Vision is the ability to interpret the environment thanks to the rays of light that reach the eye. Vision or sense of sight is one of the main sensory abilities of humans and many other animals. World Sight Day is celebrated on the second Thursday of October.
The sense of sight or vision is possible thanks to a receptor organ, the eye, which receives light impressions and transforms them into electrical signals that it transmits to the brain through the optical pathways. The eye is a paired organ located in the orbital cavity. It is protected by the eyelids and by the secretion of the lacrimal gland, it has the ability to move in all directions thanks to the extrinsic muscles of the eyeball. The essential property that makes vision possible is photosensitivity that takes place in specialized receptor cells that contain chemicals capable of absorbing light to produce a photochemical change.
When light enters the eye, it passes through the cornea, the pupil and the lens, to reach the retina, where the electromagnetic energy of the light is converted into nerve impulses that are sent through the optic nerve to the brain for processing by the visual cortex. In the brain, the complicated process of visual perception takes place, thanks to which we are able to perceive the shape of objects, identify distances, detect colors and movement. The retina is one of the most important regions of the eye and contains specialized cells called rods and rods that are sensitive to light. 
Injury to any of the structures of the visual system can cause blindness, although the rest do not present any alteration. In cortical blindness, for example, caused by injury to the occipital region of the brain, complete loss of vision occurs although the eye and optic nerve are not abnormal.  
Theories about the functioning of vision began with the pre-Socratic philosophers, according to which the eye was made up of water and fire. According to the active model of vision that has been traditionally attributed to Pythagoras and Euclid, the eye emits a beam of rays that travels through space and touches objects causing the sensation of vision. The opposite explanation is the passive model of the vision that was defended among others by Democritus and Lucretius, according to this theory, objects send images of themselves towards the space that surrounds them. The air would therefore be full of immaterial images that would move in all directions, the eye being a passive instrument with the function of capturing them. 
The scientific study of visual perception began with Alhacen, born in 965 AD. In Basra, but his ideas, which rejected the theory of emission, were slow to be accepted in the West. Isaac Newton was the main follower and follower of him in the 18th century, and in the 19th century it was Hermann von Helmholtz, a German physician who authored the Handbuch der Physiologischen Optik / Treatise on Physiological Optics. In the 21st century, the models that explain the phenomenon of vision are multidisciplinary, since they take into account both physiological and neurological and psychological aspects. The eye is now considered to act as a receptor, while the perceptual process takes place primarily in the brain.
Layers of the eye wall
The eye is the organ in charge of receiving visual stimuli, it has a highly specialized structure, the product of millions of years of evolution. The human eye has three envelopes, which from the outside to the inside are:
External fibrous tunic. It is made up of two regions, the sclera and the cornea.
Sclera: It is white and opaque, with c