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The eye has many parts that must work together to produce clear vision:
- The orbit is the bony cavity that contains the eyeball, muscles, nerves, and blood vessels, as well as the structures that produce and drain tears. Each orbit is a pear-shaped structure that is formed by several bones.
- The sclera, or white part of the eye, protects the eyeball.
The pupil, or black dot at the centre of the eye, is an opening through which light can enter the eye.
The pupil changes size to adjust for the amount of light available (smaller for bright light and larger for low light).This opening and closing of light into the eye is much like the aperture in most 35 mm cameras which lets in more or less light depending upon the conditions.
The iris, or coloured part of the eye, surrounds the pupil. It controls how much light enters the eye by changing the size of the pupil.
The iris allows more light into the eye (enlarging or dilating the pupil) when the environment is dark and allows less light into the eye (shrinking or constricting the pupil) when the environment is bright.
Thus, the pupil dilates and constricts like the aperture of a camera lens as the amount of light in the immediate surroundings changes.
The size of the pupil is controlled by the action of the pupillary sphincter muscle and dilator muscle.
- The cornea, a clear window at the front of the eye, covers the iris and the pupil. Cornea serves as a protective covering for the front of the eye.
- A clear lens, located behind the pupil, acts like a camera lens by focusing light onto the retina at the back of the eye.
- The retina is a light-sensitive inner lining at the back of the eye. Ten different layers of cells work together in the retina to detect light and turn it into electrical impulses.
- Choroid -Layer containing blood vessels that lines the back of the eye and is located between the retina (the inner light-sensitive layer) and the sclera (the outer white eye wall).
- Ciliary Body -Structure containing muscle and is located behind the iris, which focuses the lens.
- Fovea -The center of the macula which provides the sharp vision.
- Macula -The area in the retina that contains special light-sensitive cells. In the macula these light-sensitive cells allow us to see fine details clearly in the center of our visual field. The deterioration of the macula is a common condition as we get older (age related macular degeneration or ARMD).
Optic Nerve -A bundle of more than a million nerve fibers carrying visual messages from the retina to the brain. (In order to see, we must have light and our eyes must be connected to the brain.) Your brain actually controls what you see, since it combines images. The retina sees images upside down but the brain turns images right side up. This reversal of the images that we see is much like a mirror in a camera. Glaucoma is one of the most common eye conditions related to optic nerve damage.
Optic nerves: Optic nerves are of two types. These include cones and rods.
- Cones: Cones are the nerve cells that are more sensitive to bright light. They help in detailed central and colour vision.
Rods: Rods are the optic nerve cells that are more sensitive to dim lights. They help in peripheral vision.
Rods are responsible for night and peripheral (side) vision.
Rods are more numerous than cones and much more sensitive to light, but they do not register color or contribute to detailed central vision as the cones do. Rods are grouped mainly in the peripheral areas of the retina. At the junction of the optic nerve and retina, there are no sensory nerve cells. So no vision is possible at that point and is known as a blind spot.
- Vitreous Humor -The, clear, gelatinous substance filling the central cavity of the eye.
HOW EYE WORKS WHEN LIGHT ENTERS?
CHAMBERS OF EYE
The inside of the eye is divided into three sections called chambers.
| Anterior chamber | Posterior chamber | The back two-thirds |
|---|---|---|
| The anterior chamber is the front part of the eye between the cornea and the iris. | The posterior chamber is between the iris and lens. | The inner wall of the vitreous chamber is lined with a special layer of cells (the retina): millions of highly sensitive nerve cells that convert light into nerve impulses |
NOTE
LENS
- The lens is behind the iris and is normally clear. Light passes through the pupil to the lens.
- The lens is held in place by small tissue strands or fibres (zonules) extending from the inner wall of the eye.
- The lens is very elastic. Small muscles attached to the lens can change its shape, allowing the eye to focus on objects at varying distances.
- Tightening (contraction) or relaxing these muscles causes the lens to change shape, allowing the eyes to focus on near or far objects (accommodation)
NERVE SUPPLY
The optic nerve (a cranial nerve), which carries impulses from the retina to the brain, as well as other cranial nerves, which transmit impulses to each eye muscle, travel through the orbit (the bony cavity that surrounds the eyeball).
BLOOD SUPPLY
An ophthalmic artery and a central retinal artery (an artery that branches off of the ophthalmic artery) provide blood to each eye.
Similarly, ophthalmic veins (vortex veins) and a central retinal vein drain blood from the eye. These blood vessels enter and leave through the back of the eye.
