The human eye

Synonyms in the broader sense:

Medical: Organum visus

English: eye


The eye is responsible for conveying visual impressions from the environment to the brain and is anatomically still counted as an outsourced structure of the brain.

The eye consists of the eyeball (lat. Bulbus oculi; this means "the eye" in the colloquial sense) and the associated auxiliary devices, e.g. Eyelids, eyelashes, tear organs.

Anatomy and function

The eyeball has an approximately spherical shape and measures about 2.4 cm in diameter.

The light-refracting structures of the eye can be found in its front section: lens and cornea (see below), while the rear section consists of the retina, which is responsible for processing stimuli and converting them into electrical signals (retina) is formed.

The main component of the eyeball is the gelatinous-soft glass body (lat. corpus vitreum). It consists of 98% water and a fine network of connective tissue. It serves to maintain the inner shape of the eye and to protect the lens and retina from changes in position.

In old age, the vitreous body is often harmless, but annoying, which is perceived as dark spots ("mouches flies).

Are you still interested in this topic? Read more about this under: Structure of the eye

Illustration: Horizontal section through the left eyeball, seen from below
  1. Cornea - Cornea
  2. Dermis - Sclera
  3. Iris - iris
  4. Radiation body - Corpus ciliary
  5. Choroid - Choroid
  6. Retina - retina
  7. Anterior chamber of the eye -
    Camera anterior
  8. Chamber angle -
    Angulus irodocomealis
  9. Posterior chamber of the eye -
    Camera posterior
  10. Eye lens - Lens
  11. Vitreous - Corpus vitreum
  12. Yellow spot - Macula lutea
  13. Blind spot -
    Discus nervi optici
  14. Optic nerve (2nd cranial nerve) -
    Optic nerve
  15. Main line of sight - Axis opticus
  16. Axis of the eyeball - Axis bulbi
  17. Lateral rectus eye muscle -
    Lateral rectus muscle
  18. Inner rectus eye muscle -
    Medial rectus muscle

You can find an overview of all Dr-Gumpert images at: medical illustrations

The eyeball

The three-layer structure of the wall covering the eyeball is characteristic. A distinction is made between an outer, a middle and an inner eye skin.
The outer skin of the eye represents “the white” in the eye and is also known as the sclera.

In the area of ​​the anterior surface of the eye, it goes into the transparent cornea (lat. cornea) over. Opacities of the cornea are pathological (pathological) - such as cataracts. They lead to a decrease in visual acuity, which can even lead to blindness (see diseases below).

Due to its strong curvature, it is of outstanding importance for the visual process. With a refractive power that exceeds that of the lens many times over, the cornea plays a decisive role in the sharp image of the surroundings on the retina by bundling the incident light rays (focusing).

In contrast to the lens, however, its refractive power is not variable. The cornea itself is free of blood vessels and is therefore nourished by diffusion from the front from the covering tear film and from behind from the so-called anterior chamber.

The latter represents a ("chamber") fluid-filled cavity that is formed by the cornea as the front wall and the iris (iris) as the rear wall.
The transition between the two forms an acute angle, the chamber angle containing the small veins. These blood vessels ultimately form the drainage for the continuously renewed aqueous humor.

The same comes from the posterior chamber of the eye, which is connected to the anterior chamber via the iris.

If the aqueous humor cannot drain properly due to an obstacle to drainage or increased formation, the intraocular pressure increases and there is a risk of damage to the optic nerve and retina. This condition is known as glaucoma and can have a number of causes. F.

The transparency of the cornea is a masterpiece of nature: it is guaranteed by the exact arrangement of 50 layers of connective tissue fibers with precisely defined regular alignment to one another and a constant water content.

Injuries to the superficial cornea heal quickly and without scar, as a quick replenishment is guaranteed at all times by stem cells located at the transition to the white skin of the eye. These allow the surface cells to be completely renewed once a week.

This is particularly important because the cornea is exposed to environmental influences such as radiation, direct injuries, bacteria, viruses and fungi due to its location.

Components of the eye

The human eye is a complex organ that is composed of many details. Each component contributes to the proper functioning of vision, which enables the visual process.

The most important parts of the eye are presented below. More detailed information on topics is available at the click of a mouse.


The lens lies between the posterior body and the vitreous humor. It has a biconvex shape, with the back being more curved than the front. The lens is connected to the ciliary body via elastic fibers, the zonular fibers.

Features of the lens:

The task of the lens is to bundle light rays and create a sharp image on the retina. This is done through what is known as accommodation, i.e. the close-up and distance adjustment of the lens.

If you look at an object nearby, the ciliary body becomes tense. This in turn leads to the relaxation of the zonular fibers. This allows the lens to follow its own elasticity and take on a more spherical shape, which increases the refractive power.

Conversely, when viewing distant objects, the ciliary body relaxes and the zonular fibers become tense. This keeps the lens in a relatively flat shape, which lowers the refractive power.

Lens diseases:

With increasing age, the inherent elasticity of the lens decreases and it can no longer "ball itself together" as well during near accommodation. This is the reason why many old people need reading glasses.

In addition, in old age there is condensation of proteins that are located in the lens. This can thicken the lens and the disease "cataracts" can develop.

You can find detailed information on this topic at: Eye lens

Anatomical structure of the eye:

  1. Lacrimal gland
  2. Eye muscle
  3. eyeball
  4. Iris
  5. pupil
  6. Eye socket


The vitreous (Corpus vitreum) is located between the lens and the retina and takes up about two thirds of the eyeball. It consists of 98% water, the remaining 2% is made up of collagen and hyaluronic acid.

The structure of the vitreous is gel-like, and because of this and the pressure exerted on the surrounding structures, it contributes significantly to the shape of the eyeball.

In healthy people, the vitreous is translucent and transparent. In the elderly, however, there can be changes in the texture, often the vitreous becomes increasingly fluid, which can lead to an irregular structure.

A typical clinical picture are the "floaters" (German: flying mosquitoes). These are small opacities of the vitreous humor that can look like flying mosquitoes. This can be annoying due to the impairment of vision, but it is usually harmless.

You can read more about the anatomy of the eye at: Vitreous


The pupil is the opening in the center of the iris through which light can enter the inside of the eye. Together with the iris, it is responsible for regulating the incidence of light on the retina.

If it is light, it tenses Sphincter pupillae muscle and thus causes a narrowing of the pupil (Miosis). If it's dark, it tenses Dilator pupillae muscle and thus dilates the pupil (Mydriasis).

The pupil size can provide important information in medicine, which is why the "pupil reflex" is very important in many areas. The interconnection of nerve tracts leads to a narrowing of the pupil when the eye lights up (direct response). There is also an indirect reaction: the simultaneous narrowing of the other eye.

For detailed information on the anatomy of the eye, visit: pupil

Vascular skin

The vascular skin (Uvea) consists:

  • Iris (iris)
  • Ciliary body and
  • Choroid (Choroid).

It lies under the dermis (Sclera) and is primarily responsible for accommodation, adaptation and nutrition of the retina. The pigmentation of the vascular skin, which is different in every person, leads to different eye colors.


The iris separates the anterior and posterior chambers of the eye. There is an opening in the middle, the pupil. The iris acts as a diaphragm and thus, together with the pupil muscles, regulates their size and thus the incidence of light into the rear eye (Adaptation).

For detailed information on iris skin, see: Iris

Ciliary body:

The iris merges into the ciliary body. Inside is the Ciliary muscleStarting from the ciliary body, the so-called zonal fibers pull towards the lens.

On the one hand, you are responsible for hanging up the lens and fixing it in its position. On the other hand, tensing and relaxing of the Ciliary muscle and thus the state of tension of the zonular fibers the near and far setting (Accommodation) regulated (more detailed description under lens).

The ciliary body is also responsible for producing aqueous humor.


The choroid is the largest section of the vascular skin. It is located between the retina and dermis in the back of the eyeball. The choroid has numerous vessels and is the body's best blood supply.

Their main job is to supply the outer parts of the retina with oxygen and nutrients.

Are you still interested in this topic? Then read our next article: Choroid


The conjunctiva (Conjunctiva) is a mucous membrane in the front area of ​​the eye. It is the connection between the eyeball and the lids and allows the eyeball to move in all directions through various folds.

Together with the tear film, it is responsible for the smooth gliding of the eyeball.

The conjunctiva is not pigmented and is relatively thin. In addition, it is well supplied with blood, so that blood changes in the conjunctiva can also be seen.

You can find more information on this topic at: Conjunctiva


The cornea (Cornea) lies in front of the pupil in the foremost part of the eye, has no vessels and is transparent. It consists of 70% water and is covered with a tear film.

The cornea is the part of the eye that is responsible for about two thirds of the refraction of light.

You can find detailed information on this topic at: Cornea


The retina (retina) lines the inside of the back eye. Their task is to pick up light signals and then convert them into electrical signals, which are then passed on to the brain.

The retina contains different types of receptors, cones and rods. Around 7 million cones (red, green and blue cones) are responsible for seeing colors and seeing in light. The 120 million rods take over at dusk and in the dark.

You can find out more about this part of the eye at: Retina


The dermis (Sclera) surrounds most of the eyeball. It protects him and keeps him in shape. It takes on a protective function by forming a solid cover around the eyeball and almost completely enclosing it. In order to be able to guarantee this stability, it mainly consists of connective tissue.

The dermis is whitish, which is why the eyeball that is covered by it also appears white. It is opaque.

So that light can still get into the eye, the dermis leaves the central front part of the eye free. This is covered by the cornea. The dermis is also spared on the back of the eyeball, where the optic nerve enters.

If you want to dig deeper into this topic, check out our next topic: Dermis of the eye: anatomy and function


There is an upper and a lower eyelid in each eye. Their main task is to protect the eye. The eyelids cover the eye and are quickly closed in the event of an impact near the eyes ("eyelid closing reflex").

By blinking the eye regularly, the eye is moistened and cleaned with tear fluid.

Read more about this at: eyelid

Tear organs

The tear fluid is formed by the lacrimal gland and additional smaller lacrimal glands. In addition to salt, glucose and proteins, the tear fluid also contains substances that kill bacteria.

The lacrimal gland is located on the upper outer edge of the eye. The blink of an eye distributes it in the eye. Then it is transported to the inner corner of the eyelid. From there, the tear fluid flows through a small passage into the nose.

Eye diseases


A stye (Hordeolum) is inflammation of the glands in the eyelid. There are two forms of the hordeolum, depending on which glands are affected.

At the Hordeolum internum are sebum glands of the eyelid (Meibomian glands) affected. With this disease one often finds a kind of pimple that is visibly filled with pus on the lid.

At the External hordeolum are the Zeiss glands (Sebum glands of the eyelashes) or the minor glands (Sweat glands of the eyelid) ignited. This form of stye is usually less noticeable. Both inflammations are accompanied by redness, swelling, pain and overheating of the eyelid.

A stye is mostly made up of the bacteria Staphylococcus aureus triggered. They usually heal on their own; red light irradiation or warm compresses can help.

If a stye causes significant discomfort, if healing is delayed or if the pus does not drain, a doctor should be consulted. He or she may prescribe antibiotic ointments or drops or drain the pus through a small incision.

If the disease takes a severe course, it can lead to inflammation of the entire lid and abscesses. However, this is rare; it is usually a harmless condition.

For more information on this condition, see: Stye


Conjunctivitis (Conjunctivitis) is quite a common condition. It can be acute, but it heals within 4 weeks. If the disease lasts longer, it is called chronic conjunctivitis.

It is associated with reddening of the eye, pain, burning, increased sensitivity to light and a feeling of foreign bodies. The eyes sticky in the morning and the clear protrusion of the vessels of the conjunctiva are also typical (conjunctival injections). There may be discharge from the eye, which, depending on the type of pathogen, is clear to purulent.

Conjunctivitis can have various causes. Most common in bacterial diseases (e.g. Streptococci, staphylococci). This often leads to purulent discharge.

In addition, conjunctivitis is often caused by viruses (e.g. Adenoviruses), here the discharge is often watery and slimy. Also in the context of an allergy (e.g. hay fever) or irritation (e.g. solvent) of the eye, inflammation of the conjunctiva can occur.

Conjunctival treatment should be based on the trigger. Antibiotics are used locally in the form of ointments or drops against bacteria, while the symptoms of viruses are treated with decongestant drugs. In the case of allergies, antiallergic drugs can be given.

Are you still interested in this topic? Read our next article below: Conjunctivitis

Eye flicker

As eye flicker (Ciliated scotoma) is the term used to describe temporary deficits in the visual field. The flicker of the eyes is accompanied by bright zigzag lines or flashes. It occurs in both eyes and in the same area of ​​the field of vision (homonym) on. Headache, photosensitivity (Photophobia) or nausea.

Eye flicker is a symptom that can cause many different diseases. Most of them are quite harmless, such as tight neck muscles or persistent stress. Eye strain and certain medications can also trigger a shimmering scotoma.

Eye flicker usually disappears quickly on its own. However, if it lasts longer, this can provide an indication of the underlying disease. If it lasts around ten minutes, an ocular migraine can be the trigger, especially if accompanied by headaches.

A longer duration of approx. 30 minutes can indicate a migraine. Also glaucoma (glaucoma) can trigger a ciliated scotoma in its early stages.

If the eye flicker persists for a long time, it often comes back (recurrent) or if the symptoms are very distressing, an ophthalmologist should be consulted. This can examine whether a disease that requires treatment is behind the eye flicker.

Read more about this: Eye flicker - causes, symptoms and therapy

Twitching eye

Eye twitching refers to the involuntary contraction and opening of the eyelid. It can be bilateral or limited to just one eye.

It is often triggered by the nerve that supplies the facial muscles (facial nerve) or the cause is directly in the eye muscles (e.g. M. orbicularis oculi).

In the vast majority of cases, the twitching of the eyes has a harmless cause. It can be triggered by stress, tiredness, eye strain, or exhaustion while exercising. Sometimes it occurs without a trigger at all.

In addition, eye twitching can indicate a magnesium deficiency, which generally makes muscle twitching easier. Other states of malnutrition can also make themselves felt by twitching the eyes, in which case fatigue and reduced performance are often found accompanying them.

There can also be a so-called tic in the eye twitching. This is a symptom of mental or neurological illness.

If the eye twitching lasts longer than a day or if it recurs very frequently, a neurologist should be seen. This is especially true if there are other symptoms such as headache, night sweats, weight loss, fever, mood swings, changes in character or sudden clumsiness.

You can find more information on this at: Twitching in the eye

Swollen eyes

Swollen eyes often do not mean swelling of the eye itself, but swelling of the eyelid or the bags under the eyes. They are rarely associated with a disease.

Eye swelling can be caused by many different reasons. Lack of sleep, foods rich in salt, protein or alcohol, family predispositions or simply age can be the cause. Some women also experience puffy eyes as part of their monthly cycle.
For more information on this topic, see: Causes of eye swelling

However, swelling can also be triggered by an allergy, e.g. from house dust, pollen, cosmetics, food, insect bites or medicines. Also trauma (Blows, injuries) of the eye and its surroundings can cause swelling.

If other symptoms such as redness, pain and overheating are added to the swelling, this suggests an inflammation of the eye or the surrounding tissue. In this case, an ophthalmologist should be consulted.

Disorders of the lymph drainage can also lead to puffy eyes. So-called myxedemas, which also cause swelling of the eye, are often found in an underactive thyroid. Functional disorders, particularly of the heart and kidneys, can also cause swelling. These are usually accompanied by other symptoms.

In rare cases, a growing tumor can also trigger swelling. However, puffy eyes are usually harmless. If other symptoms occur, the swelling increases steadily or affects the visual field, it should be clarified by a doctor.

Read more about this under: Puffy eyes - cause, accompanying symptoms and diseases

Watery eyes

Watery eyes (Teardrop, epiphora) denote the leakage of tear fluid over the edge of the lid. There are several reasons for Epiphora.

On the one hand, too much tear fluid can be produced (Dakyrrhea), or the drain is obstructed. Too much tear fluid is e.g. produced in allergies, sinus disease, and inflammation or injury to the eye.

Also in the context of damage to the eye (endocrine orbitopathy) due to an overactive thyroid (Hyperthyroidism) it can lead to increased tearing, as well as irritation of the eye (Contact lenses, chemicals).

Watery eyes are also caused by irritation of the nerves (Trigeminal nerve), which supplies the lacrimal gland.

The drainage of the tear fluid can result from an obstruction of the drainage path, e.g. with inflammation of the lacrimal tubules (Canaliculitis), chronic inflammation of the bags under the eyes (Dacrocystitis chronica) or congenital malformations. Misalignments of the eyelids can also hinder the drainage of tears.

With Epiphora, the risk of infection for the affected eye is significantly increased. Some of the causes also require treatment. Therefore, if the tears are constantly dripping, a doctor should be consulted.

Itchy eye

Itchy eyes can have a variety of causes and are usually accompanied by other symptoms.

Allergies can e.g. Cause itching around the eyes. The eye is usually watery and swollen. Hay fever often accompanies it (e.g. with pollen allergy), or the itching sets in after using new cosmetics.

Therapy consists in identifying the allergenic substance (Allergen), in its avoidance or in the administration of antiallergic drugs.

In addition, inflammation of the conjunctiva or the edge of the eyelid can cause itching. This can be accompanied by sticky eyes, pain, redness, swelling and purulent to watery secretions. Local antibiotics are usually used here.

Itchy eyes can also be caused by chemical (e.g. chlorine), mechanical (e.g. contact lenses), biological (e.g. Insect bite near the eye) and physical (e.g. sunlight) Stimuli or overexertion. The itching usually disappears when the stimulus disappears.

A doctor should be consulted if the eye itching persists or if other symptoms occur.

Read more about the causes of itchy eyes here: Itchy eyes - what's behind it?

Eye festers - what's behind it?

Pus (Pus) occurs in the course of inflammation through the destruction of tissue (Autolysis) and the death of immune cells (neutrophil granulocytes) on. Most often, inflammation that accompanies pus is caused by bacteria.

A common cause of festering eyes is conjunctivitis (Conjunctivitis). But also inflammation of other parts of the eye, such as of the iris (Iriditis) or the cornea (Keratitis) can cause festering eyes. Barley (Hordeolum) - or hailstones (Chalazion) cause pus in the area of ​​the eye.

Obstruction and inflammation of the drainage paths of tears can also lead to pus leakage. E.g. with inflammation of the lacrimal tubules (Canaliculitis) or the eye sac (Dacryocystitis) Pus from the teardrop inside the eye.

Bacterial inflammation is usually treated with antibiotics. A doctor should always be consulted if pus emerges from the eye.

Light sensitive eye

Photosensitivity (Photophobia) manifests itself in an intolerance to light that other people do not perceive as particularly bright. When people suffering from photophobia are exposed to light, they often get headaches or eye pain.

Photophobia can have a variety of causes. E.g. especially inflammation of the conjunctiva (Conjunctivitis), but also inflammation and injuries to the cornea (Cornea) or iris (iris) to photosensitivity.

If the pupil is dilated (Mydriasis) more light can fall into the eye, which leads to photophobia. Mydriasis can be found e.g. if the eye is “dripped wide” at the doctor's, or if nerves that are responsible for contracting the pupil fail (N. oculomotorius). Even with the glaucoma (glaucoma) the eye reacts with sensitivity to light.

Sensitivity to light is also often found in migraine attacks or irritation of the meninges (Meninges). In very rare cases, photophobia can also be triggered by a tumor in the brain. In addition, it also occurs in the context of infections such as the measles.

If you are sensitive to light, your eyes can be protected with sunglasses and should not be exposed to direct light. If the person is very sensitive to light, a doctor should be consulted, especially if other symptoms such as pain in the eyes and head or redness and suppuration of the eye occur.

Find out all about the topic here: The sensitivity of the eye to light.

Eye throbbing - what are the causes?

A throbbing eye can be very uncomfortable. Often the throbbing results from noticing your own pulse. This can e.g. be the case with high blood pressure. Throbbing can also be triggered by muscle twitching, e.g. through the muscles on the lid.

They usually pass quickly and also occur in healthy people, especially when stressed.

Throbbing is also a typical symptom of inflammation. Often, e.g. Barley grains (Hordeoulum) or hailstones (Chalziomas). But abscesses on the lid or in the eye socket can also trigger throbbing.
Read more about this under: Hailstone from inflammation

If there is inflammation in the eye area, a doctor should be consulted; they are usually treated with antibiotic ointments or drops.

Throbbing in the eye can also be caused by radiating pain, for example headache or earache. If this lasts longer, a doctor should also be consulted to clarify the cause.

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  • Cataracts