Retina

Information about the retina

Hard to find Information and links about the human retina

There are lots of web pages about the retina. However, some information is particular hard to find. For instance, try to find a webpage that explains that the visual axis does not direclty pass through the fovea, or what the difference between the fovea and the macula is?

A good drawing of the eye can be found here.

A good webpage on the retina is here.

 

The name retina has its origin from the latin word "rete" which means "network". Here, is a

photo of a human retina. And here is a photo of a section of a cow's eye. The optical disc is where all the foldings meet. This is where the optical nerve leaves the eye on the other side. It is also the location where the main blood supply enters. Be aware, that the receptors are not at the inner side of the eye but are directed outwards (in wrong order from a naive design point of view).

The "network" of a single retina consist about 11 layers with over 100 million neurons in total. A nice figure that interlinks a real photo to a schematic sketch of the layers can be found here. A high resolution photo of the retinal layers is here.

The next to last of these layers serves as a projection plane and is populated with receptors of two basic types: small rods and bigger cones. Apart from some limited evolutionary variations, there basically exist three types of rods: blue, red (L-cones) and green ones (M-cones). A photo of some rods and cones is here.

Their names referring to plain colors simplifies the fact that each type has a whole range of wavelengths it is sensitive to, and the sensitivity functions have quite complex shapes, with only the maximum sensitivity correlating with its color name.  There exist few variations of the rods' sensitivity functions between differnt mammals and even among humans. The most known variations are knwon in terms of color blindness and the like but there exist other variations, too. Some of those variations are very rare, some are even only hypothetical and not or very hard to be detectable phenomenologically for the individual that is subject to the variation.  Apart those type variations among different species and individuals, the spatial distribution of receptors varies grately within the retina itself.

According to those spatial variations the retina can be divided into a peripheral region and the macula lutea, in german "Gelber Fleck" which is a translation with some semantical loss, because "lutea" seems to indicate a color that is more brown then pure yellow. The macula lueta can be divided into fovea centralis,a para foveal area a peripheral fovea. The fovea centralis has an even smaller distinctive area of only 0.33 mm which is called foveola.

|------radial topological order from optical axis---->

|macula lutea                                                          |  peripheral retina

 

|fovea centralis| para-foveal region | peripheral fovea|

|foveola|

 

Here are some links concerning the fovea:

Fovea in general: http://www.daviddarling.info/encyclopedia/F/fovea.html

Wikipedia German on the fovea centralis: http://de.wikipedia.org/wiki/Fovea_centralis

Considering the right eye the visual field that is projected onto the retina reaches 60° up, 75° down, 60° to the left and 100° to the right. The location of the fovea centralis corresponds to the location of the principal point in computer vision. It is the intersection of the center ray (ray through the center of the lens and perpendicular to the eye's surface) and the retina. The foveal centralis is only 1.5 mm wide. It has a distance of about 4mm from the optical nerve papill.

 A fascinating image of dendritic trees of Purkinje cells

 

 

 

 

 

 

 

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