Author Topic: Helmholtz vs. Schachar  (Read 5170 times)

Offline johnlink

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Helmholtz vs. Schachar
« on: June 02, 2013, 05:06:06 PM »
From http://en.wikipedia.org/wiki/Accommodation_(eye)#Theories_of_mechanism I understand that Ronald Schachar has proposed a theory of accommodation contrary to that of Helmholtz. Schachar asserts that contraction of the ciliary muscle causes the lens to flatten and focus far. I’ve looked a several diagrams of the eye but have not been able to get a clear idea of what would be the result of contraction of the ciliary muscle.

I look forward to reading whatever thoughts any of you might care to share.

Offline johnlink

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Re: Helmholtz vs. Schachar
« Reply #1 on: June 02, 2013, 07:07:23 PM »

Do you think Helmholtz was correct?


I don't know, but I sure do wish there were animations that showed how the ciliary muscle and zonules act during accommodation according to the Helmholtz and Schachar theories.

If the Schachar theory is correct, I wonder how it would explain the progress people have made in reducing their myopia, and what it implies about how myopia might be reduced more effectively.
« Last Edit: June 02, 2013, 07:15:08 PM by johnlink »

Offline johnlink

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Re: Helmholtz vs. Schachar
« Reply #2 on: June 02, 2013, 07:33:04 PM »
In the original post in this thread I wrote the following:

Quote
Schachar asserts that contraction of the ciliary muscle causes the lens to flatten and focus far.

I now see that I was mistaken in my reading of the wikipedia article, for it says the following:

Quote
...when the ciliary muscle contracts, equatorial zonular tension is increased, causing the central surfaces of the crystalline lens to steepen, the central thickness of the lens to increase (anterior-posterior diameter), and the peripheral surfaces of the lens to flatten. While the tension on equatorial zonules is increased during accommodation, the anterior and posterior zonules are simultaneously relaxing.[4] As a consequence of the changes in lens shape during human in vivo accommodation, the central optical power of the lens increases...

Thus according to Schachar, "...when the ciliary muscle contracts...the central optical power of the lens increases...". So Helmholtz and Schachar agree that near focus is accomplished by contraction of the ciliary muscle, but disagree about the mechanism.
« Last Edit: June 02, 2013, 07:44:20 PM by johnlink »

Offline johnlink

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Re: Helmholtz vs. Schachar
« Reply #3 on: June 02, 2013, 07:35:27 PM »
Hi John,

Here is a short review of what researchers have found - to avoid entry into myopia, and in some cases, get out of it.  I personally "limit" what I am willing to suggest is possible, to highly motivated people who can still objectively read the 20/50 to 20/60 lines on their "home Snellen" chart.  This is a good read for "optometrist thinking".

http://myopiafree.i-see.org/PREV73.TXT

Dr. William Ludlum wrote a paper on "submariners" developing negative status for their natural eyes - based on the time they spend in the submarine.  That seemed perfectly logical for the natural eye to behave that way - so I wrote to him.  He sent me to Dr. Raphaelson who supported the concept that threshold prevention would be possible - if the plus were worn BEFORE you go below 20/60. 

(You can call this Raphaelson's theory of the natural eye's accommodating to long-term near.   What I was interested was in what he did for his own children - and was he successful with them.  I think we have "myopic minds" when we fail to take a broad perspective - but that is my "take" on these interesting issues. )


Otis, thank you for your post but it appears to have nothing to do with the topic at hand, namely the Helmholtz and Schachar theories of accommodation. Perhaps it would be appropriate to use your post to start a new thread.

Offline johnlink

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Re: Helmholtz vs. Schachar
« Reply #4 on: June 04, 2013, 05:24:03 AM »
Hi John,

Excellent suggestion!  I have done as you requested and ask that others post their commentary on your review of these two theories of the (box-camera) eye.

But I truly wonder which of these two (almost the same) theories will lead you back to  naked-eye 20/20.  You might write up your judgment on that issue - for all who read this line-of-thought.

Your friend,

Otis


Both theories assert that contraction of the ciliary muscle brings about near focus, but that the mechanism by which they do it is different. I am thinking about what different implications the two theories have for how we might most effectively improve our vision. I have a lot of reading to do in this regard.

Offline Hillyman

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Re: Helmholtz vs. Schachar
« Reply #5 on: July 18, 2013, 10:09:44 PM »
I believe--correct me if I am wrong--that the Helmholtz theory was that the ciliary muscles relaxed for near vision, allowing the lens to "plump up" (greater + power). Schachar's theory is the other way: that for near vision, the ciliary muscles contracted, pulling the edges of the lens away from the center and causing the center to be fatter. The analogy I have read is that the latter is like pulling on the edges of a mylar birthday balloon: by exerting pressure on the edges all around, the helium is forced to the center, causing the center of the balloon "lens" to thicken.

I remember Schachar had a theory about presbyopia from this description of how the lens works: he believes presbyopia happens not because the lens stiffens, but rather because the lens in the eye is one of the few body parts that keeps growing throughout one's life (the ear lobe is another example). As a result, in one's 40's and 50's, the lens becomes large enough that the suspensory ligaments (the wonderfully named Zonules of Zinn) to the ciliary muscle ring become slack, and the action on the ciliary muscle to pull on the lens edge becomes less and less effective.  Schachar experimented with a surgical procedure to place plastic inserts at the outside of the eyeball to expand the ciliary ring itself (I may not be describing it properly here), and had some initial success. But after my first reading about this--this must have been in the late 1990s--I have not come across any other reports about it.