Also interesting: The eye’s cornea
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The structure of the eye
How is the human eye actually made up and how does vision work? Many people seem to take it for granted that we can perceive the environment with our eyes. Yet the structure of the eye is quite complex.
What is the function of the eyes?
The eyes are our most important sensory organ, with which we perceive up to 80 percent of all environmental impressions. Our eyes process at full speed all the visual information that bombards us every day. They only get a break during sleep, when they can recover.
How is the eye constructed?
The human eye consists of many individual components with different tasks. Some parts are visible from the outside, others are located inside the eyeball and the eye socket.
The Eyeball
The eyeball (bulbus oculi) has a diameter of about 24 millimetres. At first glance it appears round, but in fact it is a
slightly asymmetrical sphere. Together with the 6 eye muscles, nerves and blood vessels it lies in the bony eye socket
and is padded by fatty tissue. From the outside, the eyeball is protected by the eyelids and regularly moistened by
blinking.
The eyeball is divided into the anterior and posterior segments. The most important components of the anterior segment of the eye are:
• Sclera: The white, opaque and relatively solid layer, together with the cornea, forms the outer covering of the eyeball. It ensures that the eyeball is protected, but contains few vessels and nerves.
• Cornea: The cornea is transparent, free of vessels and traversed by many nerves. It is set into the sclera like a watch glass, but is clearly more curved. It overlays the iris and pupil and covers the anterior chamber of the eye. Due to its high refractive power, the cornea has a special importance for vision.
• Conjunctiva: The conjunctiva is a transparent mucous membrane that – with the exception of the cornea – covers the eyeball and the inner sides of the eyelids. It contains, among other things, mucous cells whose secretion is part of the tear film, thus moistening the surface of the eye and enabling the eyeball to glide smoothly.
• Iris: The iris controls the amount of light entering the eye like the shutter of a camera. The amount of light is regulated by the black, round opening in the centre, the pupil. In the dark, the pupil dilates; in strong light, it contracts. The colour of the iris depends on the number of pigments. The more pigments the iris has, the darker the eye colour.
• Lens (lens cristallina): The transparent eye lens is located directly behind the iris and, together with the cornea, is responsible for the refraction of light in the eye. It is suspended on elastic fibres, the zonula fibres, which are arranged in a circle around the lens. The curvature, and thus the refractive power of the lens, can be changed by a circular muscle, the so-called ciliary muscle. This enables vision at different distances, also called near and distance adjustment or accommodation.
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Infographic Anatomy of the eye
The most important components of the posterior segment of the eye are:
• Vitreous body (corpus vitreum): The vitreous body has a gel-like substance and consists of 98 percent water, about two percent hyaluronic acid and a highly branched network of collagen fibres. It is transparent and fills the space between the lens of the eye and the retina. Apart from its function of stabilising the eyeball, it is also involved in the refraction of light in the eye.
• Choroid (vascular layer): The choroid is located between the sclera and the retina. It is pigmented and very vascular and is primarily responsible for supplying the retina (especially the photoreceptors) with oxygen and nutrients.
• Retina: The image is projected through the pupil onto the retina. It lines the posterior inner wall of the eyeball and consists of a large number of nerve cells. The photoreceptors, the so-called rods and cones, are essential for the visual process, as they convert the incoming light into electrical signals. The central area of the retina is the area of sharpest vision. This "yellow spot" is called the macula lutea, and the fovea centralis, lies in its centre . Here, the density of photoreceptors is highest. More nerve cells transmit the signals to the brain via countless nerve fibres.
Optic nerve
The optic nerve is one of the cranial nerves and comprises over 1 million nerve fibres that transmit electrical signals from the retina directly to the brain. Because it has a slightly sinuous course, it can allow any eye movements. Within the nerve fibre bundle, the retinal artery and vein pass through, which are responsible for supplying blood to the retina. The optic nerve passes from the eye through the eye socket to the brain. Here it passes through the so-called optic nerve junction (optic chiasm), where the visual information of the right eye is arranged together with that of the left eye.
The spot where the optic nerve leaves the eye is called the "blind spot". Since there are no photoreceptors at this spot, no light can be perceived and converted. However, the brain automatically fills in the missing information so that we do not notice any impairment.
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How does vision work?
The visual process proceeds through various stages:
- When light reaches the cornea, it is refracted there and enters the eye through the pupil.
- The incoming light is refracted again in the lens and redirected directly to the inside of the eye.
- The ray of light reaches the retina through the vitreous body, which functions as a kind of screen onto which the visual image is projected. However, the image there is greatly reduced in size, right-left reversed and upside down.
- In the photoreceptors, the light is converted into electrical signals that are passed on to the brain via the afferent neurons in several intermediate steps.
- The brain turns around the upside down image and thus creates the seen image.
This process is very complex, and yet we perceive none of it because our eyes usually process all sensory impressions quickly and reliably.