Corneal Transparency, How?

[Asimov]

I'd rather this thread not drift into things like creation versus evolution or wave particle duality etc. I would like to see if I can post a non controversial thread on this forum! Maybe global warming next thread.

 

Sight historically gives a selective advantage.It depends on transparency of the media of the eye for light to reach the retina.The cornea, aqueous, vitreous gel are very transparent and get this in different ways.

 

The cornea is made up of water and an extracellular matrix of materials that one might think should be opaque.They are organized in such a way that transparency and refraction occur. The cornea does most of the eye's refraction.it is a complex tough structure of 5 specialized layers made up of many more specialized layers.Rather thin.It is very sensitive of course, far more than the white of the eye.

 

There are two complimentary theories regarding transparency.. Lattice theory states that fibrils less diameter than the wavelength of visible light cause destructive interference of any scattered waves except those in the direction of the incident light.Some think it is the relatively small distance between the fibrils related to the wavelength of light that provides transparency.Any insult to the cornea affects it' transparency.Edema, swelling.Pain and redness surrounding the cornea.

 

Animals achieve transparency by various mechanisms for different reasons in various tissues, another tread perhaps.

[Strange]

The cornea does most of the eye's refraction.

 

Surely that would be the lens?

[John Cuthber]

"The cornea is made up of water and an extracellular matrix of materials that one might think should be opaque."

Why?

Who would think that?

On what basis?

 

Most proteins don't absorb visible light. (OK that's a slight catch 22- the light that is easily absorbed by proteins is ipso facto not visible).

Essentially the optical components of the eye are made of stuff that works, but they are not that different from, for example, skin, that lets enough UV through to make vitamin D.

 

 

re. "Any insult to the cornea affects it' transparency. Edema, swelling.Pain and redness surrounding the cornea."

 

I can speak from personal experience here.

(1) don't damage your cornea.

there are two reasons for this: it hurts like hell- it's the only thing that ever sent me along to the local ER department and

(unsurprisingly) it doesn't reliably heal to give as good an optical instrument as it once was. I had 20:10 vision in that eye and it hasn't recovered better than 20:20.

 

(2) On the other hand, to a good approximation- certainly good enough to read this and reply- the eye is able to recover. The surface is marred and the curvature not as smooth as it was, but it is transparent.

 

BTW, Strange, no the cornea does most of the work, the lens does eh clever bit ,but it's in a medium with a relatively similar refractive index- so there's only so much it can do.

[Asimov]

I should have said less transparent rather than opaque. My post was off the top of my head. Research would show that active mechanisms maintain maximum transparency in the transparent components of the eye.The outer layers of the cornea act as pumps.I don't believe translucent skin has any such mechanisms. Doesn't need it. The slightest fluid imbalance affects the transparency of this very transparent tissue. The epithelium heals very rapidly. The location of your abrasion is also important. Off the pupillary axis may not affect your acuity after healing. Iron foreign bodies on the pupillary axis leave rust that must be drilled out, often to Bowman's membrane.I hate this. This leaves an acuity affecting scar THE PATIENT MUST BE TOLD OF THIS IN ADVANCE. People that had 20/10 vision don't consider 20/20 normal.

 

The crystalline lens gives about 1/3 of the refraction of the eye and before 40 adjusts the focus of the eye from far to near. A ring of muscles around the lens change the thickness and power of this lens. Then readers eventually are needed as the lens stiffens with time{presbyopia} unless the person is myopic enough to remove their glasses to read up close.

A good thread? Why does everything stiffen with age except what we want to? Backs, lens, knees etc.