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Eva Rudolf-Müller is a freelance writer in the medical team. She studied human medicine and newspaper sciences and has repeatedly worked in both areas - as a doctor in the clinic, as a reviewer, and as a medical journalist for various specialist journals. She is currently working in online journalism, where a wide range of medicine is offered to everyone.

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Lisa Vogel studied departmental journalism with a focus on medicine and biosciences at Ansbach University and deepened her journalistic knowledge in the master's degree in multimedia information and communication. This was followed by a traineeship in the editorial team. Since September 2020 she has been writing as a freelance journalist for

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The human eye is the most complex sense organ in the body. It consists of the optical apparatus - the eyeball, which reacts to light - as well as the paired eye nerve (optic nerve) and various auxiliary and protective organs. Read everything you need to know about the eye as a sensory organ: structure (anatomy), function and common diseases and injuries of the eye!

How is the eye structured?

The structure of the eye is - like its function - highly complex. In addition to the eyeball, the optic nerve, the eye muscles, the eyelids, the tear system and the eye socket are also part of the visual system.

eyeball

The eyeball (Bulbus oculi) has an almost spherical shape and lies in the bony eye socket (orbit), embedded in fatty tissue. It is protected in front by the upper and lower eyelids. Both are covered on the inside with a transparent, mucous membrane-like layer of tissue - the eyelid conjunctiva. This merges into the conjunctiva at the upper and lower crease.

The eyelid and conjunctiva connect the eyelids with the front of the eyeball. You can read more about this layer of tissue in the article Conjunctiva.

The eyeball is made up of several structures: In addition to the three wall layers, these are the lens and chambers of the eye.

Wall layers of the eyeball

The wall of the eyeball is made up of three onion-shaped skins superimposed on one another - the outer, middle and inner eye skin.

Outer eye skin

The outer skin of the eye is also called "tunica fibrosa bulbi" by doctors. It consists of the cornea in the front part of the eyeball and the sclera in the back part:

Leather skin (sclera): The porcelain-white sclera consists of coarse collagenous and elastic fibers and has hardly any blood supply. It has several openings (including for the optic nerve). The function of the dermis (sclera) is to give shape and stability to the eyeball.
Cornea: It rests on the front of the eyeball as a flat bulge, is transparent and plays a key role in refracting the incident light rays. You can find out more about the structure and function of the cornea in the article Eye: Cornea.

Middle eye skin

The medical term for the middle skin of the eye is "Tunica vasculosa bulbi" or "Uvea". This wall layer of the eyeball contains blood vessels (hence the part of the name "vasculosa"), has a recess for the pupil at the front and one for the optic nerve at the back. Their color is similar to that of a dark grape, hence the name uvea (Latin uva = grape).

The middle skin of the eye consists of three sections - in the front part of the iris and the ciliary body, in the back part of the choroid:

Rainbow skin (iris): This pigmented layer of tissue is responsible for the color of the eyes (e.g. blue, brown). It surrounds the pupil and acts as a kind of diaphragm that regulates the incidence of light into the eye.
Ciliary body (Corpus ciliare): It is also called a radiation body. On the one hand, its function is to suspend the lens of the eye. On the other hand, the ciliary body is involved in the adaptation of the eye to distance and near vision (accommodation) as well as in the production of aqueous humor.
Choroid: It supplies the underlying retina with oxygen and nutrients.

Inner eye skin (tunica interna bulbi)

The innermost wall layer of the eyeball is called "Tunica interna bulbi" in technical terms. It consists of the retina, which is divided into two sections: The front, light-insensitive section of the retina covers the back of the iris and the ciliary body. The back section of the retina contains the light-sensitive sensory cells.

You can read more about the function and structure of the retina in the article Retina.

Eye lens

The lens of the eye - together with the cornea - is responsible for refracting and thus bundling the light rays falling into the eye. It is arched on both sides, slightly weaker in front than on the back surface. It is around four millimeters thick and around nine millimeters in diameter. Due to its elasticity, the eye lens can be deformed by the eye muscles. This is important for the refraction of light: The greater or lesser curvature of the surface changes the refractive power of the eye lens. This process is called accommodation (see below).

The lens is made up of:

Lens capsule
Lens cortex, which contains the lens epithelial cells in the front area
Lens nucleus

The lens capsule is elastic and structureless. It envelops the soft interior of the lens (lens cortex and lens nucleus) and protects it from clouding and swelling from the surrounding aqueous humor (in the anterior and posterior chambers of the eye). Its front surface is thicker, about 14 to 21 micrometers (µm), and borders the back of the iris. The rear surface is significantly thinner at four micrometers and borders the glass body. Up to about the age of 35, the back surface of the eye lens increases in thickness.

The lens cortex is the outer area of the lens of the eye inside the capsule. It merges continuously (i.e. without a recognizable border) into the lens nucleus. This is significantly less watery than its surroundings.

Eye chambers

If you look at the structure of an eye, you will notice three separate rooms inside.

Anterior chamber of the eye (anterior chamber)
Posterior chamber (posterior chamber)
Vitreous body (corpus vitreum)

The anterior chamber of the eye lies between the cornea and the iris. It is filled with aqueous humor. In the area of the chamber angle (transition from the back surface of the cornea and the iris) there is a mesh-like structure made of connective tissue. Through the cracks in this tissue, the aqueous humor penetrates from the anterior chamber into a ring-shaped canal, the so-called Schlemm's canal (sinus venosus sclerae). From there it is diverted into venous blood vessels.

The posterior chamber of the eye lies between the iris and the lens. It absorbs the aqueous humor formed by an epithelial layer of the ciliary body. The aqueous humor flows into the anterior chamber via the pupil - the junction between the anterior and posterior chambers of the eye.

The aqueous humor has two tasks: It supplies the lens of the eye and the cornea with nutrients. It also regulates intraocular pressure. In a healthy eye, this is around 15 to 20 mmHg (millimeters of mercury). If the pressure increases due to illness, glaucoma can develop.

The vitreous makes up about two thirds of the eyeball.It consists of a clear, gelatinous substance. Almost 99 percent of it is water. The small remainder is made up of collagen fibers and water-binding hyaluronic acid. The task of the vitreous is to maintain the shape of the eyeball and stabilize it.

Optic nerve

The optic nerve (Nervus opticus) is the second cranial nerve, part of the visual pathway and actually an upstream component of the white matter of the brain. It forwards the electrical impulses from the retina to the visual center in the cerebral cortex.

You can find out more about the structure and function of the optic nerve in the article Optic nerve.

eyelid

The eyelids are movable folds of skin above and below the eye. They can be closed - to protect the front eyeball from foreign objects (such as small insects or dust), too bright light and dehydration.

You can find out more about the structure and function of the upper and lower eyelids in the article Eyelid.

Lacrimal system

The sensitive cornea is constantly covered with a protective tear film. This fluid is mainly produced by the lacrimal glands. You can read more about their function and structure in the article tear gland.

The tear system also includes tear-draining structures. They distribute and dispose of the tear fluid:

Tear points (punctum lacrimale)
Lacrimal tubules (canaliculi lacrimales)
Lacrimal sac (Saccus lacrimalis)
Lacrimal duct (ductus nasolacrimalis)

Eye muscles

The anatomy of the eyes also includes six eye muscles that ensure the mobility of the eyeball - four straight and two oblique muscles. The so-called ciliary muscle has a different task: it can change the shape of the eye lens and thus change the refractive power of the eye lens.

You can find out more about the structure and function of these muscles in the article Eye muscles.

How does the eye work?

The function of the eye consists in the optical perception of our environment. This “seeing” is a complex process: the eye must first convert incident light into nerve stimuli, which are then passed on to the brain. The human eye only perceives electromagnetic rays with a wavelength of 400 to 750 nanometers as "light". Other wavelengths are invisible to our eyes.

Considered in detail, two functional units are involved in the process of "seeing": the optical (dioptric) apparatus and the receptor surface of the retina. In order to be able to see optimally, the eye must be able to adapt to different lighting conditions (adaptation) and to switch between distance and near vision (accommodation). You can read more about this in the following sections.

Functional unit optical apparatus

The optical device (also known as a dioptric device) ensures that the rays of light falling into the eye are refracted and bundled and hit the retina. Its components include:

Cornea
Eye lens
Vitreous
Aqueous humor

The cornea has the greatest refractive power of the eye (+43 diopters). The other structures (lens, vitreous humor, aqueous humor) are less able to break the light rays. In summary, this results in a total refractive power of normally 58.8 diopters (applies to the eye at rest and focused on distance vision).

Functional unit retina

The light beams bundled by the optical apparatus hit the receptor surface of the retina and create a scaled-down and upside-down image of the object being viewed. Suppositories and rods - into electrical impulses, which are then passed on from the optic nerve to the cerebral cortex. This is where the perceived image is created.

adaptation

The eye has to adapt to different light intensities during the visual process. This so-called light-dark adaptation takes place via various mechanisms, including above all:

Change in pupil size
Alternation between rod and cone vision
Change in rhodopsin concentration

Change in pupil size

The iris of the eye changes the pupil width in adaptation to the light intensity:

When stronger, brighter light hits the eyeball, the pupil narrows so that less light falls on the delicate retina. Too much light would be blinding. In contrast, when the light intensity is low, the pupil expands so that more light hits the retina.

A camera works in a similar way: The diaphragm here corresponds to the iris, the aperture to the pupil.

Alternation between rod and cone vision

The retina can adapt to different light conditions by switching between rod and cone vision:

In the twilight and darkness, the retina switches to seeing with the rods. This is because these are much more sensitive to light than the cones. However, you cannot see any colors in the dark because the rods are unable to do so. In addition, you cannot see clearly at night. At the point of sharpest vision in the retina - the fovea centralis - there are no rods, but only all around in the rest of the retina.

On the other hand, on a bright day, the retina switches to cone vision. The cones are responsible for color perception - that's why you can see colors during the day. In addition, sharp vision is then also possible because the cones are particularly close at the point of sharpest vision (pit of vision), while they become rarer towards the edge of the retina.

Change in rhodopsin concentration

Rhodopsin (visual purple) is a pigment in the rods that is made up of two chemical components: opsin and 11-cis-retinal. With the help of rhodopsin, the human eye can distinguish between light and dark. It does this by converting light stimuli into electrical signals - a process called light transduction (photo transduction). It works like this:

When a light stimulus (photon) hits the rhodopsin, its component 11-cis-retinal is converted into all-trans-retinal. As a result, rhodopsin is converted into metarhodopsin II in several steps. This sets a signal cascade in motion, at the end of which an electrical impulse is created. This is transmitted to the optic nerve by certain nerve cells in the retina (bipolar cell, ganglion cell), which are connected to the rods.

After exposure - i.e. in twilight and darkness - the rhodopsin regenerates so that it is available again in larger quantities. This increases the sensitivity to light again (dark adaptation).

The degradation of rhodopsin (when exposed to light) takes place quickly, its regeneration (in the dark) much more slowly. Therefore, changing from light to dark takes a lot more time than changing from dark to light. It can take up to 45 minutes for the eye to "get used" to the darkness.

Accommodation

The term accommodation generally stands for the functional adaptation of an organ to a specific task. In connection with the eye, accommodation refers to the adaptation of the refractive power of the eye lens to objects at different distances.

The lens of the eye is suspended in the eyeball on the radiation body (ciliary body), which contains the ciliary muscle. From this, fibers pull into the lens of the eye, the so-called zonular fibers. If the tension of the ciliary muscle changes, this also changes the tension of the zonular fibers and subsequently the shape and thus the refractive power of the eye lens:

Long-distance accommodation

When the ciliary muscle is relaxed, the zonular fibers are taut. Then the eye lens is drawn flat at the front (the back remains unchanged). The refractive power of the lens is then low: rays of light falling into the eye are refracted and combined on the retina in such a way that we can see distant objects clearly.

The farthest point that can still be seen clearly is called the far point. In the case of people with normal vision, it is infinite.

Remote adjustment of the eye also means that the pupil dilates and the eyes diverge.

Near accommodation

When the ciliary muscle contracts, the zonular fibers relax. Due to its inherent elasticity, the lens then changes to its rest position, in which it is more curved. Your refractive power is then higher. Thus, light rays incident on the eye are refracted more strongly. As a result, nearby objects appear sharp.

The near point is the shortest distance at which something can still be seen clearly. In normally sighted young adults, it is about ten centimeters in front of the eyes.

With closer focus, the pupil also narrows, which improves the depth of field, and both eyes converge.

Accommodation resting point

In the resting state, if there is no accommodation stimulus at all (e.g. in absolute darkness), the ciliary muscle is in an intermediate position. As a result, the eye is focused at a distance of about one meter.

Accommodation width

The range of accommodation is defined as the area in which the eye can change its refractive power when switching between distance and near vision. A young person's range of accommodation is around 14 dioptres: their eyes can see objects at a distance of between seven centimeters and “infinitely” sharp, whereby the ophthalmologist understands “infinite” to mean a distance of at least five meters.

From the age of 40 to 45, the ability to accommodate - i.e. the ability of the eye lens to change its shape and thus its refractive power - steadily decreases. The reason: the rigid core of the lens becomes larger with age, while the deformable lens cortex becomes less and less. Finally, as people get older, the range of accommodation can drop to around one diopter.

So naturally, as people get older, they become increasingly farsighted. This age-related, inevitable farsightedness is called presbyopia).