Sight is probably one of the most complex sensory systems that living beings have, probably because of its strong implication in survival . Therefore, this sense is of vital importance. To talk about the parts of the human eye is to talk about intricate structures that range from the outside of the head to the innermost part of the skull.

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What is the human eye
It is convenient that, before delving into the analysis of the parts of the human eye and their functions, we make a brief review of the definition of these organs and their function.
The eyes are responsible for capturing visual information and processing it, in the form of electrical impulses, to the occipital lobe of the brain, located at the back of the head. Once it reaches this area, the brain will decompose the data received according to the shapes, colors, textures, depth and movement captured; allowing us to create an understandable mental image.
It must be said that, as technically it is the human brain that “sees”, each individual will perceive the reality that surrounds him in a different way, so that two people who contemplate the same thing will not really be seeing it in the same way.

7 external parts of the human eye
At the beginning we commented that there are parts of the human eye located on the outside, due to their role in capturing the information that surrounds us , as well as the protection of those most sensitive areas.
Let’s see, from the outside in, in which areas the parts of the external eye are divided.

1. Eyelid
The eyelids are the folds of skin that cover the eyesyes Its function is to keep the eyeballs moist with each blink (uncontrollable reflex actions), in addition to cleaning them of all solid particles that are suspended in the air and that can damage the most sensitive areas of the external eye.

2. Eyeballs
The scientific name by which what we colloquially call “eyes” is designated: the eyeballs are nothing more than the spherical organs that contain the rest of the parts of the eye that we will see below. They are located in the eye sockets (also called ‘orbits’), two separate holes located in the front part of the skull where they fit.
Both eyeballs are divided into what is called ‘anterior chamber’ and ‘posterior chamber’. The first section is located between the cornea and iris and contains the aqueous humor. The second chamber contains the vitreous humor and the other parts of the eye. These two fluids must adjust the pressure inside the eye and keep each zone in its correct position.

3. Sclera
The eyeballs have this protective membrane, which surrounds them and stains them with their distinctive white pigment , in addition to serving to protect the internal parts of the eye.
The sclera reaches from the cornea (front of the eye and what we see when they are open) to the back of the eye, which we do not see with the naked eye. There, the sclera is not completely closed, as it allows the retina (the membrane below) to connect to the optic nerve and the optic nerve to continue its journey into the skull.

4. Cornea
The cornea is the part of the central eye, located in such a way that it is in constant contact with the outside, at least while the eyes are not closed. Due to its location, its main role is basically protective , something easily deducible if we look at its domed and transparent shape.
Another task of the cornea is to protect the iris and the lens, although it is also responsible for refracting and filtering the light captured towards the rest of the ocular structures.

5. Iris
If we colloquially call the sclera “the whites of the eyes”, when we talk about the iris we are referring to that part of the eye that gives it color and which we look at when we say that someone has a beautiful eye color. This coloration is possible thanks to the action of melanin.
It is a disc-shaped muscular structure, which has the pupil in its central part, so that the iris will perform contraction (miosis) or dilation (mydriasis) movements , depending on the amount of light received from the outside.

6. Pupil

Central opening of the iris with a circular appearance and black color . Depending on the intensity of the light beam that penetrates along the previous parts of the eye, and as a result of mydriasis or miosis, the pupil will react by dilating or contracting.

7. Lens
We have included the lens in the subgroup of external eye parts, although its location is already internal. However, if we were to make a virtual cut to an eye, we would find it in the anterior-medial area of ​​these organs, so we will leave it here.
As we said, the lens is a lenticular structure, colorless and transparent, which is behind the iris. Its job is to focus the vision according to the distance at which the objects are.(short, medium and long). Even more important than this function, the lens must divert light from the outside to the retina, so that it spreads through the following parts of the eye, on its way to the brain.

6 internal parts of the human eye
After going through the outer layers of the eye, we go with the parts located inside, both the eyeballs and the skull itself.

8. Aqueous humor
We have already talked about this liquid in the point referring to the eyeballs, but let’s go a little deeper.
We mentioned that the aqueous humor is in the anterior chamber of the eyes, between the back of the cornea and the front of the lens. This privileged position makes it one of the most essential parts of the eye, due to itsmoisturizing function : neither the lens nor the cornea are vascularized, so if it is not for the aqueous humor, they would not be lubricated.
In addition, the aqueous humor has a pressure-stabilizing role , thanks to which the rest of the structures always remain firm in the same position.

9. Conjunctiva
We insist that vision is a vital sense that is important to keep practically intact, otherwise the survival of many species that depend on it would be strongly threatened. For this important reason, when it comes to eye protection, the more layers the better.
The conjunctiva is a transparent layer that surrounds the sclera. This thin sheetkeeps the eye hydrated practically all the time , from the segregation of natural oils that contribute to this end. As the ocular walls are always moist, the filtration of microbes into the eye is prevented.

10. Choroids
The sclera and the retina are separated by an intermediate membrane called ‘choroids’, whose function is to nourish and oxygenate the second for its optimal functioning. Likewise, this highly vascularized structure maintains a constant temperature in the eyes .

11. Vitreous humor
The aqueous humor is found in the anterior chamber of the eye, while the vitreous humor is in the posterior chamber of each eyeball, between the lens and the retina. This liquid fills this space, is thicker than its counterpart and gives rigidity to the visual organ, keeping the other parts of the eye in place.

12. Retina
It is the innermost membrane that covers the eyeball. It contains the cells in charge of processing visual stimuli , the ‘cones’ and ‘rods’, dedicated to daytime vision and the first chromatic distinction; and night vision and black and white images, the latter.

13. Optic nerve
Network of nerve fibers whose main task is the transmission of light stimuli(in the form of electricity) to the occipital lobe, where they will be recognized and will give meaning to what has been processed.

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