ABSTRACT
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Eyesight is unquestionably the most important modality of perception that humans possess.
It is so central to our way of life that irrational fear of losing vision, termed Scoptophobia or
Scotomaphobia, is totally incapacitating for some individuals. Creationists suggested that an
organ as special as the human eye could not have come about by chance; it is therefore prima
facie evidence of divine intervention. More rational people point out design imperfections: for
example, the invertebrates seem to have acquired the services of a better architect. In the squid
and the octopus, the photoreceptor cells point forward toward the incoming light; whereas in
humans, the structural elements-rods and cones-are aimed backward, away from the light
source. Moreover, the nerve fibers that must carry signals from the retina to the brain must
pass in front of the receptor cells, partially impeding the penetration of light to the receptors [1].
Whatever the anatomical failings at inception, this wonderful structure is also subject to all the
vicissitudes of time and disease. Preservation of sight as the mean age of the population
increases, and thus as blindness becomes more prevalent will remain a key healthcare objective.
The human eye is a spherical structure that, in three dimensions, has the outside profile of a
smaller sphere embedded within a larger sphere. The organ is the biological embodiment
of a very familiar physical object-a camera, with the ability to control the focus, light
intensity, and depth of field of an image through movement of the external and internal
musculature under intentional and autonomic nervous control. The film of this camera is the
brain itself, the stimuli being carried by the afferent pathways from the photoreceptors to the
optic nerve that hands on the information to the visual cortex at the back of the brain through
the optic tracts. As the information is relayed to the brain, a few fibers leave the main tracts to
enter the nerve nuclei that control the visual reflexes. The purpose of this feed helps both eyes
work in concert to improve the stereo image-the consensual response-and also to adjust
light input and lens power via a separate pathway.
