The eye is our most important sensory organ. Over 80% of the sensory information is picked-up as optical stimuli by our eyes. They enable us to process the many impressions from our surroundings, such as colours, shapes and brightness, and to assemble them into an image with the help of our brain. 1, 2
The eye can be considered an extension of the brain and the central nervous system. It sits protected in the eye socket and is held there by a group of muscles. Four straight and two oblique eye muscles are connected to the eyeball and form the eye's movement apparatus. They afford the eyeball a great range of mobility so that it can track movement and focus on objects adjusting its position in the eye socket.
The optic nerve exits the inner eye in the posterior segment and forms the connection to the brain. In the anterior part, the upper and lower eyelids are directly attached to the conjunctiva on the inside. This mucous membrane forms the foremost barrier between the eye and the environment. The eyelids ensure that the eye is sufficiently moistened with every blink. They also prevent dust and particles from entering the eye.
The outer fibrous layer
The outer layer of the eye consists of two sections, the light-impermeable sclera, which is recognisable as the white of the eye, and the light-permeable cornea (a), which covers the circular area of the iris (b) and pupil (c) as the outermost layer. While the solid, sturdy sclera (d) gives the eyeball its shape and stability, the transparent cornea allows for light to enter the eye.
The pigmented choroid
The pigmented choroid (e), together with the iris (b) and the ciliary body (f), forms the uveal tract. While the choroid is permeated by blood vessels, which supply the eye with oxygen and nutrients, the iris has the function of a lens within the eye: The muscle fibres in the iris make sure that its central opening, the pupil (c), constricts or dilates depending on light intensity. Behind the pupil is the lens (a), which is held in position by the suspensory structure of the ciliary body (f). Under the influence of the ciliary muscle, which is embedded in the ciliary body, the elastic lens extends and contracts to change its degree of refraction. This allows us to focus our vision across all distances.
The retina (h) forms the inner sensory layer of the eye. It contains highly specialised sensory cells, which capture the incoming light signals and convert them into electric signals. Once these signals have been processed by the overlying cells in the retina, they are transmitted to the brain by the optic nerve.
The optic nerve
The human optic nerve consists of approximately one million nerve fibres, which lead out of the eye at the rear of the eyeball and connect the retina to the brain. While it is the purpose of the retina to convert visual stimuli into nerve signals, it is the responsibility of the optic nerve to channel these signals to the brain after preliminary processing by the inner retina. Only the brain is capable of analysing the information contained in the electrical signals and of converting it to an image. 2, 3, 4, 5