How The Eye
Works
Sclera
Cornea
Choroid
Ciliary Body
Iris
Pupil
Retina
Macula
Optic Nerve
Optic Disc
Aquaeous Humor
Vitreous Humor
Lens
Conjunctiva
Lacrimal Glands
Muscles
Protection
Eye photography
Eye Care
Accident Prevention
First Aid For The Eye
Eye Health Nutrition
HIV/AIDS & the eye
A Global Picture of
Childhood Blindness
Thinking of the eye as a processing machine also helps to understand how it works. The job of the machine is to refract light rays to the retina at the back of the eye, where they are converted into electrical and chemical energy. From here, they are sent along the optic nerves to the brain for interpretation.
This is just a brief tour through the mechanics of the eye, but we do hope it will be an eye-opening experience.
Sclera Cornea
Choroid Ciliary Body
Iris Pupil
Retina Macula
Optic Nerve Optic Disc
Aquaeous and Vitreous Humor
Lens Conjunctiva
Lacrimal Glands Muscles
Protection for the Eye
The tough, outermost casing of the eye, this is the white part that you can see. The Sclera maintains the shape of the eye, and gives protection to the inner elements.
Approximately the one fifth of the Sclera is clear. This section is called the cornea, and it is the windscreen in your car, allowing you to see, but also protect your eye from the world outside. The cornea is the door through which all the rays of light must pass to enter the eye. Much focusing of light is also carried out by the cornea, as the light travels through its many layers. The first of these is the self-regenerating epithelium, which is usually cut or removed during refractive surgery, and other procedures involving the cornea.
Also known as the uveal tract, this is the second layer of the eye, and contains ocular blood vessels. Directly behind the cornea, the choroid houses the ciliary body and iris.
The ciliary body contains muscles which attach to the lens. adjusting its size in order to focus the lens.
The iris is the coloured, ring surrounding the pupil, containing two muscles which alter pupil size. Iris colour is derived from the amount of pigment within the connective tissue. Blue eyes have less pigment, whilst brown eyes have much more.
The sphincter pupillae muscle enlarges the iris whilst contracting the pupil, which allows less light to enter the eye.
The dilator pupillae muscle contracts the iris. As the pupil grows larger, more light is allowed to enter the eye;
This is the black circle the we see in the middle of the iris. The muscles of the iris help it to contract and enlarge in response to the level of light perceived. The pupil can grow from 2 millimetres to 8 millimetres in diameter, enabling the eye allow as much as 30 times more light to enter it.
This is the light-sensitive layer at the back of the eye. It contains millions of rod cells and cone cells, which help to convert light into visual messages for interpretation by the brain.
Rod cells assist with vision in low light conditions. They contain a chemical called rhodopsin (visual purple), which converts light into electrical impulses for the brain to interpret.
Cone cells process bright light, and therefore assist with visual perception, colour vision and detail. They contain photopsins, which convert light into electrical energy for transfer to the brain.
When a retinoblastoma tumour develops, it prevents light from reaching the retina, causing the light to be reflected back as a white glow, like a cat’s eye.
The optic nerve is a collection of retinal nerve fibres which act as a conveyorbelt, carrying the newly formed electrical impulses to the brain for interpretation.
The highly sensitive centre of the retina, the macula is responsible for detailed vision. The centre of the macula is called the fovea centralis and it only contains cones, enabling us to clearly see very fine detail.
This is the point at which the optic nerve and blood vessels meet and exit the retina. This spot on the retina contains no rods or cones, meaning it cannot perceive or interpret light. Therefore it is known as “the blind spot”. However, both eyes and the brain work as a team, filling in the gaps for each eye’s blind spot.
When we take a photograph, we often see a tiny dot of light in the middle of the pupil. This occurs because light cannot be processed at the point of the optic disk, so the eye reflects it back as a white dot. When a flash hits the optic disk head on, the white dot in the eye may be more prominent.
Below is a diagram of what an ophthalmologist sees when looking at the retina with an ophthalmoscope.
The centre of the eyeball contains two fluid sections, which are divided by the lens. The small watery front section is called the aqueous humor, and consists of two sections called the anterior chamber (in front of the iris) and the posterior chamber (behind the iris). The watery fluid is produced by the ciliary body and discharged through the canal of Schlemm. Glaucoma is a painful condition which occurs when the canal become blocked, and the fluid cannot drain away.
Behind the lens is the larger, gel-filled section called the vitreous humor, whose job it is to provide shape for the eyeball. When tiny fragments break off from a retinoblastoma tumour, they can float into the vitreous humor, and are called vitreous seeding.
Just 1cm (0.4 inches) in diameter, this is the clear, bi-convex structure directly behind the pupil, which changes shape to focus light to the retina, and fine-tune vision.
This is a moistening mucous membrane covering the sclera and the inside surface of the eyelids. A conjunctivitis infection causes the eye to look pink, leading to the more common name of “pink eye”.
Located above the outer section of each eye, this is the tear factory. From here, tears drain into the lacrimal sac at the inner corner of the eye, and eventually drain through the nasal duct. This is why we sniff when we cry.
A person who has had radiotherapy to the eye is more likely to have difficulty producing tears, and this can lead to a painful dry eye. This can be corrected by lubricating drops.
As an organ, the eye is unique in its ability to move in every direction. In so doing, it is able to maximizes visual field. Eye movement is controlled by three pairs of extraocular (outside the eye) muscles, which are attached to the sclera.
The Medial and Leteral Rectus move the eye inwards and outwards, whilst the Superior and Inferior Rectus rotate it up and down. The Superior Oblique and Inferior Oblique move the eye diagonally.
When an eye is removed (enulceated), an implant is used to fill the space where the eyeball used to be. Sometimes, the muscles can be swen into the implant, allowing them to move the implant, and the artificial eye which sits on top of it.
Below is a diagram of the different eye muscles.
With all its intricate mechanics, protection of the eye is essential. The bony orbital cavity helps protect against injury, and the surrounding fat provides cushioning to absorb blows or vibrations.
Eyebrows help prevent airborne moisture and other particles from entering the eye, whilst blinking eyelids and eyelashes help to dislodge and sweep away debris that may injure it. They also help disperse lubricating tears across the surface of the eye.
Eye Care Accident Prevention First Aid For The Eye
Eye Health Nutrition HIV/AIDS & the eye Eye photography
A Global Picture of Childhood Blindness
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