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Human Eye


The human eye is an organ that reacts to light and has several purposes. As a sense organ, the mammalian eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth.



The front part of the eye (the part you see in the mirror) includes:

  • The iris (the pigmented part)
  • The cornea (a clear dome over the iris)
  • The pupil (the black circular opening in the iris, which lets light in)
  • The sclera (the white part)
  • The conjunctiva (an invisible, clear layer of tissue covering the front of the eye, except the cornea)


Functions of the Eye
Although we may take it for granted, seeing is one of the most complex functions our bodies perform and it requires the cooperation of many small and intricate parts. The human eye functions much like a digital camera. Both devices gather, focus, and transmit light through a lens to create an image of the surrounding environment. In order to see, we must have light.

  • Light enters the eye through the cornea, the clear front surface of the eye, which acts like a camera lens.
  • The iris of the eye functions like the diaphragm of a camera, controlling the amount of light reaching the back of the eye by automatically adjusting the size of the pupil, which acts like an aperture.
  • In dark conditions, the pupil widens. In bright conditions, the pupil constricts.
  • The eye’s crystalline lens, located directly behind the pupil, helps the eye automatically focus on near and approaching objects, like an autofocus camera lens.
  • The light then travels through the vitreous humor, a clear gel-like substance that fills the middle of the eye.
  • Light then reaches the retina, a sensitive inner lining of the back of the eye. The retina acts like camera film, converting optical images into electronic signals.
  • The image reflected on the retina is upside down. The optic nerve transmits signals to the visual cortex in the brain, which flips the image right side up and creates one composite image.


Perception, Color, and Image
The eye’s retina contains millions of tiny light-sensing nerve cells called rods and cones, which are named for their unique shapes.
Cones are responsible for perceiving color and detail.
Rods and responsible for night vision, peripheral or side vision, and detecting motion.


Parts of the Eye and Their Functions
There are several physical and chemical elements that make up the eye. The eye is also heavily involved with the nervous system, which allows the brain to take in information from the eyes and make the appropriate decisions on how to act upon this information. The nerves must be kept in prime condition or the brain may start to receive false images, or you will not take in enough information to get an accurate perception of your environment.


Eye Parts Description and Functions


Cornea The cornea is the outer covering of the eye. This dome-shaped layer protects your eye from elements that could cause damage to the inner parts of the eye. There are several layers of the cornea, creating a tough layer that provides additional protection. These layers regenerate very quickly, helping the eye to eliminate damage more easily. The cornea also allows the eye to properly focus on light more effectively. Those who are having trouble focusing their eyes properly can have their corneas surgically reshaped to eliminate this problem.
Sclera The sclera is commonly referred to as the “whites” of the eye. This is a smooth, white layer on the outside, but the inside is brown and contains grooves that help the tendons of the eye attach properly. The sclera provides structure and safety for the inner workings of the eye, but is also flexible so that the eye can move to seek out objects as necessary.
Pupil The pupil appears as a black dot in the middle of the eye. This black area is actually a hole that takes in light so the eye can focus on the objects in front of it.
Iris The iris is the area of the eye that contains the pigment which gives the eye its color. This area surrounds the pupil, and uses the dilator pupillae muscles to widen or close the pupil. This allows the eye to take in more or less light depending on how bright it is around you. If it is too bright, the iris will shrink the pupil so that they eye can focus more effectively.
Conjunctiva Glands These are layers of mucus which help keep the outside of the eye moist. If the eye dries out it can become itchy and painful. It can also become more susceptible to damage or infection. If the conjunctiva glands become infected the patient will develop “pink eye.”
Lacrimal Glands These glands are located on the outer corner of each eye. They produce tears which help moisten the eye when it becomes dry, and flush out particles which irritate the eye. As tears flush out potentially dangerous irritants, it becomes easier to focus properly.
Lens The lens sits directly behind the pupil. This is a clear layer that focuses the light the pupil takes in. It is held in place by the ciliary muscles, which allow the lens to change shape depending on the amount of light that hits it so it can be properly focused.
Retina The light focuses by the lens will be transmitted onto the retina. This is made of rods and cones arranged in layers, which will transmit light into chemicals and electrical pulses. The retina is located in the back of the eye, and is connected to the optic nerves that will transmit the images the eye sees to the brain so they can be interpreted. The back of the retina, known as the macula, will help interpret the details of the object the eye is working to interpret. The center of the macula, known as the fova will increase the detail of these images to a perceivable point.
Ciliary Body Ciliary body is a ring-shaped tissue which holds and controls the movement of the eye lens, and thus, it helps to control the shape of the lens.
Choroid The choroid lies between the retina and the sclera, which provides blood supply to the eye. Just like any other portion of the body, the blood supply gives nutrition to the various parts of the eye.
Vitreous Humor The vitreous humor is the gel located in the back of the eye which helps it hold its shape. This gel takes in nutrients from the ciliary body, aqueous humor and the retinal vessels so the eye can remain healthy. When debris finds its way into the vitreous humor, it causes the eye to perceive “floaters,” or spots that move across the vision area that cannot be attributed to objects in the environment.
Aqueous Humor The aqueous humor is a watery substance that fills the eye. It is split into two chambers. The anterior chamber is located in front of the iris, and the posterior chamber is directly behind it. These layers allow the eye to maintain its shape. This liquid is drained through the Schlemm canal so that any buildup in the eye can be removed. If the patient’s aqueous humor is not draining properly, they can develop glaucoma.
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