones and rods are stimulated to produce a chain of split-second chemical reactions converting light to nerve impulses.
- Finally, nerve impulses are sent through the optic nerve from the retina to the brain where they are interpreted in the primary visual cortex.
Eye movement 
- Each eye has 6 muscles attached to the sclera, thus enabling us to move our eyes and track objects without turning our head.
- The six muscles are: the lateral, medial, superior and inferior rectus muscles, and the superior and inferior oblique muscles.
- These eye muscles work either individually or together, therefore allowing us to shift our field of gaze left, right, up, down and diagonally.
Jia Qian (5) (: