How the retina works
mtyner at mindspring.com
Fri Jul 2 21:40:31 EST 2004
"r norman" <rsn_ at _comcast.net> wrote
> I don't think the retinal layers are different in the fovea -- light
> still does pass through all the neural cell layers. What is different
> is that in the rest of the retina light also has to dodge the bundles
> of axons and the blood vessels supplying the retina. These tend to
> skirt around the fovea.
A cross-section micrograph shows the retina _does_ thin significantly at the
fovea, forming a concave pit that produces the "foveal light reflex" seen
with the ophthalmoscope. IIRC the cross-section shows thinning in the two
nuclear layers, fewer cell bodies, as well as fewer axons. Fewer axons
The axons are transparent. And actually, the blood vessels are pretty
transparent too. When you see vessels with an ophthalmoscope, you're seeing
the "blood column," not the vessel walls.
The blood column is essentially opaque, and casts a distinct shadow. If it
were a simple camera, the film would have a superimposed blood-vessel
pattern on every picture. Instead, the retina "filters out" static images,
those without moving edges. If you wiggle a penlight against the sclera and
cause the shadows to move, the vessels become dramatically visible - a
phenomenon called Purkinje's tree.
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