Human eyes-all you wanted to know

Human eyes-all you wanted to know

Although the size of the human eyes is relatively small, they are very important to our perception of the environment. As we know, the eyes are the organs that allow us to take in the light and perceive the color, details and depth of the objects around us. They are considered one of the most complex organs in the human body, so much that if any of its parts suffers damages or does not work properly, the person loses his/her ability to see effectively or can even lose the vision completely.

human-eyes

What are the parts of the eye and what are their functions?

The eyes of some microorganisms and species are composed of simple cells that only allow to differentiate between light and darkness. However, the human eyes are complex organs composed by different lenses, supporting structures, filters, nerves and light-sensitive tissues. All the chemical and physical elements that make up the eyes work together to accomplish a good vision and to avoid injuries and infection in the more internal structures.

The central nervous system is closely related to the human eyes anatomy, so the brain can directly receive the information obtained from the eyes, process it and generate a response accordingly. In fact, if the eyes’ nerves suffer any damage, the brain will receive false images, so there will not be an appropriate perception of the environment.

From outside to inside, the parts of the eye include the following:

The Conjunctiva

These are clear and mucous layers that cover the inner eyelids and the eyes’ surface. Their main function is maintaining the outside of the eye properly lubricated. The human’s body produces several substances (mucous, oils and watery solutions) that are poured on the conjunctiva to keep it lubricated. This also protects the eyes from irritating agents. When they are dry, the person can experience itch and pain in the eyes.

Being the outermost layer, the conjunctiva is also the part of the human eyes that is more prone to develop damages or infections. In fact, we all know someone, even ourselves, that have suffered a conjunctival inflammation/infection, which is called conjunctivitis or pink eye. 

The Cornea

The cornea is a transparent structure located on top of the iris and pupil. It has multiple layers that give it the necessary hardness to bring additional protection to the inner structures of the eyeball.

These cornea layers have a great capability to regenerate themselves, which can help the eye to eliminate certain injuries or damages more easily. Likewise, the cornea has a lot of nerve fibers to detect any scratch, irritation, dryness or infection, conditions that can be really painful. On the contrary, the cornea has almost no blood vessels in it to avoid interference with the pass of the light, but this is a disadvantage when trying to fight an infection or heal a wound in this area.

However, the main function of the cornea is helping to correctly project the beams of light that penetrate the eye on the retina. The surgeries performed to help the eyes to get a well-focused image, consist of reshaping the cornea. The contact lenses have the same principle, they augment or reshape the cornea to properly focus the light in the retina.

human-eyes-anatomy

The Sclera

The sclera is the smooth and flexible layer that surrounds the iris and the pupil and extends to the back of the eyeball. This part of the eye does not collect visual information itself, but it is very important because it is strong enough to act as a tough and protective membrane for the rest of the structures inside the eyeball.

This is the part of the eye commonly known as “the white of the eye”, although only the outer part of the sclera is white. The inner layers of the sclera are brown and cover (do not go through) the inner chambers of the eye to allow the light to reach the retina. The sclera layers also have grooves where the tendons of the muscles that move the eye can insert properly.

The Iris

The iris is the ring-shaped structure around the pupil which brings “the color of the eye”. Its color can range from black to very pale blue or green according to the amount of melanin or pigment that its cells have. However, this is not its main function as we will see next.

Actually, the iris has a sphincter muscle able to expand or contract to regulate the amount of light that enters the eyes. When we are in a very bright environment, the iris will shrink the pupil to reduce the amount of light that enters, so the eye can focus in the objects with more effectiveness. On the contrary, the iris will increase the pupil diameter if there is low light.

This explains why when we go to the ophthalmologist, and he/she artificially dilates our pupils, we can experience blurred vision, light sensitivity and even the brighter lights can produce pain.

The Pupil

Many people think of the pupil as the “black dot in the middle of the eye”. However, it is the “window” or aperture between the inner structures of the eye and the environment.

It is black because the light which enters to the eye through it, does not return. In fact, once the light has entered to the eye through the pupil, it travels through the other eyeball structures to the retina, and it is transformed in signals which are understandable for the brain.

The Lens

It is the eye structure located just behind the pupil. It is also transparent, and the ciliary muscles are in charge of keeping it in its place and give it the necessary form to focus the light properly on the retina according to the amount of light that the pupil filters.

The lens can be defined as an elastic capsule that contains proteins and water to refract the light that goes through it. Its nucleus is firm, but the layers around it are softer to change its shape as needed. This is what gives it the “adjustable” character. When the ciliary muscles lose their capacity to change the lens shape, the person develops a condition called presbyopia, which is very common in people older than 40 years and makes it difficult for the person to read small letters without glasses.

The Aquous and Vitreous Humor

The aquous humor is a watery substance located in the eyes’ chambers, which means between the iris and the cornea (anterior chamber) and between the iris and the lens (posterior chamber). When the aqueous humor is not properly drained, the person can develop glaucoma.

The Vitreous humor is a gel that occupies the back of the eyeball to hold its shape. It also takes the nutrients from the blood vessels to maintain the other structures healthy.

The Retina

It is the layer of the eye that covers the inner-back part of eyeball. It has numerous cells which are sensitive to the light, and they transform the light stimuli into chemical and electrical pulses that can be interpreted by the brain to generate the images we see of the environment; this is why the retina is directly connected to the optic nerves. The two major types of cells in the retina are the cone cells, which perceive the colors, and the rod cells, which perceive the light.

The retina also has an area called macula, that helps to interpret all the details of the objects that our eyes are looking at. In fact, the center of this area, called fovea, increases the number of details of those images even more.

The Optic Nerve

As its name says, it is a bundle of neural fibers that travel from the eye to the brain. The optic nerve of each eye receives the information from the retina and sends it as nervous signals that the brain can read.

In conclusion, the human eyes are complex organs that improve our capability to interact with other people and the environment. It is very important to keep them healthy to be able to perceive all the details of the objects around us, its specific location and even imagine their texture before touching them.

The eyes health is of great importance to carry out all daily activities, so it is essential to have a check up with an ophthalmologist periodically.

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Aurelysmar Grimandecollantes

Author: Dr.Aurelysmar Grimandecollantes

Dr.Aurelysmar is 29 years old MD and medical writer. Medical degree from the Universidad Centroccidental "Lisandro Alvarado", Venezuela, 2014. Diploma in Occupational Health and Safety, Venezuela, 2015. Course: Specialized Health Training for Physicians, Spain, 2017.