Eye is one of the sense organs found in human body. It is equivalent to a photographic camera.
Did you know that the eye, just like our Earth, is not a perfect sphere? It is fused by two units or components called anterior segment, and posterior segment.
In this article, we are going to take a detailed look at the human eye structure by breaking it down in five broad chapters.
So, without further ado, let us start our learning process of structure of eye.
Chapter 1: Human Eye Structure – The EYEBALL
The eyeball is made up of three layers which are as follows:
- The Outer Layer
- The Middle Layer
- The Innermost Layer
Let us take a look at each of these three layers separately.
Eye Structure: The Outer Layer of the Eyeball
The outer layer is also known as the fibrous tunic. It consists of two regions which are: Cornea and Sclera. The outer layer also consists of the Canal of Schelmm and the Conjunctiva. Let us take a quick look at each of these parts of the eye.
It is the transparent and exposed part of the fibrous tunic. It covers one-sixth part of the eyeball.
Because cornea is avascular, it became one of the earliest transplanted organs of human body.
The first successful corneal transplant happened in 1905. It was performed by Eduard Zirm of Olomouc Eye Clinic (now in Czech Republic).
It is the remaining part of the tunic to which the extrinsic muscles (which move the eyeball) are attached.
It is the white part of the eye. It contains a lot of collagen fibers.
Most of the eyeball (5/6th part of the eyeball) is covered by Sclera. It helps in protecting and maintaining the shape of the eye.
Canal of Schelmm
At the junction of cornea and sclera, a structure called Canal of Schelmm is present.
The aqueous humor (described later) which is produced continuously in the anterior chamber is drained into the Canal of Schelmm. It is through this canal the aqueous humor ultimately drains into the blood.
The continuity and oneness of the sclera and cornea is apparent but not real. This is because sclera is the product of mesoderm and cornea is the product of both ectoderm and mesoderm.
Conjunctiva is a thin and clear membrane which covers the front surface of the eye and the underlining of the eyelids.
Conjunctiva consists of two segments which are: Bulbar Conjunctiva and Palpebral Conjunctiva.
It is the part of conjunctiva which covers the anterior (upper) part of sclera. It stops at the Canal of Schelmm.
This part covers the underlining of both upper and lower eyelids. It is also known as Tarsal Conjunctiva.
These two parts (the Bulbar and Palpebral Conjunctiva) are continuous which is why contact lenses doesn’t get lost behind our eyes!
Conjunctiva helps in keeping the front part of the eye and inner part of eyelids moist and well lubricated, and in protecting the eye from foreign particles.
Eye Structure: The Middle Layer of the Eyeball
The middle layer of the eyeball contains many blood vessels. It is also known as Uvea because of its bluish hue.
This middle layer consists of three parts, which are: Choroid, Ciliary Body and Iris.
It is the vascular, pigmented layer completely adhered to sclera.
The blood vessels of choroid supply nutrients and oxygen to other parts or tissues specially to the retina.
The pigmented cells help in absorbing the light and prevent the light from being reflected in the eyeball.
Ciliary body which is the Uvea’s muscular part can be found at the transition level between cornea and sclera.
It consists of ciliary muscles and ciliary processes (finger-like projections).
Aqueous humor is secreted by ciliary processes.
Ciliary muscles are involuntary or smooth muscles and are of two types ‑ circular ciliary muscles and meridional ciliary muscles.
Ciliary body is connected to the capsule (see below at #?) which surrounds the lens through suspensory ligaments.
The ciliary body, the capsule and the suspensory ligaments together hold the lens in place.
From the ciliary body, the uvea moves away from sclera. It moves inwards and joins the margin of the innermost layer (which is the retina) to form the third part of Uvea, the Iris.
It contains the pigment which gives eye its particular color.
Iris separates aqueous humor in anterior region and posterior region.
There is an opening in the center of iris which is called the pupil.
Iris contains smooth muscles which are sensitive to light.
There are two types of smooth muscles ‑ sphincters or circular muscles and dilators or radial muscles. These muscles are of ectodermal in origin.
So, the iris controls the light entering the eye.
Pupil changes its diameter because of the smooth muscles of the iris.
When there is bright light, sphincters or circular muscles of iris contracts so that pupil decreases in its diameter which in turn allows less light to enter the eye.
When there is less light, dilators or radial muscles of iris relax.
Because of this the pupil’s diameter increases and allows more light to enter the eye.
Constriction of pupil is due to the stimulation of parasympathetic nervous system and dilation of pupil is due to the stimulation of sympathetic nervous system.
Eye Structure: The Innermost Layer of the Eyeball
The innermost layer is also known as retina. It borders all along the uvea. It is 200 µm thick.
The part where retina is in contact with choroid is thick and it is the region which is sensitive to light.
The remaining part is thin and not sensitive to light and contributes to iris and ciliary body.
Retina’s outer part is connected to the choroid, and the inner part is connected to the vitreous humor.
The retina consists of the following layers:
- Outer pigmented layer
- Sensory layer
Outer Pigmented Layer
It is attached to the choroid layer. It contains a single layer of cells which contain pigment. These cells are rectangular. They produce projections called pigmented processes which breach into the next layer.
The Sensory Layer
It is the nervous layer which contains certain specific light receptors and succession of neurons in synaptic series.
The cellular components of this layer are further divided into three layers which are:
- A layer of Photoreceptors
- A layer of Bipolar Neurons
- A layer of Ganglion Cells
A Layer of Photoreceptors
It consists of two photoreceptors – rods which are long and narrow and cones which are short and thick.
Rods contain a pigment called rhodopsin or visual purple. It contains opsin and retinene.
Opsin is a protein or amino acid and is known as scotopsin. It is present in visual purple.
Retinene is an aldehyde of vitamin A and it is termed as retinal.
Rods majorly help the organisms to see in the dark. Hence, they are present in higher numbers in nocturnal animals than in diurnal animals.
There are three types of photosensitive pigments in cone which are porpyrosin (for red light), iodopsin (for green light, also known as visual violet), and cyanopsin (for blue light).
Organisms can see different colors because of the combinations of these 3 types of cones and their pigments.
When all the types of cones are equally stimulated, sensation of white light is formed.
Cone contains a protein called photopsin which is different from scotopsin of rods.
They serve as the primary recipients of visual stimulation.
The free ends (processes) of the photoreceptors are present towards the outer pigmented layer.
Rods are nearly 120 million in number and cones are only 7 million in each eye. This means that there are nearly 17 times more rods than cones in our eyes.
There is another layer called External Nuclear Layer which contains the cell bodies and nuclei of rods and cones.
A Layer of Bipolar Neurons
The photoreceptors – rods, and cones send processes to the outer layer where these processes form synapses with bipolar neurons and eventually form a network or plexus of nerve fibers.
This makes a connection with photoreceptors to the neurons. But what about the integration of rods and cones?
There is something called horizontal integration of photoreceptors where the rods and cones are connected via a layer of horizontal cells (a variant of interneurons).
The processes of these interneurons connect with the terminals of neighboring rods and cones.
Note: The region where the synapse takes place is known as External Plexiform Layer or Outer Synaptic Zone.
A Layer of Ganglion Cells
In the inner synaptic layer, the terminal of each bipolar neuron is connected to one or more ganglion cells and other horizontal integrative components called amacrine cells.
The axons of these ganglion cells move over the inner surface of the retina.
All these axons finally converge to form optic nerve. This optic nerve then moves through choroid and sclera and reaches the brain.
The spot where these axons converge, the neurosensory cells are absent and therefore this spot is called blind spot or optic disc in the visual field.
There is another area where there is a greater visual activity than any other area of retina and this area is called Fovea centralis.
Fovea centralis is a specialized depression where many thousands of closely packed cones are present. However, there are no rods in Fovea centralis!
There are no blood vessels in this region and ganglionic nerve fibers diverge in such a way that the focused light falls directly on the cones.
Did you know why our retina is termed as “inverted”? It is because light has to pass through layers of bipolar neurons and ganglionic cells to reach the rods and cones (except for Fovea centralis).
In the same way the stimuli have to pass through these layers to reach optic nerve.
In simple words, the light sensing cells are present in the back of the retina and hence light has to pass through the retina to be sensed!
Note: The region where the synapse takes place (between dendrites of ganglion and amacrine cells and axons of bipolar neurons) is known as Internal Plexiform layer or Inner Synaptic Zone.
Apart from the above-mentioned cells, Retinal Gliocytes or Cells of Muller are also present in the retina. These cells form many protoplasmic processes that spread throughout the retina and form internal and external limiting membranes called Membrana limitans interna and Membrana limitans externa respectively.
These cells support neurons and some may even cover the neurons. It is also found that they have nutritive function.
Astrocytes, a type of glial cells are also seen.
This completes our first chapter of human eye structure. Before we move on to chapter 2, we suggest that you go back at the top and give a second reading. This will ensure that your knowledge of the structure of human eye becomes thorough and extensive.
If you are ready, here is the second chapter…
Chapter 2: Eye Structure – Eye Parts – The COMPONENTS of EYEBALL
The eyeball generally include lens, aqueous humour, vitreous humour, extrinsic eye muscles. Let us take a detailed look at each component is details.
The Lens is the interior of the eye present behind the iris. It is glassy and clear in its appearance.
It is elastic and biconvex and has a refractive index of 1.42. Therefore, the lens bends the light waves as they pass through it.
It is thickest near the circumference.
As mentioned earlier, the ciliary body, capsule and suspensory ligaments (aka suspensory zonule) hold the lens.
The lens is covered anteriorly by epithelium. At the center of anterior surface of the lens, the epithelium is made up of a layer of cubical cells but at the periphery of the lens, the cells elongate to form fibres of the lens.
These fibers are arranged concentrically to form the substance of lens. The center of the lens consists of older fibers and hence it is stronger and the periphery of lens consists of new fibers and hence it is softer.
It separates aqueous humor and vitreous humor.
It changes its shape as required so that it can focus light waves to the retina from the objects present at varying distances from the eye.
It also changes as one ages, and hence there is change in vision as well. The capsule of lens is a clear membrane which covers the entire lens. It consists of layers of intercellular protein.
The total refractive power of eye is 59 diopters while the total refractive power of lens is only 20 diopters.
Aqueous chamber is the space present between cornea and lens. The fluid filled in the aqueous chamber is called Aqueous Humour.
Ciliary processes constantly secrete this fluid.
Aqueous Humour helps giving much-needed nutrients to lens and cornea and in maintaining the shape of the eye’s front part.
As stated earlier, the fluid drains into blood through the Canal of Schelmm.
The intraocular pressure of the eye is mainly produced by the aqueous humour.
Vitreous chamber is the space present between the lens and the retina. The chamber is filled with a transparent gel called the Vitreous Humour.
Like aqueous humour, vitreous humour helps in maintaining the pressure of the eye, called intraocular pressure and maintaining the shape of the eye.
However, vitreous humour is not constantly produced and cannot be replaced if significant amount of it is lost.
Extrinsic Eye Muscles
There are six extrinsic eye muscles which are attached to the eyeball. Two of them are oblique and four are straight.
The names of the six muscles are:
- inferior rectus
- superior oblique
- median rectus
- superior rectus
- lateral rectus
- inferior oblique
Chapter 3: Eye Structure – Accessory Structures of the Eye
The eyebrows, eyelashes, eyelids and lacrimal apparatus form the accessory structures of eyes.
Eyebrows are arched projections of the skin present above the eye.
Many hairs grow obliquely from the skin’s surface. Eyebrows protect the anterior part of the eye from foreign particles.
Eyelashes are small hairs that grow at the eyelid. They have various functions including preventing the entry of small particles like debris, sand and dust into the eye causing damage.
The eyelashes are also very sensitive to touch and work as a warning instrument telling us that something has come too close to the eye.
Eyelashes are also regarded as emphasis of beauty.
In case of an embryo, the eyelashes develop somewhere between 22nd week and 26th week.
Later in life, if someone pulls out the eyelashes, they take around 7 to 8 weeks to grow back.
Eyelids are the folds (movable) which are present below and above the front of the eyes. The hair that grows on their free ends are called eyelashes.
In some animals there is a third eyelid called nictitating membrane or plica semilunaris. It is vestigial in humans.
Conjunctiva (which is a mucous membrane) is the underlining of the eyelids.
Modified sebaceous glands called Glands of Zeis open into the follicles of the eyelashes.
Tarsal or Meibomian glands are modified sebaceous glands which are present along the edges of the eyelids.
The oil secreted by tarsal glands and Glands of Zeis help in lubricating the surface of cornea and to hold a thin layer of tears over the cornea.
Glands of Moll, present at the edges of the eyelids, are nothing but modified sweat glands.
The Lacrimal Apparatus is a physiological system. This system consists of orbital structures which are responsible for the production as well as the drainage of tears.
The whole apparatus is made of several parts which includes lacrimal glands, lacrimal canaliculi, lacrimal sac and nasolacrimal duct.
Lacrimal glands are present in the orbit of the eye on the superior, lateral surface of the eyeball.
They produce tears which contains salt, water and lysozyme. The tears flow into inferior and superior canaliculi and reach the lacrimal sac.
From the lacrimal sac, they reach into the nasal cavity through nasolacrimal duct.
Tears help in keeping the cornea moist thereby removing any foreign particle or bacteria etc. The lysozyme kills any microorganisms, if present.
Did you know why you get runny nose when you cry? It is because the lacrimal glands produce a lot of tears and nasolacrimal ducts cannot simply carry them. And the result? You have a runny nose!
Gland cells present in conjunctiva also help in secreting mucous like substance which is a constituent of tears.
All right! You made it to the end! Yes, we have completed the human eye structure.
We are yet to tell you a few interesting tidbits about human eye and of course, the function of the human eye.
Let’s start then…
Chapter 4: Human Eye Structure – Few Interesting Tidbits
1. A thin layer of adipose tissue is present all around the eyeball. It works as a soft shockabsorber.
2. The ability of eyes to adapt from bright light conditions to dark condition is called dark adaptation. It is the retina which enables the eye to see in the dark.
3. Adaptation of cones from bright place to darkness happens quickly. It takes just 10 minutes.
4. However, the adaptation which happens after the 10 minutes happens slowly and with a greater degree of sensitivity and degree.
5. It is to be noted that if someone is being exposed to bright light for a long period of time, this adaptation takes some years.
6. Rhodopsin is formed in higher quantities when there is either dim light or darkness. This in turn increases the sensitivity of the rods.
7. Vitamin A is necessary for the proper production of rhodopsin. So, dark adaptation depends indirectly on vitamin A as well.
8. There is something called Light Adaptation. It is the ability of the eyes to adapt or adjust to bright light conditions immediately after coming out from darkness.
9. Though the earlier signs of this adaptation are sort of dazzle, they quickly decrease as the eye become less sensitive.
10. When a person comes into a brightly lit room from darkness, the rhodopsin level decreases.
11. The other two ways through which the eyes adapt to either dark or light adaptation are change in pupillary size and neural adaptation. Change in pupillary size is mentioned earlier.
12. Neural adaptation makes use of the neurons in successive stages of the visual chain. They take place in retina and in the brain. These two adaptations take few steps and are quick whereas the photochemical one takes from minutes to several hours.
Chapter 5: The Functions of Eye
Now that we are done with the human eye structure, it is time that we divert our attention to the function of the human eye.
Short and simple, the main function of the eye is to help us see.
But, if you are hungry for some details, read on…
The functions of the eye can be classified into three parts:
- Creating a sense of sight.
- Perception of color, details and motion.
- Production of tears.
Creating a Sense of Sight
To create a sense of sight, our eyes act as a digital camera. As a matter of fact, our eyes are far more advanced than the most advanced digital camera we have managed to create till date.
So, how does our eye act as a digital camera to create the sense of sight? Here is how it does so:
First, the light enters our eyes through the cornea. So, the cornea is like the lens of the camera.
Second, the iris controls the amount of light that reaches the back of our eyes. So, the iris works as the diaphragm of the camera. The iris manages to control the amount of light by adjusting the pupil size. So, the pupil works the a camera’s aperture.
Third, the eye lens that sits right behind the pupil focuses on objects that are close or are approaching towards us. So the lens works very much like the autofocus camera lens.
Fourth, the light which is focused together by the cornea, the iris, the pupil and the lens eventually reaches the retina, which is nothing but a light-sensitive lining, which is located at the back of the eye. So basically, the retina is like the electronic image sensor you find in digital cameras.
The retina converts the images into electrical signals and then send them to the optic nerve.
Fifth, the job of the optic nerve is to then transmit these electrical signals to the brain’s visual cortex. The brain then creates the sense of sight.
Perception of Color, Details and Motion
While discussing the structure of the eye, we talked about rods and cones. There are millions of those in our eyes. These rods and cones have special functions.
The rods are responsible for detecting motion and for our side (or peripheral) vision and night vision.
The cones on the other hand have the job of perceiving details and color.
These rods and cones are responsible for converting the light from the retinas into electrical impulses which are then sent to the brain through optical nerve. The image is produced in the brain.
Our retina has a part known as macula. This part is responsible for giving us the central vision.
Production of Tears
Our eyes also produce tears which are responsible for lubricating and nourishing eye surface and are also responsible for washing away any any debris that enter our eyes.
That’s it! We have completed our detailed study of human eye structure. The eye structures and functions mentioned here are simplified for ease of understanding. In case you face any problem, do not hesitate to drop a message and we will try our best to clarify your doubts.
Sources Used for Eye Structure Article…
1. Singh H.R. and Kumar N. (2017) Animal physiology and biochemistry. Jalandhar, Delhi: Vishal Publshing Co.
2. Chaurasia B.D. (2006) Human anatomy volume 3 head, neck and brain. New Delhi: CBS Publishers and Distributors