Structure of Eye

Human Eye: Anatomy, Structure and Functions

The eye is a sensory organ. It absorbs light rays from our surroundings and converts them in such a way that the facts in the brain can be treated further. The eye and brain form a component that has developed together in the course of evolution (visual system). The procedure of processing is so-called "seeing”, “watching" or "looking". The visual imprint is basically generated from visual memory, in which only a little new information from the eye is combined.



Part of the human eye structure:

Cornea



The forward-facing side of the eye is the cornea. The cornea is transparent and is made up of six layers. The ring-shaped shift from the cornea to the sclera is named limbus (from Latin for "border"). With the help of the stem cells present there, the cornea is everlastingly renewed. It is somewhat thinner in the center than the outer areas. This is particularly important in the case of eye lasers when a portion of the cornea is removed in order to enhance the refractive power. The curvature of the cornea refracts the light around 45 dioptres. The cornea is surrounded by tear fluids, which is produced in the tear glands and act as supply and protect the eye.

Anterior and posterior chamber, intraocular fluid



Anterior eye chamber is present behind the cornea, which is occupied by intraocular fluid. Behind the iris, the posterior chamber of the eye starts. The intraocular fluid is made and then released at the ciliary processes. It then slowly flows out through the pupil into the anterior chamber of the eye.

Iris



The iris is situated in the center of the cornea. It contains many fine muscle pathways that can contract or expand. The resulting round opening in the middle is known as the pupil. The darker it is, the more light is required for vision - the pupil becomes respectively larger in darkness. In bright light, the pupil is only small. The iris is colored by certain pigments such as (blue, brown, green, grey or corresponding mixed values).

Lens



Eye lens (Phakos) is present behind the pupil. It is liable for about 15 dioptres of the refractive power, but it can alter its refractive power. Thanks to its ability (accommodation), the eye can watch sharply to objects present both near and far. The eye lens is a type of liquid sphere. They can be associated with a water-filled balloon. In the center called lens equator - the lens is hung up on the zonular fibers, which arises from the ciliary muscle. The lens fluid solidifies over the years’ cataract. Often, the old lens is removed during cataract surgical treatment and a non-natural lens is simply implanted - which guarantees a clear view even in old age.

Ciliary muscle



The ciliary muscle is situated behind the cornea in a ring-shaped figure present inside the eye. It can energetically influence the curving of the eye lens. In a stress-free state, the lens is flat and drawn out - so you can see well in the vicinity. But, if the ciliary muscle constricts (contract tone), the diameter of the ring is reduced. The zonula fibers relax and the lens takes on a somewhat bulbous, spherical shape. This causes changes in the refractive power of the lens so that you can see well in the surrounding area. This process is called accommodation.

Vitreous chamber



The inner space of the eyeball is completely filled by the vitreous body. It has of a gel-like clear liquid and is particularly important for the stability of the eyeball: the liquid creates a pressure, called intraocular pressure. This makes sure that the surrounding layers do not peel off and breakdown. In the absence of intraocular pressure, the eye would be much more sensitive to outside pressure impacts that affect the cornea. An abnormal intraocular pressure can give rise to many eye diseases, such as glaucoma.

The dioptric apparatus

All the elements through which the rays of light travel before they hit the retina causes a dioptric effect. They are also known as dioptric apparatus. The aqueous humor and the vitreous body are transparent so that there is barely any measurable refraction of single photons. The cornea and eye lens are accountable for the actual dioptric effect. The entire eye has a refractive power around 59 dioptres (dpt), of which is about 43 dpt (75%) is on the cornea and about 19 dpt (25%) on the lens (in a relaxed).

Sclera



The eyeball is covered by three layers. The outer or exterior shell is called sclera. It is the white color part of the eye which surrounds the cornea. The scera forms more than 80% of the eyeball’s surface area. it extends from the cornea all the way to the optic nerve that exists on the back side of the eye.

Choroid



Inside the protecting sclera follows the choroid, which, as the name suggests, is permeated by several blood vessels and capillaries. The blood delivers the retina with nutrients and oxygen. Retina



The retina is situated on the back or inside of the eye. It is made up of different cell layers: the photoreceptors transform the light impulse into an electrical nerve impulse. The light information is bundled in named receptive fields, enlarged and transmitted to the brain via the visual pathway.

The genuine "visual process" then occurs on the retina. The retina is made of a number of different cell kinds with different responsibilities. First of all, the sensory cells are vital. They convert the light into an electrical impulse.

There are two kinds of vision cells:

  • The rods (light-dark vision, active in evening or darkness)

  • The cones (accountable for color vision)

  • Three different kinds of cone cells are essential for color vision:

  • Pins for red-visibility (about 46% of all pins)

  • Cones for green vision (about 46% of all cones)

  • Cones for blue vision (about 8% of all cones)

  • The three types of cone react to light of different wavelengths. 

    If a photon or a light partial with a wavelength in the red region hits a red cone, then it "produces" an impulse at the following cells. The other two cone kinds remain inactive for a "red photon". They react accordingly when photons reach with their exact wavelength. If one of these cone types is not properly made due to a genetic defect, there will be a color vision injury or color blindness we can say as red-green weakness. The genetic irregularities of the eye's color are as follows:

  • Protanomaly: Red vision weakness

  • Protanopia: Red blindness

  • Deuteranomaly: Green vision weakness

  • Deuteranopia: Green blindness

  • Tritanopia: Blue blindness

  • Tritanomaly: Blauseh weakness

  • Further processing on the retina



    The retina involves a large number of other different cells that develop the electrical impulses directed by the visual cells. The visual information from neighbouring regions is bundled, compared and enhanced in contrast. It can be said that only "new" and "relevant" info from the "image" is delivered on to the brain This "filtering" of data is very effective and economical. Evolution has advanced the eye in such a way that it uses as little energy as possible. Rationally, you don't have to see everything to live, but only what is vital.

    Stereoscopic vision



    The pre-structured optical evidence is then conveyed to the brain with the help of the optic nerve. The info is collected from both eyes and forwarded together. This is where the "stereoscopic vision" is formed. The information from the left and right eye is somewhat different since the angle of incidence is marginally different. From this difference, the brain can conclude something like space. The spatial visual imprint is therefore produced from the different information of the two eyes. Correctly, still, one has to say that the learned information like perspective, sizes and so on have a bigger share of the three-dimensional visual imprint than stereoscopic vision.

    Lastly, the information reaches the brain with the help of optic nerve - and here it is spread over large areas that are stored to fluctuating degrees (via linked synapses of the single nerve cells). Eventually, this "neural pattern" is what we see as a visual image of reality. This pattern has been continuously developed and modified since the first day of opening the eyes.