Eye Anatomy – Exeter Eye

The human eye is remarkable. Although it is small in size, the eye arguably provides us with the most important of the five senses – vision.

Vision occurs when light enters the eye through the pupil. With help from other important structures in the eye, like the iris and cornea, the appropriate amount of light is directed towards the lens.

Just like a lens in a camera sends a message to produce a film; the lens in the eye refracts incoming light onto the retina, where messages are encoded. The retina, which is made up by millions of specialised cells known as ‘rods’ and ‘cones’, transforms the image into electrical energy and this is sent to the optic disk on the retina, where it will be transferred via electrical impulses along the optic nerve to be processed by the brain.

The eyeball contains three layers:

• The outer layer, formed by the cornea and sclera
• The middle layer, holding the primary blood supply for the eye and containing the iris and pupil
• The inner layer, comprised of the retina

The eyeball also contains three chambers of fluid:

• Anterior chamber, between the cornea and iris
• Posterior chamber, between the iris and the lens
• Vitreous chamber, between the lens and the retina

The anterior and posterior chambers are filled with aqueous humour, which is a watery fluid that provides nourishment to the interior eye structures and helps to keep the eyeball inflated. The vitreous chamber is filled with a thicker fluid called vitreous humour, a transparent gel which is 99% water, which helps the eyes to stay inflated.

What makes up an eye?

• Choroid: the middle layer of the eye between the retina and the sclera. It also contains a pigment that absorbs excess light so preventing blurring of vision.

The choroid is the spongy middle layer of your eye located between the sclera and the retina. Filled with blood vessels, the choroid’s function is to nourish the outer layers of the retina.

• Ciliary body: the part of the eye that connects the choroid to the iris.

The ciliary body is located behind your iris, near the crystalline lens. This structure has two functions. The aqueous fluid that fills the front of your eye is made inside the ciliary body. Also, the ciliary body is made up of muscles that allow the eye to focus at different distances.

• Cone cells are the second type of light sensitive cells in the retina of the eye. The human retina contains between six and seven million cones; they function best in bright light and are essential for acute vision (receiving a sharp accurate image). It is thought that there are three types of cones, each sensitive to the wavelength of a different primary colour – red, yellow or blue. Other colours are seen as combinations of these primary colours.

• Cornea: the transparent circular part of the front of the eyeball. It refracts the light entering the eye onto the lens, which then focuses it onto the retina. The cornea contains no blood vessels and is extremely sensitive to pain.

The cornea is the clear front window, representing one-sixth of the outer layer of your eye. The primary function of the cornea is to focus and transmit light onto the retina.

• Conjunctiva: the thin, moist, clear membrane that covers the sclera – the white part of the eye. It is the skin that lines the eye socket and protects and lubricates the eyeball.

• Crystalline Lens: The transparent structure inside of the eye located directly behind your iris. The sole function of your lens is to focus light rays onto the retina.

• Fovea: forms a small indentation at the centre of the macula and is the area with the greatest concentration of cone cells. When the eye is directed at an object, the part of the image that is focused on the fovea is the image most accurately registered by the brain.

• Iris: regulates the amount of light that enters your eye. It forms the coloured, visible part of your eye in front of the lens. Light enters through a central opening called the pupil.

The iris is the colored portion of your eye. Located behind the cornea, and in front of the crystalline lens. This structure separates the anterior and posterior chambers of the eye. The function of the iris is to help regulate the amount of light that enters your eye.

• Lens: a transparent structure situated behind your pupil. It is enclosed in a thin transparent capsule and helps to refract incoming light and focus it onto the retina. A cataract is when the lens becomes cloudy, and a cataract operation involves the replacement of the cloudy lens with an artificial plastic lens.

• Macula: a yellow spot on the retina at the back of the eye which surrounds the fovea.

The macula is located roughly in the center of the retina. It is a small and highly sensitive part of the retina responsible for detailed central vision. The fovea is the very center of the macula. The macula allows us to appreciate detail and perform tasks that require central vision such as reading or driving a car.

• Optic disc: the visible (when the eye is examined) portion of the optic nerve, also found on the retina. The optic disc identifies the start of the optic nerve where messages from cone and rod cells leave the eye via nerve fibres to the optic centre of the brain. This area is also known as the ‘blind spot’.

• Optic nerve: leaves the eye at the optic disc and transfers all the visual information to the brain.

The optic nerve is located in the back of the eye. This structure is responsible for transmitting the images we see from the retina to the brain. The front surface of the optic nerve, which is visible on the retina, is called the optic disk. There are millions of nerve fibers that pass through the retina and converge to form the optic nerve. When light hits the retina it is converted into electrical impulses and carried along these fibers through the optic nerve and to the brain.

• Pupil: the circular opening in the centre of the iris through which light passes into the lens of the eye. The iris controls widening and narrowing (dilation and constriction) of the pupil.

The pupil is the dark center in the middle of the iris. The pupil’s function is to regulate how much light enters the eye. The pupil’s size is automatically varied to regulate the amount of light entering the eye.

• Retina: a light sensitive layer that lines the interior of the eye. It is composed of light sensitive cells known as rods and cones. The human eye contains about 125 million rods, which are necessary for seeing in dim light. Cones, on the other hand, function best in bright light. There are between six and seven million cones in the eye and they are essential for receiving a sharp accurate image and for distinguishing colours. The retina works much in the same way as film in a camera. The retina’s function is to sense light and create impulses that are sent through the optic nerve and to the brain.

• Rod cells are one of the two types of light-sensitive cells in the retina of the eye. There are about 125 million rods, which are necessary for seeing in dim light.

• Sclera: the white part of the eye, a tough covering with which the cornea forms the external protective coat of the eye.

The sclera is the white portion of your eye, making up the back five-sixth’s of the eyes outer layer. The function of the sclera is to provide protection for your eye, and to serve as the attachment for the extra ocular muscles which move the eye.

• Tear Layer: Understanding the structure of tears is important in order to understand how the tears and tear film provide the eye with a number of specialized functions.

Our tears are formed by tiny glands that surround the eye. Tears are comprised of three layers: oil, water, and mucous. The lower mucous layer serves as an anchor for the tear film and helps it adhere to the cornea. The middle layer is comprised of water and the outer layer seals the tear film and prevents evaporation. The tear film serves several purposes. It keeps the eye moist, creates a smooth surface for light to pass through the eye, nourishes the front of the eye, and provides protection from injury and infection. When we blink, the eyelids smooth and spread the tear film so that it is uniform across the surface of the cornea. Excess tears flow out of the eye into two tiny ducts, which then drain into the nasal passage.

• Vitreous Body: A clear, jelly-like substance that fills the center of the eye.

It is composed mainly of water and comprises about two thirds of the eye’s volume. The vitreous helps the eye maintain a round shape and is attached to the retina at various points, including the macula and the optic nerve.

• Zonules: a series of fine fibers that connect the crystalline lens to the ciliary body. The function of your zonules is to hold the lens in place in the eye.

If you are worried about the health of your eyes, take a look at the range of symptoms you may experience, call us on 01392 699969, arrange a call back or get in touch online.

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Eye Anatomy: Diagram & Human Eye Anatomy

Eye anatomy can be pretty intimidating when seeing it for the first time. However, when we break it down into varying sections, we can start learning this anatomy bit-by-bit and working out what the different parts of the eye do.

Human Eye Anatomy

Fig. 1 – Overview of eye anatomy

The human eye is slightly different anatomically compared to other animals and primates. The most significant difference between human eye anatomy and the anatomy of other animals is that our eyes have three distinct colours. We have our:

• sclera – the white area on our eyeball
• iris – the part of your eye that determines your eye colour
• pupil – the black part in the centre

Other animals do not have this much difference in the colours of their eyes.

The reason human eye anatomy is different from the eye anatomy of other primates is fascinating! The whites of our eyes, i.e. the sclera, are the main difference between our eyes and other primates. The whites of our eyes allow us to see which direction the people we are talking to are looking. If someone stands across the room from you, you may not be able to tell what colour their eye is, but you can tell which direction their eyes are looking in because of their white sclera! This helped humans evolutionarily, as it allowed us to hunt in packs more efficiently and avoid predators when in groups.

Eyeball Anatomy

The anatomy of the eyeball can be divided into the external eye anatomy and internal eye anatomy. We will detail the anatomy of these two different sections in the ‘external eye anatomy’ and the ‘internal eye anatomy’ sections.

The anatomy of the eye follows the function of the eye; when we understand the function of the eye, we can understand its anatomy. The eye’s primary role is as a sensory organ. It sends information to the brain so that we can interact with our environment. For this reason, the eye needs to have photoreceptors. These are the cone cells and rod cells found in the retina, which are broken down by light so that a message can be transmitted to the brain. Now that we understand the role of these photoreceptors, we can understand why the different aspects help to focus and refract light onto the retina (where the photoreceptors are found).

The eye also plays an important immune function. The conjunctiva and sclera protect the eye but also help to produce tears and mucus. These tears contain an enzyme called lysozyme, which helps to break down pathogens such as viruses and bacteria.

Remember that enzymes are proteins that play an essential role in many metabolic reactions. They have active sites which are specific to substrates. Enzymes also have optimum pHs and temperatures at which enzyme activity is at its highest.

We can break down each part of the eye into whether it plays a role in maintaining the eyeball’s shape, protecting the body from microbes, or whether it helps in refracting light and sending the message to the brain.

The eye isn’t the only place where we can find photoreceptors. Plant cells, such as those that make up chlorophyll, are known as photoreceptors. Their roles are similar but not identical to the eyes’ photoreceptors. During photosynthesis, the photoreceptors found in chlorophyll absorb light energy, which can then convert to chemical energy for the plant. The name of one of these photoreceptors is chlorophyll a. This is the primary photoreceptor in plants.

External Eye Anatomy

The external eye refers to the parts of the eye that are visible outside the body. You can see these parts of the eye when looking at someone’s eye. We will break down each of the features of the eye and describe their function.

There are many functions carried out by the external eye, including refraction. This is important as it allows our eye to ensure that light hits the correct part of the inner eye (the retina) to send clear messages to the brain via the optic nerve.

Remember, refraction is the process by which the direction that light travels changes.

Table 1. The structures of the external eye

The sclera and conjunctiva protect the eye, whilst the iris and pupil’s primary function revolves around ensuring enough light hits the correct location at the back of the eye. These are the main functions of the external eye.

Conjunctivitis is the most common infection of the eye. This is when the blood vessels next to the conjunctiva – the layer lining the sclera on our external eye – become infected by either a viral or bacterial pathogen. You may have more commonly heard of this condition as pink eye, as it sometimes causes the sclera of the affected person to turn from its usual white colour to a pink/red colour.

Accommodation

Accommodation is the process by which the lens of the eye becomes thicker or thinner to either focus the eye on a distant object or focus the eye on a nearby object.

Accommodation is vital as it helps us to adjust our focus to objects at different distances.

If you’re looking at your phone and someone from across the street calls your name, your eyes need to adjust their focus from looking at the nearby phone to the person standing a distance away.

To achieve this, two components of the eye are required. These are the ciliary muscle and the suspensory ligaments of the eye. The ciliary muscle is a circular ring of muscle that surrounds the lens; suspensory ligaments attach the lens and ciliary muscle.

Remember that the Ciliary muscle is Circular because they both start with the letter C!

The ciliary muscles and suspensory ligaments work in an antagonistic way. This means that when one contracts, the other relaxes. When our ciliary muscles contract, the suspensory ligaments slacken (relax) and vice versa. Let’s have a look at what happens when the ciliary muscles relax and contract;

• When the ciliary muscle relaxes, it becomes wider. This causes the suspensory ligaments to tighten, which pulls the lens thinner. The ciliary muscle relaxes, and suspensory ligaments become taught when we focus on a distant object.

• When the ciliary muscle contracts, it becomes smaller. This causes the suspensory ligaments to slacken, which causes the lens to return to a thicker shape. This sequence of events happens when we focus on a nearby object.

The Ciliary muscles Contract when focusing on an object Close to us!

Fig. 2 – Pupil dilation and constriction

Internal Eye Anatomy

We can split the aspects of the internal eye into different categories depending on their function. The aqueous and vitreous humour plays a role in maintaining the eyeball’s shape. The retina, lens, and optic nerve are essential in vision, as they help refract light and send a message to your brain.

Fig. 3 – Refraction of light to the retina

Table 2. Structures of the internal eye

The optic nerve is the only nerve in the body that technically is not a nerve. Instead, it is a projection from the brain.

Eye Anatomy Diagram

Now that we have discussed the internal eye anatomy and the external eye anatomy, make sure you can label these different areas of the eye on a blank diagram.

Fig. 4 – Overview of eye anatomy

Eye Anatomy – Key takeaways

• The eye can be divided into the internal and external eyes, separated by the lens
• Behind the lens, we find the vitreous humour, the retina, and other substances.
• In front of the lens, we find the aqueous humour and the cornea, iris, and pupil.
• Each aspect of the eye plays a role in vision, protection or maintaining shape.
• The retina contains rod and cone cells that allow for vision.

diagram and photo with description

The human eye is a complex paired organ that makes it possible to receive most of the information about the world around. The eye of each person has unique characteristics, but has structural features. Knowing the structure of the eye allows you to understand how the visual analyzer works.

The visual analyzer has a very complex structure, characterized by a combination of various tissue structures that provide its main function – vision.

The human eye is spherical or spherical, hence the name “eyeball”. The eyeball is located in the eye socket – the bone structure of the skull, thanks to which it is protected from damage. Its front surface is protected by the eyelids.

Movement of the eyeball is provided by six external muscles. Their well-coordinated work provides the possibility of binocular vision – vision with two eyes. This allows you to get a three-dimensional image (stereoscopic vision).

The surface of the eyeball is constantly moistened with tears produced by the lacrimal glands. The outflow of tear fluid is carried out through the tear ducts. Tears form a protective film on the surface of the eye.

Membrane of the eye

Conjunctiva . The outer transparent membrane that lines the surface of the eye and the inner surface of the eyelids. When moving the eyeballs, it provides sufficient sliding.

Fibrous membrane of the eye . Most of it is the sclera – the white shell, which is the most dense, the role of which is to provide a supporting function, protection. The fibrous membrane in the anterior part is transparent, has the appearance of an hour glass. This part of it is called the cornea. It is abundantly innervated, therefore it has a high sensitivity. Due to its spherical shape, the cornea is an optical refractive medium. Its transparency allows light rays to penetrate into the eye. On the border of the sclera with the cornea there is a transitional zone – the limbus. Here are the stem cells that ensure the regeneration of the outer layers of the cornea.

Choroid . Provides blood supply, trophism of intraocular structures. Consists of the following structures:
– the choroid itself – in close contact with the retina, sclera, performs trophic and shock-absorbing functions;
– ciliary body – neuro-endocrine-muscular organ, participates in accommodation, produces aqueous humor;
– iris – this part of the choroid determines the color of the eyes, depending on the pigment content, its color can vary from pale blue, greenish to dark brown. In the very center of the iris there is a pupil – a hole that limits the penetration of light rays.
Despite the fact that the iris, ciliary body and choroid belong to a single structure, they have different innervation and blood supply, which determines the nature of many diseases.

Retina . This is the innermost shell, which is a highly differentiated multi-layered nervous tissue. Lines 2/3 of the back of the choroid. This is where the fibers of the optic nerve begin, along which impulses through the complex visual tract enter the brain. Impulses are transformed, analyzed, perceived as an objective reality. The most sensitive thin part of the retina, the macula, provides central vision.

Chambers of the eye

There is a space between the cornea and the iris – the anterior chamber of the eye. Between the peripheral part of the cornea and the iris is the angle of the anterior chamber. There is a complex drainage system that provides the outflow of intraocular fluid. Behind the iris is the lens, which has the shape of a biconvex lens. The lens is fixed to the ciliary body with many thin ligaments. Between the posterior surface of the ciliary body and the iris, as well as the anterior surface of the lens, is the posterior chamber of the eye. Behind the lens is the vitreous body, which fills the cavity of the eyeball, supporting its turgor.

The chambers of the eye are filled inside with aqueous humor – an intraocular colorless liquid that bathes the internal eye structures, nourishes the cornea, the lens, which do not have their own blood supply.

Optical system of the eye

The human eye is a complex optical system that enables vision. This system has important optical structures. The perception of objects of the external world is provided by the functioning of light-conducting and perceiving structures. The clarity of vision depends on the state of transmissive, refractive, perceiving structures.

• Cornea. Having the shape of a convex hour glass, the cornea most of all affects the refraction of light rays. The refracted rays then pass through the pupil, which is a kind of diaphragm. The pupil regulates the amount of light entering the eye. The refractive media are the anterior and posterior surfaces of the cornea.
• Crystal. The surface of the lens refracts light rays, which then fall on the light-perceiving section – the retina.
• Aqueous moisture and the vitreous body also have refractive properties. Their transparency, lack of blood, opacities determines the quality of vision.

The light rays passing through the light-refracting media fall on the perceiving part – the retina. Here, a real reduced inverted image is formed.

Further along the fibers of the optic nerve, the impulses enter the brain – the occipital lobes. Here the final analysis of the information takes place, and the person sees the real image. Such a complex structure of the organ of vision provides the possibility of a clear perception of information about the world around.

For more details on the individual structures of the eye, see the relevant sections:

Structure of the eye

The human eye is a complex system, the main purpose of which is the most accurate perception, initial processing and transmission of information contained in the electromagnetic radiation of visible light. All individual parts of the eye, as well as the cells that make them up, serve the fullest possible fulfillment of this goal.

Below are graphic images that give an idea of ​​the appearance of the eye and its internal structure. The eye is a complex optical system. Light rays enter the eye from surrounding objects through the cornea. The cornea in the optical sense is a strong converging lens that focuses light rays diverging in different directions. Moreover, the optical power of the cornea normally does not change and always gives a constant degree of refraction. The sclera is the opaque outer shell of the eye, so it does not take part in transmitting light into the eye.

Refracted at the anterior and posterior surfaces of the cornea, the light rays pass unhindered through the clear fluid that fills the anterior chamber, up to the iris. The pupil, the round opening in the iris, allows the centrally located rays to continue their journey into the eye. More peripherally turned out rays are retained by the pigment layer of the iris. Thus, the pupil not only regulates the amount of light flux to the retina, which is important for adapting to different levels of illumination, but also filters out lateral, random, distortion-causing rays. The light is then refracted by the lens. The lens is also a lens, just like the cornea. Its fundamental difference is that in people under 40 years of age, the lens is able to change its optical power – a phenomenon called accommodation. Thus, the lens produces more accurate refocusing. Behind the lens is the vitreous body, which extends all the way to the retina and fills a large volume of the eyeball. Rays of light focused by the optical system of the eye end up on the retina. The retina serves as a kind of spherical screen onto which the surrounding world is projected.

We know from the school physics course that a converging lens gives an inverted image of an object. The cornea and lens are two converging lenses, and the image projected onto the retina is also inverted. In other words, the sky is projected onto the lower half of the retina, the sea is projected onto the upper half, and the ship we are looking at is displayed on the macula. The macula, the central part of the retina, is responsible for high visual acuity.