The eye is built like a analog camera. Like the lens in the camera lens, the human lens focuses the incoming light. The retina is like a film: the visual impressions are shown on it.
ANATOMY OF THE EYE
The transparent cornea protects the eye from external influences and (together with the lens) focusses the incoming light on the retina. the iris and the pupil behind it act like an aperture, the pupil changes size depending on brightness.
LENS
Behind the iris is the lens of the eye connected to the ciliary muscle via the zonula fibers. The lens can change its shape and thus its refractive power. This makes it possible to see not only objects in the far but also in the near clearly (accommodation). However, from the age of about 45 this ability gradually diminishes. This is also the reason for the need for reading glasses with increasing age. If the lens of the eye becomes cloudy, this is referred to as a cataract.
VITREOUS
Most of the eye is filled with a clear, gel-like substance called the vitreous body. It consists of 98 percent water. Protein components and fine connective tissue fibers are distributed in it. With increasing age, the vitreous body can thicken. These condensations are perceived as “floaters” (“mouches volantes”).
RETINA
Incident light passes through the cornea, lens and vitreous body, and then focus on the retina, which fulfills the function of the “film in the camera”. Millions of light-sensitive sensory cells (photoreceptors) are located in the retina. The light and thus incoming images from our surroundings, are converted by the sensory cells into nerve impulses, which can then be transmitted to the brain. There are two types of sensory cells: cones (6 million) and rods (120 million). In the center of the retina it is primarily the cones that enable color vision. The rods are responsible for mesopic vision and are located in the periphery of the retina. The cones are particularly densely packed in the macula. This “yellow spot” measures about 3-5 mm in diameter and is the point of sharpest vision. Age-related changes such as age-related macular degeneration are therefore associated with a loss of central visual acuity.
OPTIC NERVE
Next to the macula is the optic nerve head (optic disc). This is where the optic nerve leaves the eye. This 1.2 million nerve fiber ‘data line’ connects the eye with the brain. In case of glaucoma, the optic nerve among other things can be damaged.
Ametropia
Clear vision requires that the focal point (focus) of incident light is exactly on the retina. However, many people suffer from ametropia: in the myopic or short-sighted eye, the focal point is in front of the retina. Objects that are further away are then perceived as blurry. In the hyperopic or farsighted eye it is the other way around: the focal point is behind the retina. In the case of astigmatism the incident light rays are not combined in a focal point, but in a focal line. So objects can appear distorted.
EYE MOVEMENTS
Each eye is moved by six eye muscles in the eye socket (orbit). If both eyes are not looking in the same direction, this is referred to as squinting (strabism).
