A highly sophisticated organ that allows us to perceive the world through sight.
The eyes receive information from the outside world through light, which is transformed into nerve impulses to the brain where it is interpreted.
Next we are going to talk about the parts of this organ that is so important for human beings, in addition to describing the functions they carry out and some diseases that they can suffer from.
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What is the eye?
The eyes are known to be one of the most complex organs in the human body. Its main function is to detect light and send signals through the optic nerve (second cranial nerve) so that they reach the brain , specifically the cortex of the occipital lobe , where they will be interpreted in the form of images.
Each eye is spherical in shape, with an approximate diameter of 2.5 cm and a weight of between 7 and 8 grams. The movements of the eyes are due to the fact that they have 6 muscles inserted in the most external part, the sclera. These muscles act in such a way that the eyes are coordinated when looking at a specific place.
The eyes are divided into two sections: the anterior chamber and the posterior chamber, with the aqueous and vitreous humors.
During embryogenesis, the eye forms partially from the central nervous system . The retina is formed from the diencephalon, an embryonic structure that also gives rise to the cerebral hemispheres, thalamus and hypothalamus in more advanced stages of development.
The parts of the human eye
The parts that make up the eyes can be classified according to the three layers that these two organs present:
- Outer layer: sclera and cornea
- Middle layer: choroid, iris, ciliary body and lens.
- Inner layer: retina, fovea, macula and aqueous and crystalline humors.
All the parts that make up the eyeballs are described below.
1. Sclera
The sclera, also called the sclera or popularly known as the white of the eyes, is a fibrous and opaque structure whose function is to protect the internal content of the eye , giving it shape and acting as a kind of skeleton.
It is in the sclera where the muscles that allow the saccadic movements of the eyes are inserted.
The anterior part of the sclera is connected to the cornea, while the posterior part has an opening that allows the connection between the retina and the optic nerve.
2. Cornea
The cornea is a kind of spherical cap with a convex shape that is located in the front part of the eye.
It is a very transparent tissue, without blood vessels but with a lot of nervous innervation, which explains why the eyes are so sensitive to external stimuli. The function of this structure is to refract and transmit light towards the back of the eye.
If the shape of the cornea presents abnormalities, this can affect vision, since it can happen that the light introduced into the eye is projected at different points on the retina, instead of just one. This is what causes astigmatism.
3. Choroid
It is a layer made up of blood vessels and connective tissue that separates the sclera and the retina. This structure contains a pigment, a pigment that absorbs excess light and prevents blurred vision.
It is also responsible for nourishing the retina, in addition to maintaining a constant temperature in the eye.
4. Iris
The iris separates the anterior and posterior chambers of the eye, and forms the rim of the eye’s central opening , the pupil.
It protects the eye from the entry of too much light, working as if it were the diaphragm of a camera. If there is a lot of light, it contracts (miosis), and if there is little, it dilates (mydriasis).
The iris has the remarkable characteristic of being able to be of various colors. When we say that a person has blue, green or brown eyes, we refer to this specific part of the eyes.
The color of the iris varies depending on the amount of melatonin and the pigment it contains.
5. Pupila
This is a dark, circular hole in the center of the iris that allows light to enter the eye .
6. Ciliary body
It is a structure that joins the iris with the choroid . It produces the aqueous humor and is responsible for the accommodation of the eye, that is, making the lens change shape.
7. Crystalline
The lens is known to be the natural lens of the eye . It is biconvex, transparent and elastic, located behind the iris.
It does not contain blood vessels nor is it innervated, and is supported by the ciliary body.
Thanks to the fact that it can change its shape (accommodation), it works as if it were the zoom of a camera, bulging or flattening in order to focus the images that are located at different distances.
Age affects the lens. With the passage of time and, above all, after the age of 45, this structure loses transparency and increases in thickness, reducing its accommodation capacity.
This problem makes it increasingly difficult to read up close, resulting in presbyopia and eyestrain.
8. Retina
The retina is the most important inner layer of the eye . It is a photosensitive layer in which light is transformed into nerve impulses that are sent to the brain. It contains the photoreceptors that allow vision: the rods and cones.
The cones are cells that allow color vision and there are three types, each with the ability to detect the colors red, blue and green.
If there is an abnormality in the cones, one of the three primary colors of light may not be seen or be mistaken for one of the other two. This problem is called color blindness.
The rods are the photoreceptors responsible for night vision. They are more sensitive to light and produce black and white vision.
Some vision problems can be the result of the light beam introduced into the eye not being projected properly onto the retina.
In myopia, the light is not projected on the retina, but in front of it, while in hyperopia, the light is projected behind it.
9. Macula and fovea
The macula is located in the center of the retina, it is yellow in color and elliptical in shape . In its center is a small depression that is the fovea.
This is where the highest concentration of photoreceptors is found. This part is responsible for central vision, allowing daily actions such as reading or distinguishing people’s faces.
10. Aqueous humor
The aqueous humor is a substance found in both the anterior and posterior chambers of the eye . Its main function is to nourish and oxygenate the cornea and lens, as well as allowing eye pressure to remain constant.
The main components of this substance are water, vitamin C, glucose, proteins and lactic acid.
If the aqueous humor does not exit the eye correctly, it can lead to an increase in intraocular pressure, which can cause glaucoma.
11. Vitreous humor
The vitreous humor is a transparent, gelatinous tissue found inside the eye . It occupies about two thirds of the eyeball and supports it.
It fills the entire eye behind the lens, crushing the retina and protecting it from impact.
Floaters are a phenomenon that occurs in the vitreous humor that can be quite annoying. These are cells or small pieces of the same humor that project shadows on the retina, giving the sensation of having them in front of the eyes.
Bibliographic references
- Starr, C. & Taggart, R. (2008). Biology. The unity and diversity of life. Cengage Learning Publishers
- Land, M. F.; Fernald, R. D. (1992). The evolution of eyes. Annual Review of Neuroscience 15: 1-29.
To the classic question “what do you do?” I always answer “basically I am a psychologist”. In fact, my academic training has revolved around the psychology of development, education and community, a field of study influenced my volunteer activities, as well as my first work experiences in personal services.