Changes

The retina has many specialized cells (photoreceptors) that are able to react to light and send their signals via the optic nerve to the brain for interpretation. There are two types of photoreceptor cell called rods and cones. Rods are by far the most numerous (around 120 million) and are sufficiently sensitive to allow basic black/white vision even at very low light levels. Cones (6-7 million) only function at higher light levels, but enable us to perceive colour. <ref>http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html</ref>  

The retina has many specialized cells (photoreceptors) that are able to react to light and send their signals via the optic nerve to the brain for interpretation. There are two types of photoreceptor cell called rods and cones. Rods are by far the most numerous (around 120 million) and are sufficiently sensitive to allow basic black/white vision even at very low light levels. Cones (6-7 million) only function at higher light levels, but enable us to perceive colour. <ref>http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html</ref>  

Typically eyes come in pairs, to allow binocular vision and hence depth perception, but [[spider]]s (for example) have eight eyes, and [[scallop]]s may have several dozen.

Typically eyes come in pairs, to allow binocular vision and hence depth perception, but most [[spider]]s (for example) have eight eyes (though some have less or none at all), and [[scallop]]s may have several dozen.

== Diseases of the eye ==

== Diseases of the eye ==