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Anatomy and Physiology of Human Eye

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The eyes are an important part of the human body. It is an essential sense organ of humans. There are five major sense organs are present in the human body. They are Eye, Ear, Nose, Tongue & Skin. The eyes provide a sense of vision. Without vision, no animal can have proper navigation. It is the most useful part of the human body. Human eyes have the feature to look around without moving the neck. As eyes have the power to move their lens. Eyes & Optic Nerves both together work & develops an image of the things which we are seeing. The light comes from the outside & it develops the picture of that object in the eyes. Hence, we can see the image of the object.

Anatomy Of Eye

Eyes are round in shape. But the Eye Leaves cover most of the structure of the eye. Hence, we can see the eye as in a convex structure. But the Eye Leaves are not part of the eye. There are some components in the eye. Those structures together build the anatomy of the Eye. Also, along with the components, there is the secondary structure which helps the eye to smoothly look around.

Human Eye


Primary Components Of Eye

  1. Cornea: It is the circular structure present in front of the eyes. We can’t able to see the cornea directly from the outside. As it is circular, we can’t able to identify its position normally. It is just situated at the front of the eye. It is completely transparent. There are no blood vessels present at all in this area. It is a highly sensitive region.
  2. Sclera: This is the whitest region found in the eye. It covers most of the eye portion which we can look at from the outside.
    Aqueous Humour: It is the jelly-like structure present between the Cornea & the lens. There is 99% of water in this portion. Nearly 1% is vitamin, protein & other materials may present. As the name suggests “Aqueous” means the water. And it defines the watery nature of this area.
  3. Iris: It is made with a fiber-like structure. It is the circular region present in the middle of the eye. There are some pigments present. According to the inheritance, these pigments are inherited from the parents of any individual. These pigments may be Black, Blue, Brown, etc. as per the heredity. That is why Iris may differ from individuals as per the availability of the pigments.
  4. Pupil: This is the circular hollow position present in the center of the Iris.
  5. Lens: It is olive shape structure present behind the Pupil. It is enclosed in a transparent capsule. There are no blood vessels in this area. This is the main component of the eye.
  6. Ciliary Muscles: These are the muscles that hold the lens at its position.
  7. Vitreous Humour: This is another jelly-like structure. It is present inside of the eyeball. It is completely transparent. But it is not soft like jelly. The term “Vitreous” means Glass like structure. But it is not hard as glass. But it is transparent like glass.
  8. Retina: This is another main component of the eyes. Inside the wall of the eyes, the retina is present all over the wall. This is the receipting component of the eye. But the Retina consists of two different kinds of cells. They are Rod Cells & Cone Cells. Around 130 million cells are present here. Also, only this part has the blood vessels. Also, this part is connected with the nerve.
    • Cone Cells: These cells work as light-sensing cells. There are nearly 5 million Cone Cells present in the eye. There are three types of cone cells. These divisions are made based on the color they can accept. There are Red Light Sensing Cone Cell, Green Light Sensing Cone Cell & Blue Light Sensing Cone Cell. Among them, Red light-sensing cells are present most of the number & Blue light-sensing cells are present least in number. All other colors can be accepted by the combination of those cells.
    • Rod Cells: These are also the light-sensing cells present in Retina. There are 125 million Rod Cells present in Retina. These cells can only able to accept low bright light. They can’t able to accept high-intensity lights.

Secondary Components of Eye

  1. Optic Nerves: This is the main component of the secondary eye components. This is very important for visualizing anything. Optic nerves merged with the Retina in the eyes. Nearly one million nerve fibers merged with the Retina. Optic Nerves are part of the Central Nervous System (CNS). They carry the messages from the eye to the brain.
  2. Layers of Eye: There are mainly three layers present in the eye. The different components of the eye merged & define such layers.
    • Fibrous Tunic: It is the outer layer of the eye. It is made with tissues. And it covered the outer layer of the eyes. The outer layer of the eye is also made of Sclera & Cornea of the eye.
    • Vascular Tunic: It is the middle layer of the eye. Though it is not surrounded the eye. But it divides the eye into Interior & Posterior portions. It is also made with tissues & fibers. The Ciliary Body & Iris are two components that developed the Vascular Tunic.
    • Inner Layer: The inner layer of the eye means the Retina of the eye. There are no tissues present in this area.

Functions Of Eye Components

  1. Cornea: The cornea helps to bend the light rays. As it is convex in shape, it will bend the outer light source & make convergent beams of light. It helps to pass through the Pupil of the eyes.
  2. Sclera: It helps to provide the shape to the eyeball. It also helps to protect the eyeballs from any external threats. Also, it has the muscle Conjunctiva, which helps to move the eyeball up & down.
  3. Aqueous Humour: It helps to maintain the shape of the Cornea. As Cornea is convex, it provides support to the Cornea. So, the Cornea will never get misshaped. Also, it helps to reduce the friction while moving the Lens of the eye.
  4. Iris: The light beams enter via Cornea. But the complete light beams are not necessary to get inside the eye. Iris absorbs the extra light beams & reflects them. Depending upon the pigments available in the Iris, we can able to see some colored Iris for individual humans. Also, Iris helps to maintain the size of the Pupil. Depending upon the environment the Pupil size gets changed.
  5. Pupil: It has only a single task to enter the convergent light beams inside of the eyeball. With the help of Iris, Pupil can change its size. Like in darkness, when there is a need to get more light beams inside of the eyeball, Iris helps to get stretched the Pupil. So by the Pupil, more lights enter the eyeball. In more bright light, Iris contract the Pupil. So that minimum light beams can enter the eyeball. Otherwise, more bright light can harm the inside of the eyeball. It can able to damage the cells also.
  6. Lens: This helps to make a clear picture of the Retina. The lens works the same as the camera lens. As it is convex it can make convergent light beams. It helps to focus all the convergent light beams to a particular point of the Retina. So a clear picture can be available at Retina.
  7. Ciliary Muscles: These muscles hold the lens in its place. Also, depending upon the light beams can change the structure of the lens. Like if the incoming light beams are convergent, it will elongate the lens. So that, Lens can make a convergent light beam easily. If the incoming lights are divergent, so the ciliary muscles will compress the Lens. So it can easily focus the convergent light to Retina.
  8. Vitreous Humor: It is the fluid-like structure present on the inside of the eyeball. It helps to support the shape of the eyeball from the inside. Without it, the shape of the eyes makes gets misshaped due to injury.
  9. Retina: This helps to collect the light beams from the outside. Then, it will make the picture of the image from where the light is coming. Depending upon the environment the light receipting cells act. After making the image of the object, it provides the details to the Optic Nerve.
  10. Layers of the Eyes: It helps to protect the eyeball from any injuries. Also, it helps to provide the shape of the eyeball.
  11. Optic Nerves: These nerves collect information from the retina. Then it decodes the information to the message or commands. Then these messages will transfer to the optic lobe of the brain. There the messages are being interpreted by the brain & the picture of the object can be visualized by the human.

Mechanism Of Eye

The light beams from any of the outer objects first land on the Cornea. The makes them convergent light beams & try to focus them on the Pupil. Then via Aqueous Humour, the light beams come to the Pupil. Some radio of the light beams gets absorbed by the Iris. The remaining light beam directly enters the eyeball. Now, depending upon the nature of the light beam, the Lens tries to make a convergent light beam & focus it on a certain point at Retina. After being convergent by the Lens, the light beam travels via Vitreous Humour & then lands on the special region of the Retina. This is a special region as this region can able to make a clear picture. The other region of Retina can also be able to make the picture, but they are not so clear. But the picture developed in the Retina is the reverse of the actual object. Then this image is converted to the message & received by the Optic Nerves. The Optic Nerves then took the messages & transfer them to the Optic Lobe of the Brain. Their brain reads the message & develops the reverse of the accepted image. Hence, it will develop the actual image of the object. The working of the eye is as same as the camera. Here, Iris works as aperture & Retina works as a film of the camera.

What is Eye Movement?

Inside our eyes, there is only one type of muscle is present. They are the ciliary muscles. Along with that, there are some muscles present outside of the Sclera. These muscles help to rotate the eye. There are six muscles present. They are plays the main role to rotate the eyeball. Eyes have the Optic Nerve. By that Eyes sends the signal to the brain. By which we can visualize anything. The same comes Cranial Nerves are there. These nerves connect the brain with the Eye in a different form. Through these nerves, Barin sends signals to the muscles of the Eyeball. Hence, they can rotate themselves to a certain degree of position.

According to scientists, there are mainly four types of Eye Movements can be found. Based upon the action & muscles interaction these names are generated.

  1. Saccades: This movement is used to see a rapid change in the position of an object. This movement involves when we don’t have the option to move the head & neck. At that point of time of gazing at a certain object which moves very fast, this Eye Movement helps us to look at that.
  2. Vergence: This is a special kind of Eye movement. This type of movement can visualize when we need to focus on a certain object. It helps to make a clear image in the Retina. The eye moves in such a way that light beams from that object land on the focus area.
  3. Smooth Pursuit: This is a very soft kind of movement in the Eye. This is a very smooth movement rather than other movements in the Eye. When we have to continuously gaze at an object which is moving in a small amount, then this movement helps us.  In this movement, Eyeball rotates in a very small amount. Hence, there is no extra work have to be done by the Eye muscles.
  4. Vestibulocular: This is another type of Eye movement. In some fields, where the heads move drastically. But the eye needs to focus on any object to see it. Then this type of movement can be observed. This type of movement can observe in a crowded place. These individuals need to observe certain interesting things. But due to the huge crowd, they can’t properly see them. As a result, they move their head at two different sides of their body. With that, the Eyes need to move to focus on the object.

Near Response of Eye

This type of response in the Eye can observe when individuals sit in a car or any moving things. Here, at some point in time, an object was situated at a distance. But with time it comes closer to the individuals. Then Eye needs to refocus on it to get a clear image of it. For that purpose, the Eye needs to change its Lens structure. This is called a Near Response.

When a light beam is coming from a distance, it creates a divergent light beam. To focus divergent light beams at a certain focal point, Eye makes some changes to the Lens. The Ciliary muscles contrasted the Lens. Hence, the width of the Lens gets increased. So it can easily make convergent light beams with such a Lens structure. But when the object comes nearer to the individuals, the incoming divergent lights become nearly parallel light beams. So, to make the focus of such parallel light beams Ciliary muscles elongated the Lens structure. Hence, it can easily get the image of the object.

Along with that when the object is situated far from the individual, the Iris also gets contrasted & helps the Pupil to get extended. It helps to collect more divergent light beams from the object. And when the object comes nearer to the individual, the Pupil gets contrasted to stop more incoming Parallel light beams. These actions of the Eye are known as the Near Response of the Eye.

Importance Of Eye

The eye is an important sensing organ in our body.  It helps to visualize anything which we are seeing. Also, along with that, it helps to navigate us in any situation. Without looking at anything it is very difficult to know where is anything situated. It is the main part of the body that helps to identify anything. Some lower-class animals don’t have a powerful eye. But they pose a sensing organ that helps to navigate them. The importance of the eye doesn’t need to clarify. Without it, we can never spend a single day. It is not only a sensing organ & also a navigating organ. The individuals who don’t have a working eye or have any difficulty in vision know the importance of the eye. In every aspect of our daily life, we use our eyes. From typing a message to playing a game, the eye is being used in every field. It helps us to live our life easier. It helps to visualize the world.

Eye Irritation

Eye irritation is a term that defines some common problems related to the Eyes. In such cases, individuals can feel itches, pain, vision problem, etc. These cases are very common in humans. We can note some problems like:

  • Dryness of Eye: It is a problem in the eye. Our eye has lacrimal glands. These glands wet our eyes. Due to long use of mobiles, laptops for a while. These cases can develop. Long use of electronic devices leads to dry eyes. As the lacrimal glands can’t work properly in such cases.
  • Itches of Eye: Some common problems with the eye are related to foreign elements. Some pollen or chemical can lead to itches in the eye.
  • Allergic Reaction: Human Eyes are very sensitive with the allergy. Due to some allergies to the eye, the eye can get red. It can lead to severe itches in the eye. Also, if the human body is affected by allergies, then the eye can also be a symptom in such cases.

Disorders With Eye

There are several disorders are present with the eye. Some of them are very common in humans. But some of them are very rare in case. Some of them don’t completely make a human blind. But some of them can make visual impairment.

  • Common Disorders with Eye
    • Myopia: It is a disorder when the individuals can able to see an object which is situated near to them. But they can’t able to see the objects which are far from them. Due to a problem with the Eye Lens this type of problem may arise. In such cases, individuals need a spectacle to remove the problem.
    • Hyperopia: It is the objects which are situated far from individuals can able to see clearly. But those objects which are situated nearer to them are not been seen. In such cases, individuals need to get a spectacle to remove the solution.
    • Cataract: It is a very common disorder in older aged people. A thin layer of protein developed on the Lens of the Eye. Thus it makes a cloudy-like structure in front of the eye. In these cases, clear visions are distracted. In this case, the Lens of the eyes is being removed. And in place of that, an artificial lens is provided to the individuals.
  • Rare Disorders with Eye
    • Coloboma: It is a disease in the eye where the tissues which protect the eye from injuries are missing. It is a very rare disease. In this disease, the chance to get injuries to the eye is much higher than the normal cases.
    • Anophthalmia and Microphthalmia: These are inborn diseases. In the case of Anophthalmia, a child takes birth without the eyes. The child doesn’t have an eye. In the case of Microphthalmia, the child has the eyes. But they are not working in nature.
    • Usher Syndrome: It is a disease where a child has both hearing & vision impairment. As well as in this disease the child may have a problem with balance. This disease occurs due to genetic impairment in a child.  This is a very rare disease. Nearly, 4-17 children get affected per 100000.

FAQs on Human Eye

Question 1: What is Blind Spot?


The blind spot is the region in the eye. It is the region where the Retina gets connected with the Optic Nerves. In this region, light doesn’t get to land. The light which will land in this area will not be received by the Optic Nerves. Thus this area does not develop any picture at Retina. That is why it is called Blind Spot.

Question 2: What is Yellow Spot?


Yellow Spot is often known as Macula. It is the region in the eye. It is on the opposite side of the Pupil. Rarely light beam gets land in this area. This area has a high concentration of Cone cells. If light gets landed in this area a yellowish color image will appear.

Question 3: What is Optic Disc?


The Optic Disc is the region where the Optic Nerve gets connected to the Retina. It is the starting point of the Optic Nerve. Sometimes this area can be visualized from the outside of the eyes. It is often called a Blind Spot.

Question 4: What is Fovea?


It is the region in the eye where the light gets focused by the Lens. In this region, a clear image of the object is developed. This is the region where the clearest, correctly developed picture can be available. From this region, the Optic Nerves take the messages.

Question 5: According to Scientists, why are the cone cell intensity high in the human eye?


Human Eyes have nearly 125 million Cone Cell. These cells are only used to receive dim light. And Rod Cells are nearly 5 million in number. Scientists believe that in the ancient era when humans only lived in the caves, they mostly use the Cone Cell for vision. As there was not any bright light available at those time. According to the rule of evolution, the Cone Cells developed most. And in this way, Cone Cells are large in numbers than Rod Cells.

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Last Updated : 02 Sep, 2022
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