Editing Human eye development (section)

==Retina==

[[Image:Fig retine.png|thumb|right|300px|Diagramatic representation of cell layers in the retina. Light enters the retina from the left and passes through several layers of axons and non-light sensitive cells before reaching the rods and cones at the right. The pigmented epithelium layer (not shown here) would be further to the right.]]

During embryonic development the [[w:Retina|retina]] forms [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=retina+AND+dbio%5Bbook%5D+AND+131821%5Buid%5D&rid=dbio.section.2920#2921 from the brain]. The part of the neural tube that forms the retina folds over another nearby part that forms the [[w:retinal pigment epithelium|retinal pigment epithelium]]. The [[w:Rod cell|rods]] and cones are metabolically very active and they shed a large part of their structure every day. The adjacent cells of the pigment epithelium act like a garbage disposal to [[w:Phagocyte|phagocytize]] the discarded chunks of the visual cells. Thus, it is very useful to have the photo-detector cells "inside". The other cells of the retina are essentially transparent, so it hurts nothing to have them on the "outside", with light having to pass through most of the retina to get to the light-sensitive part. 

===Optic nerve===

[[Image:Schematic diagram of the human eye.svg|thumb|left|200px|(14) Optic nerve. (11) Retina and axons leading to the optic nerve.]]

[[Image:Gray773.png|thumb|right|Optic nerve]]

It is a simple matter for axons of the optic nerves to follow the "stalk" of the retina back to the rest of the brain. In an animal like humans with over-lap in the visual fields of the two eyes, [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=optic+nerve+AND+neurosci%5Bbook%5D+AND+231345%5Buid%5D&rid=neurosci.section.816 not all] of the axons in the [[w:Optic nerve|optic nerves]] cross to the other side of the brain. This allows information from the two eyes about the same object to be combined in the brain.

===[[Visual cortex]]===

The target for the [[axons]] of the optic nerves is not the back of the [[brain]]. The [[w:Lateral geniculate nucleus|lateral geniculate nucleus]] of the thalamus is a major target for retinal axons in humans. In evolutionary terms, it was a later "invention" to send additional axons to the back of the brain and use for vision the vast amount of cerebral cortex that is available there.
[[Image:Functional magnetic resonance imaging.jpg|thumb|left|Activated visual cortex at the back of the brain.]]

===Your turn===

Read [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=eye+development+AND+dbio%5Bbook%5D+AND+131820%5Buid%5D&rid=dbio.section.2920 this] and then edit this page to add more information about eye development.