Luminous stimuli that originate from the outside world are perceived by the photoreceptor cells, distinguished in cones and rods and located in the outer part of the retina. The stimuli are then transformed into electrical impulses and transmitted through the optic nerve to the brain, where the processing and perception of the visual image happens.
It is the space between the cornea and the lens and is filled by the aqueous humour, a transparent liquid, produced by the ciliary epithelium.
The posterior chamber is located between the iris and the formations that attach the lens to the ciliary body; it is smaller than the anterior chamber and serves as an initial storage area for the aqueous humour produced by the ciliary processes. In normal conditions the aqueous humour moves from the posterior chamber to the anterior, where it is drained through the trabecular meshwork. This process is impaired in the case of glaucoma.
The cornea is a transparent membrane without blood vessels, located in front of the iris. It is rich in nerve fibres and is constantly wet by the tear film, which sticks to its surface. The cornea is the most powerful converging lens of the human eye: it carries out the function of transmitting on the light radiations that reach the eye giving them the necessary convergence to be diverted on the lens and subsequently focused on the retina.
It is a vascular membrane lining the retina. Its function is to feed and oxygenate the retinal pigmented epithelium, the outer retinal layers (in particular the photoreceptors) and is involved in the blood supply to the optic nerve.
The choroid contains a dark pigment that captures light rays that if reflected within the eyeball would interfere with vision.
It is a 3-4 mm tissue, located behind the iris, which in the inner surface contains the ciliary processes, about 70, which serve to produce the aqueous humour.
It consists of a gelatinous material and occupies about 4/5 of the total volume of the eyeball. Along with the sclera, it contributes to preserve the shape of the eyeball, ensuring at the same time a certain elasticity and ability to absorb shocks. It is crossed by light rays and helps to converge them on the retina.
It is the natural ocular lens behind the iris, with the function to clearly converge on the retina the light rays coming from both near and far away due to its ability to adapt the image focus depending on the distance.
To carry out this function, the lens must be absolutely transparent. The process of partial or complete opacification of this natural lens determines the onset of cataracts.
Located on the side wall of the orbital cavity, they are the leading producer of tears and aqueous layer of the tear film. The lacrimal fluid, consisting of salts, proteins, and of a number of substances with antibacterial and antioxidant actions, performs many functions in support of the surface of the eye (cornea and conjunctiva): it carries oxygen and major nutrients; expels waste products; helps to prevent infections, moisturises and lubricates the tissues.
It works like the diaphragm of a camera, thanks to two small muscles: the pupillary sphincter, which serves to constrict the pupil (miosis), and the pupillary dilator, which serves to dilate the pupil (mydriasis).
The iris determines eye colour and can be clear (from blue to green) or brown (from brown to black). Its colour depends on both the amount of pigment and by optical phenomena of reflection and diffraction of light. In clear irises, poor in pigment, the light passes up to the deep layers where it is reflected taking on a light colour. Unlike in brown irises, rich in pigment, light does not penetrate into the deeper layers and is not reflected or diffracted.
It is the central area of the retina, which provides the ability to recognise objects and colours, to read and write, and other visual abilities known as "fine visual discrimination".
In the macula, there are three types of cones (the photoreceptors providing the ability of colour discrimination ), each of which is sensitive to light beams of different wavelengths, respectively red, green and blue. The result from the integration of stimuli coming from different types of cones is the coloured visual image. Most of the visual receptors are concentrated in the central fovea, a highly specialised structure providing in conditions of high brightness, the best visual acuity from far and near, colour discrimination and contrast sensitivity.
It consists of nerve fibres that originate from the retina and come out the eyeball in one point called the optic disc. This is the area directly affected by alterations caused by glaucoma. The two optic nerves leave their orbits via the optic canal towards the optic chiasm where there is a partial decussation of the nerve fibres. Most of the fibres that make up the pair of optic nerves terminate in the brain where visual information is transmitted to the visual cortex.
It is the hole located in the centre of the iris that allows the entry of light into the eyeball. Its diameter varies depending on the intensity of the light. The dilation (mydriasis) and constriction (miosis) of the pupil happen thanks to a system of muscles, activated by the central nervous system.
It is a thin membrane that lines the inner surface of the eyeball and is divided into two areas:
- a central area, called macula, which contains the central fovea, rich in cones
- a meridian and peripheral area where are located most of the rods and provides the twilight and nocturnal vision.
The retina transforms light stimuli into electrical impulses that are then transmitted through the optic nerve to the brain.
It is an opaque and fibrous membrane that covers much of the eyeball except for the front area, which gives way to the cornea. It consists of a dense network of collagen fibres, arranged in an irregular way. Due to the intra-ocular pressure, the sclera stretches and assumes a spherical shape. Intra-ocular pressure and the sclera together stabilise the shape of the eye, even in case of intensive mechanical stresses, such as rapid eye movements.
The portion of the sclera exposed to air is protected by a transparent mucous membrane, called the conjunctiva.
It is a thin membrane, located under the sclera. It owes its name to a dense latticework, full of blood vessels, which give it the appearance of a bunch of grapes. The uvea consists of the iris, ciliary body and choroid.
