How Do the Eyes Work?
As basic as your eyes may seem, their ability to see makes them one of the most valuable assets in the human body.
The eyes work in the same way as cameras. When you focus on an object, light is reflected and enters the eye through the cornea. As the light passes through, the dome-shaped nature of the cornea bends light, enabling the eye to focus on fine details.
In addition to the retina, other visual components such as the pupil, iris (colored part of the eye), and lens help focus light rays on the retina. The retina is the light-sensitive membrane covering the back of the eye.
When light entering the eye hits the retina, special cells known as photoreceptors convert it into electrical signals.1
The optic nerve carries these electrical impulses from the retina to the visual cortex (in the brain). The brain then interprets the impulses into the images you see.
Eye Anatomy (16 Parts of the Eye & What They Do)
The following are parts of the human eyes and their functions:
The conjunctiva is the membrane covering the sclera (white portion of your eye). The conjunctiva also covers the interior of your eyelids.
Conjunctivitis, often known as pink eye, occurs when this thin membrane becomes inflamed or swollen.
Other eye disorders that affect the conjunctiva include:
- Pinguecula (accumulation of protein and fat deposits in the conjunctiva)
- Pterygium (a noncancerous growth that develops on the conjunctiva)
- Subconjunctival hemorrhages (broken blood vessels)
By generating mucus and tears, the conjunctiva helps lubricate the eyes. It also aids in immunological monitoring and prevents microorganisms from entering the eye.
The sclera is sometimes known as the "whites" of the eye. It covers more than 80% of the eyeball's surface.2
On the exterior, the sclera is smooth and white, but on the inside, it is brown and has grooves that help in the proper attachment of the eye tendons. Tendons are strong, flexible tissues that join muscles to structure, such as bones and, in this case, the sclera.
The episclera is a thin layer of tissue that lies on top of the sclera. The episclera has tiny blood vessels that supply the sclera with nutrients.
The sclera provides stability and protection for the eye's inner workings, but it is also flexible, allowing the eye to move as needed to see different objects.
When the sclera gets inflamed, the condition is referred to as scleritis.3 If the episclera is inflamed, the resulting condition is known as episcleritis. These conditions can cause redness, eye pain, blurry vision, and lid swelling.
The iris is the colored part of the eye and is unique to each person. This structure is located in the front of the eye, between the cornea on the outside and the lens on the inside.
The iris primarily regulates how much light reaches the retina by controlling the size of the eye's "window," or pupil.
As a result, it exhibits what is known as a "pupillary light reflex," in which it narrows in bright light while opening up in low light.
The iris also performs what is known as the "accommodation reflex." This is the eye's instinctive ability to shift focus from nearby to distant objects.
This action requires adjusting the pupil's aperture (opening), the shape of the lens, and convergence (the ability of the eyes to work together).
The pupil is seen as a black dot in the center of the iris.
The pupil is essentially a hole that allows the eye to focus on the things in front of it. They open and close to regulate the amount of light that enters the eye.
Light enters the eye via the lens which focuses light rays through the pupils into the retina.The difference between the centers of your pupils is called your pupillary distance.
When it's dark, our pupils dilate or expand wider to let in more light. This increases the scope of our view. In bright light, our pupils contract to a small diameter to protect our retina's delicate photoreceptors.
The cornea is the clear and protective outer layer of your eye. Together with the sclera (the white of your eye), the cornea acts as a barrier against dirt, infectious microorganisms, and other substances that can damage the eye.
In addition to protection, the cornea also plays a significant role in vision. Considering the dome shaped surface of the cornea, this structure refracts (bends) light as it passes through the eye. This enables it to focus on objects effectively.
The cornea has the ability to filter out the harmful ultraviolet (UV) light from the sun. This prevents some UV light from reaching other structures inside the eye. However, to prevent damage to the cornea itself and other parts of the eye, you should wear sunglasses.
Chronic exposure to UV light may lead to inflammation and other complications, including cancer.4
The uvea is the eye's middle layer. It is located underneath the white part of the eye (the sclera) and is composed of three parts:
- The iris
- Ciliary body
These structures control some eye functions, such as adapting to varying levels of light or object distances.
If any of the structures get inflamed, the resulting condition is referred to as uveitis.
This vascular layer is located between the sclera and retina of your eye. It delivers nourishment (through blood and oxygen supply) to the retina's outer layers.
In short, the choroid is the source of life that keeps the retina functioning effectively.
The choroid reflects light, which causes the red-eye effect in photographs.
The retina is a sensitive membrane that covers your eye's rear surface. Images are transmitted to the retina when your eye picks up the images.
The retina converts these images into impulses, which are sent to your brain through the optic nerve. This enables you to see and interpret what you see.
Some of the ocular conditions that affect the retina include:
- Diabetic retinopathy. Diabetes complication caused by damaged blood vessels.
- Retinal detachment. When the retina detaches from its normal position.5
- Retinitis pigmentosa. Deterioration of special light sensitive cells in the retina.
- Retinoblastoma. Formation of cancer cells in the retinal tissues.
9. Eye Muscles
The eye has six muscles. These muscles arise from the eye socket (orbit) and work to move the eye up and down, side to side, or in a circular motion.
- The superior rectus. Attaches to the top of the eye and moves the eye upwards.
- The inferior rectus. Attaches to the bottom of the eye and allows downward eye movement.
- The medial rectus. Attaches to the side of the eye adjacent to the nose and helps the eyes to shift inwards towards the nose.
- The lateral rectus. Attaches to the outer side of the eyes and moves the eyes towards the temples.
- The superior oblique. It originates from the back of the eye socket and attaches to the top of the eye. It rotates the eye inwards (front to back) and downwards.
- The inferior oblique. It arises from the front of the socket near the nose and travels inwards, attaching to the bottom surface of the eyeball. It rotates the eye outwards (front to back) and upwards.
10. Macula Lutea
Light rays are focused on the macula lutea when an eye is looking directly at an object.
The macula lutea is a yellow oval area in the retina's center (back of the eye). The center of the macula is known as the fovea.
It is the section of the retina that is in charge of sharp, detailed central vision (also called visual acuity).
The macula lutea has a high concentration of cones. These are the light-sensitive retinal cells that provide high visual acuity.
11. Eye Lens
The lens of the eye (or crystalline lens) is the transparent lentil-shaped structure inside your eye. This is the natural lens.
It is located behind the iris and to the front of the vitreous humor (vitreous body).
The vitreous humor is a clear, colorless, gelatinous mass that fills the gap between the lens and the retina in the eye.
The lens is held in place by a fibrous membrane known as zonule of Zinn or the lens suspensory ligaments.6 The lens changes its thickness and curvature, allowing the eye to focus on objects from varying distances.
If your lens has an irregular curvature, you're more prone to develop astigmatism. Cataracts are another lens-related visual disorder in which the lens becomes opaque or hazy, impairing vision.
If your natural lenses have an irregular curvature, your eye doctor may prescribe artificial lenses such as prescription glass or contact lenses to correct vision.
12. Aqueous Humor
Aqueous humor is a fluid substance that fills the eye. It's divided into two chambers. The anterior chamber is situated in front of the iris, whereas the posterior chamber is situated right behind it.
These layers enable the eye to keep its shape. This liquid is evacuated via the Schlemm canal to eliminate any accumulation in the eye.
Glaucoma may develop if a person's aqueous fluid does not drain adequately.
13. Ciliary Body
The ciliary body is a ring-shaped tissue found behind the iris. It attaches to the lens through the zonular fibers (fibers of Zinn).
The ciliary body holds and regulates the movement of the eye lens, hence keeping the lens shape intact.
This structure is also involved in the production of aqueous humor.
14. Optic Nerve
The optic nerve is a bundle of about 1.2 million nerve fibers that transmit visual information to the central nervous system (brain).7
There is one nerve per eye connecting each eye to the brain. Vision loss may occur if any of the nerves is damaged.
However, the consequences of optic nerve damage depend on the extent of the damage.
15. Optic Disc
The optic disc is the place at which the axons of retinal ganglion cells join together and mark the beginning of the optic nerve (second cranial nerve).
The optic disc also serves as the entrance site for major blood vessels that nourish the retina.
In the average person, the optic disc carries about 1.2 million nerve fibers from the retina to the brain.
16. Fovea Centralis
The fovea Centralis (central fovea) is a tiny depression in the retina that houses cones that help with proper vision. It is located within the macula.
Approximately 80-90% of the optic nerve fibers convey visual information from the fovea, while the other portion convey information from the peripheral retina.
If there are any issues with the fovea of the cones, you may experience blurry vision.
Now that you know the anatomy of the eye, you certainly understand the complex working mechanism and unique structure involved in each task. Any defects in the eye structures are likely to affect your ability to see and overall eye health.
For this reason, it's important to take good care of your eyes. If you notice any unusual changes in your ability to see, a visit to your eye doctor (optometrist) will be the best step to take.
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