Albino people have clear blood

Anatomy and function of the eye

The eyes are our most valuable sense organs. No other sense organ gives us so much important information from the environment in order to survive in everyday situations. The visual system, under this term all organs and organ functions that are involved in visual performance, functions so naturally that one quickly forgets the admirable achievements that are achieved here through the interaction of eyes, visual pathways and parts of the brain. Seeing, i.e. the perception of light, is a very important sensory function that enables us to perceive and understand objects and processes in our environment; colloquially one also says "to get an idea".

Visual system

Although the eye is a complicated organ, it impresses with its clearly structured structure, its anatomy and its functions, the physiology. The anatomy and physiology of the eye have optimally adapted to their habitat and needs in the course of human evolution.
Interactive anatomy

The eye is embedded in the bony eye socket of the skull, the orbit. The eye socket provides space to accommodate the eyeball like the head of a ball and socket joint. The optic nerve and blood vessels run through an opening in the back of the eye socket. With the four straight and two oblique eye muscles, the eye can be rotated like a ball in the eye socket.

Eye socket and eye muscles

In the schematic cross-section of the following figure you can see the eyeball, which is embedded in fatty tissue in the eye socket. The eyeball itself is spherical and its outer layer is essentially formed by the dermis (sclera), the “skeleton” of the eyeball. The eye muscles have grown on the dermis. At the back, at the point where the optic nerve emerges, the sclera is open. The cornea is enclosed in another round opening in the sclera in the anterior segment of the eye.

Eyeball and eye socket

From the outside, the eye only shows itself in a small section and is enclosed by the eye rims and lids. You can see the front part of the white dermis, which is covered with a conjunctiva. At the front, the transparent cornea is framed in an opening in the dermis, through which one can see the iris. In the middle you can see the pupil, an opening in the iris through which the ophthalmologist looks into the inside of the eye.

Eye supervision

Eye chambers

The inside of the eyeball is divided into three cavities, all of which are filled with translucent clear liquids so that the incident light can be projected through them onto the retina:

  • the anterior chamber of the eye,
  • the posterior chamber and
  • the vitreous space.

The anterior chamber of the eye is the space between the cornea, iris and the anterior part of the lens surface in the area of ​​the pupil. The iris is a circular sheet, the outer edges of which are attached to the radiation body. The edges of the iris in the area of ​​the pupil rest on the lens.

Anterior and posterior chambers of the eye

The posterior chamber of the eye begins behind the iris. The lens "floats" in this chamber, which is suspended from the radiating body by means of fine zonular fibers. The radiation body is a sphincter muscle, whereby the fibers can be loosened by a contraction of the muscle and thus the refraction of the lens can be changed. The posterior area of ​​the posterior chamber is formed by the vitreous humor.

Lens, zonular fibers and radiation body, trabecular structure

The vitreous is a "balloon" that completely fills the vitreous space. In its outer thin shell it contains a crystal-clear, jelly-like liquid. The vitreous body rests mostly directly on the retina, at the front it touches the back of the lens, its suspension straps, and the radiation body.

Vitreous in the vitreous space

Aqueous humor

The anterior and posterior chambers of the eye are both filled with the clear aqueous humor. They are much smaller in size than the vitreous space.

The aqueous humor is formed in the radiation body (ciliary body) and released into the posterior chamber of the eye. It consists of electrolytes, protein, sugars, ascorbic and hyaluronic acids and other ingredients. It is used to supply internal structures of the eye that are not supplied with blood, such as the lens and cornea. Approx. 2-3 µl of aqueous humor are produced per minute. Some of the aqueous humor is released into the vitreous humor. Most of the aqueous humor flows from the posterior chamber along the lens and iris through the pupil into the anterior chamber. It passes through the gap in the area of ​​the pupil that is created when the edge of the iris rests on the lens, thus preventing these structures from sticking together.
In the chamber angle, that is the angle in the anterior chamber between the iris and the cornea, the aqueous humor seeps through the tiny crevices of the trabecular system into a small canal, the Schlemm's canal, and from there into the venous plexus of the choroid.

Aqueous humor flow

Schlemm's canal is a circular canal system that surrounds the anterior chamber in the area of ​​the chamber angle.

Schlemmscher Canal

The following figure shows the schematic representation of the drainage system in the chamber angle region. The aqueous humor seeps through the trabecular network (a) to Schlemm's canal (b) and collects there. The aqueous humor flows from Schlemm's canal via smaller collecting ducts (c) to the veins of the choroid membrane, from which it is absorbed.

Drainage path in the chamber angle region

The following figure shows an anatomical cross-section through the drainage system in high magnification. The water contained in Schlemm's Canal and the drainage canals is colored blue. The trabecular meshwork can be seen between the chamber angular space (bottom right in the picture) and Schlemm's canal.

Trabecular structure and Schlemm's canal

The trabecular meshwork is constructed like a multi-layer sieve. The aqueous humor flows through its mesh from the corners of the anterior chamber to Schlemm's canal. The following picture shows a micrograph, the cavities between the meshes are not empty, but filled with larger molecules.

Microscopic image of the trabecular network

Changes in the trabecular meshwork are the cause of the increase in intraocular pressure in chronic open-angle glaucoma.

A smaller proportion of the aqueous humor leaves the eye via a second drainage path, the uveoscleral drainage. At the base of the retina, the aqueous humor flows between the cells of the base of the retina and the radiating body and enters the space between the choroid and the sclera. There it is absorbed by the vessels of the choroid or seeps through the dermis into the eye socket. This drainage route is increasingly used, especially at night. The uveoscleral outflow is increased by prostagladins.

Aqueous humor and intraocular pressure

In addition to supplying the lens and cornea, the aqueous humor fulfills a second very important function. The balance between production and drainage of the aqueous humor creates a pressure in the inner eye, the so-called intraocular pressure. The intraocular pressure keeps the eye in its shape. The eye is mobile and is therefore not firmly grown into the surrounding tissue in the eye socket. It is held in the eye socket by the eye muscles, but its round shape is largely maintained by the intraocular pressure. The intraocular pressure is also responsible for the even tension of the cornea.
Intraocular pressure Depending on the age of the person, the intraocular pressure can be between 10 and 30 mmHg. Normally, a middle-aged adult is assumed to have an average intraocular pressure of 21 mmHg. One also speaks of intraocular pressure, and IOP for short.

The ball-like eyeball is bounded on the outside by the opaque leather skin (sclera) and in the front part by the transparent cornea. The white dermis, together with the intraocular pressure, is responsible for the shape and stability of the eye. It consists of collagen and elastic fibers.

The conjunctiva is a delicate mucous membrane with many fine blood vessels. It lines the space between the lids, the eyeball and the eye socket. This protects the eyeball against foreign bodies and pathogens. If the eyes appear reddened from drafts, smoke or the sun, the cause is often irritation of the conjunctiva (conjunctivitis).

The cornea is slightly more curved than the dermis and is inserted into it like a watch glass. It is a clear tissue with no blood vessels. Blood vessels would block the otherwise clear view. As a transparent, evenly curved window, the cornea is the most important refractive medium in the human visual apparatus. Its task is to focus the incident light like a magnifying glass. The refractive power of the cornea is usually around 43 diopters (D) and the refractive power of the lens around 19 D. Because the cornea makes up a much larger proportion of the total refractive power of the eye, the transparency and regularity of the corneal surface are of the utmost importance for clear vision.

The tear film, the aqueous humor and the vessels of the conjunctiva ensure the nutrition of the cornea. The exchange of substances takes place through osmosis. This means: the cornea itself constantly pulls the substances it needs from its "watery" environment. At the same time, it gives off "waste products", the smallest traces of which are dissolved in water.

The lens is a crystal clear body, i.e. there are no blood vessels or nerves running through it. Your metabolism is only maintained through the aqueous humor. The front surface of the lens is slightly less curved than the rear surface. It lies behind the pupil in a plate-shaped pit in the vitreous humor. The lens is stretched onto a muscle by a wreath of hanging threads, the so-called zonular fibers, the radiating body (ciliary muscle).

The lens and cornea are responsible for the bundling of the light rays and their sharp image on the retina. While the cornea always refracts light in the same way, the shape of the lens is adapted to focus near or distant objects (accommodation). If you read a newspaper, the lens bends; if you look at the horizon, it flattens out.

Refraction of light through the cornea and lens

The shape adjustment necessary for focusing is done with the help of the ciliary muscles. If the ciliary muscle contracts, the tension of the zonular fibers on the lens decreases, the lens arches more strongly due to its own elasticity, its refractive power increases and the eye is thus adjusted to near vision. Relaxation of the ciliary muscles causes the lens to flatten and thus distant accommodation.

Ametropia

The light travels around 24 mm from the cornea to the retina. However, if the eyeball is too short or too long, the light rays do not meet exactly on the retina and the resulting image is out of focus.

If the eyeball is too short, the incident light only combines behind the retina. This type of ametropia is called farsightedness (hyperopia). Far-sighted people can see everything well in the distance, while in the vicinity everything is blurred.

Farsighted Eye Anatomy

With nearsightedness (myopia), the eyeball is too long and the light rays meet in front of the retina. Distant targets are no longer recognized correctly. Both visual defects can be compensated for with lenses placed in front (glasses or contact lenses).

Anatomy of the nearsighted eye

The so-called presbyopia is not based on a length of the eyeball that deviates from the norm, but is triggered by an astigmatism. The refraction of light changes so that the focus is no longer on but in front of the retina. For far-sightedness, the lens can usually compensate for a part, or precise focusing plays a lesser role here. In close vision, however, compensation is no longer possible when the ciliary muscle is contracted and the lens shape is relaxed.

The cohesive skin consisting of the iris, the radiating body and the choroid is called uvea.

The iris is a soft, circular membrane, the outer edge of which is fused with the radiation body at the chamber angle. The light falls into the eye through its central circular hole, the pupil. Here the edge of the iris rests on the lens.

The iris acts like a light screen. It regulates the amount of light entering the eye by using its muscles to make itself wide or narrow, i.e. by changing the size of the pupil. The pupil size can vary between approx. 8 and 1.5 mm. The pupil is usually large in the dark and small in bright light. Disorders of the pupil size can indicate different diseases.

The iris determines the color of the eyes. Depending on the number of pigment cells, the eye appears blue (few pigment cells) or brown (many pigment cells).

Low pigmentation iris

The cohesive skin consisting of the iris, the radiating body and the choroid is called uvea.

The radiating body has a ring-shaped structure and contains a muscle. The lens is suspended from its villi by zonular fibers. By contracting the muscle, the lens is adjusted to near vision (accommodation).

Radiant body

In addition to this task, it fulfills another important function, because the aqueous humor is produced in the two-layered skin (epithelium) that surrounds the radiation body. Under the epithelium there are special blood vessels from which ions and medium-sized molecules can escape through small openings. This creates a layer soaked with fluid between this layer of blood vessels and the epithelium. The epithelium then actively withdraws the fluids from this layer and releases this aqueous humor into the posterior chamber of the eye.

Aqueous humor production

Thus, the radiation body is essentially responsible for the production of aqueous humor. The intraocular pressure is regulated by the amount of aqueous humor produced. When the intraocular pressure rises, the production of aqueous humor only stops when it reaches such high values ​​(> 70 mmHg) that the blood flow to the radiating body is impaired.

In the anterior chamber in the area of ​​the chamber angles, the iris has grown onto the radiation body. The trabecular network, through which the aqueous humor flows out of the anterior chamber of the eye, also adjoins there. Outward and further back, the radiation body is firmly fused with the sclera.

The cohesive skin consisting of the iris, the radiating body and the choroid is called uvea.

The choroid (choroid) lies directly under the dermis (sclera). It is part of the external blood supply to the eye. Through the sclera, the supplying and draining blood vessels enter the eye. The choroid is mainly supplied by the short posterior ciliary arteries.

Blood supply to the eye

The venous and arterial vessels form a narrow vascular network that spans the entire inner eye up to the iris and the radiating body. The choroid is the tissue in the body with the most blood supply.

Model of the choroid

The choroid has the important task of supplying the outer retinal layer (this is where the light-sensitive rod and cone cells of the retina are located). It also helps regulate the temperature in the eye.

Choroid

The small venous and arterial choroidal vessels form lobular-like vascular areas.

Vascular areas of the choroid

Vascular areas of the choroid (scheme)

The pigment epithelium shields the choroid from the retina and the incidence of light in the inner eye. The choroid is therefore not visible in the ophthalmoscope. With increasing age, the density of the network of the choroidal membrane decreases. In the area of ​​the choroid there are neuronal cells whose importance for a possible regulation of the blood flow in humans has not yet been clarified.

The inside of the eyeball lines the retina. Essential parts of the retina belong to the diencephalon. Their innermost layer consists of ganglion cells, the long appendages of which bundle up, form the optic nerve and run along the visual pathways directly into the brain.

Visual system

The ophthalmologist can look at the retina directly through a microscope (slit lamp) to assess the retinal blood vessels, the nerve fiber layer, and the papilla. In the middle of the image is the macula, the area of ​​greatest visual acuity, and on the left side you can see the papilla. The optic nerve emerges at the papilla and with the optic nerve also the blood vessels of the retina. The blood vessels form a network that encompasses the entire fundus of the eye. The nerve fiber layer appears orange, individual nerve fibers cannot be seen.

Fundus of the eye, papilla visible on the left

The retina contains the sensory and nerve cells that receive the light stimulus, process it and transmit it to the brain as nerve impulses. The light penetrates the innermost layers of the retina and hits the rod and cone cells (receptors).There the light is converted into nerve impulses, the nerve impulses pass through the inner layers of the retina and are conducted via the ganglion cells to the optic nerve and ultimately via the visual pathways to the brain.
Blood supply to the eye

Structure of the retina

A thin membrane, known as Bruch's membrane, lies directly on the choroid, and the retinal pigment epithelium lies on top of it. The pigment epithelium contains the pigment melanin and shields the sensory cells of the retina from the choroid. The pigment epithelium absorbs the light that penetrates the inner layers, thus preventing backscattering. In albinos the pigment is missing, so you can see the choroid and the inside of the eye "glows" red. The pigment epithelium and Bruch's membrane together form the so-called blood-retina barrier.

Layers of the retina

Furthermore, the retina is divided into three neural layers:

In the first outermost layer are the light-sensitive receptors, the rods and cones. They convert the incident light into electrical impulses. Each eye has about 120 million rods and 6 million cones. The cones make it possible to see colors and shapes in high resolution and are responsible for seeing in brightness. The rods mainly come into action at night because they enable vision in poor lighting. They are essentially responsible for peripheral vision.

The rods and cones are supplied by the choroid, whereas the layers further inside the retina are supplied by the blood vessels of the retina, which run through the second neural retinal layer.

The second neural retinal layer consists of the bipolar cells, the horizontal cells and the amacrine cells. The bipolar cells are synaptically connected to the rods and cones (first synapse zone).

The inner ends of the bipolar cells are synaptically connected to the third neuronal layer, which consists of the ganglion cells (second synapses zone). The ganglion cells have long nerve cell processes and these axons combine to form the optic nerve and leave the eye with it.

Information processing of the retina

Information processing is already taking place within the retina within the two synapse zones between the three retinal layers. These circuits improve the perception of contrasts and the differentiation of colors. A significant part of the adaptation to the brightness conditions in the environment is also carried out in this neural network.

In the first synapse zone, the horizontal cells, in particular, are also involved in the processing, which build up additional overarching horizontal interconnections. Cross-connections have the effect that neighboring neuronal cells receive inhibitory or reinforcing signals and take them into account for the transmission of their own signals. However, the horizontal cells themselves do not pass on information directly to the third neuronal layer; this only happens via the bipolar cells. In a similar way, the amacrine cells in the second synapse zone interconnect the bipolar cells and the ganglion cells.

The distribution of the nerve cells and their interconnections are not the same across the retina. The point of sharpest vision is found in the center of the retina. The macula, also known colloquially as the “yellow spot”, is approximately 3.5 millimeters from the papilla. In the central area of ​​the macula, the fovea centralis, there are almost only cone cells and very many bipolar cells. There is approximately a 1: 1 transmission, since each pin contacts a bipolar cell and this in turn is connected to a ganglion cell. This structure enables the highest resolution and differentiation when seeing.

In the peripheral areas of the retina there are many rod cells and fewer bipolar cells. Each bipolar cell is connected to several rod and cone cells. This supports seeing in the dark, because individual light information from many rods is combined in this way.

However, the exact mechanisms of this complicated neural network are not yet understood.

Blood supply to the retina

The outer parts of the retina are supplied by the choroid via the retinal pigment epithelium (see choroid). The inner retinal layers (ganglion cells, bipolar cells), on the other hand, are supplied by a vascular network that is fed by the central artery and covers the retina.

Blood supply to the eye

The central artery enters the eye with the optic nerve and branches into the retinal vascular network. Larger arterial and venous vessels run in relative proximity and almost parallel to one another.

Vascular network of the retina

As the branching increases, the diameter of the vessel becomes smaller and smaller.

Vascular network in the area of ​​the macula

The smallest vessels are called capillaries. Their diameter is so small that even a single blood cell deforms when it flows through a capillary.

Capillary blood vessel

The optic nerve, like the retina, is part of the brain. The optic nerve consists of around 1,000 bundles of optic nerve fibers. The optic nerve fiber bundles are created by bundling approx. 1 million nerve cell processes of the ganglion cells, which are distributed in the outer retinal layer. The optic nerve is surrounded by three connective tissue sheaths, with the outer sheath continuing into the dermis, which also encloses the eyeball. The blood vessels that supply the retina run inside the optic nerve.

Exit of the optic nerve from the eye

Outside the eye socket, the optic nerve initially runs in an S-shape on its way to the brain and can therefore adapt to extreme eye movements. The optic nerves of both eyes cross at the optic chiasm and the nerve fiber bundles are forwarded from here in two lines to the right or left visual cortex to the brain, depending on whether they contain information from the right or left field of vision.

Optic chiasm

The optic nerve leaves the inside of the eye at the papilla, an opening in the dermis, in which there is a sieve plate (lamina cribrosa). All nerve fiber bundles and blood vessels penetrate the sieve plate. The papilla is filled with the bundled optic nerve fibers, the blood vessels and supporting cells. There are no sensory cells in the area of ​​the papilla, which is why a “blind spot” arises in the perceived image.

The first section of the optic nerve in the area of ​​the papilla, where the nerve fibers are bundled, is called the optic nerve head. The optic nerve head is particularly interesting for glaucoma disease and its diagnosis, as pathological changes can be seen here and the ophthalmologist can directly examine this area of ​​the optic nerve when examining the fundus.

In the microscopic longitudinal section and in the following schematic drawing you can see that the nerve cell bundles form a hilly bulge when they emerge through the papilla from the inside of the eye, the edge of the rim. The nerve fiber cushion (4) forms the physiological (healthy) cavity (3). You can also see the central artery (2) and the structures of the lamina cribrosa (sieve slit).

Anatomical longitudinal section through the optic nerve

Scheme drawing of the optic nerve head in section

If increased intraocular pressure exerts constant mechanical pressure on the optic nerve head or if there is a reduced blood supply to the optic nerve head for other reasons, the result is usually the death of the nerve fibers and the shape of the optic nerve head (the papilla) changes. Losses in the field of vision have usually already occurred.

Excavated papilla

When examining the fundus, the ophthalmologist assesses the shape of the optic nerve head, the course of the blood vessels on the retina, the color of the structures and much more. Important clues for clarifying a suspicion of glaucoma are, for example, bleeding in the edge area of ​​the papilla.

Fundus image of a healthy papilla

The blood supply to the optic nerve

The blood supply to the optic nerve does not take place via the blood vessels that run inside the optic nerve (central artery) and which serve exclusively to supply the retina, but comes from blood vessels that accompany the optic nerve laterally, the short posterior ciliary arteries. The ciliary arteries also feed the choroid, which is responsible for the blood supply to the outer sections of the eye. The smaller vessels supplying the optic nerves run in the connective tissue sheaths that surround the optic nerve on the outside.

The blood flow to the optic nerve head is particularly critical, as this is already in the inner eye and the intraocular pressure can influence the blood flow. Here, too, small vessels, which are part of the choroidal system, are responsible for blood flow. When the blood vessels enter the inside of the eye, there is an abrupt drop in pressure, as the tissue pressure outside the eye (approx. 5 mmHg) is significantly lower than the pressure in the inner eye (approx. 15-22 mmHg).