Apple #6: Eyeballs

EYEBALLS

Last night I was tired and rubbing my eyes. I thought, what the heck are eyeballs made of, anyway? Not the lens and the iris and all that, but the white stuff. So today I looked it up. Here's what I learned:


Diagram from Eye Topics

The outside of the white stuff is the sclera, which is a firm fibrous membrane that holds the innards in its spherical shape. This membrane is white and smooth on the outside, and it's thicker at the back of the eye than at the front. If you could turn the membrane inside out, you would see that it is stained brown, and grooved where nerves and vessels are attached.

My Gray's Anatomy has this disturbing observation about the sclera: "It yields gelatin on boiling."

The stuff that the sclera holds together is the vitreous humor. This forms about 4/5 of the entire eyeball. It is transparent and has the consistency of a thin jelly. Its role is to fill the concave space between the retina and the lens, and to hold everything in place.

The vitreous humor is made of:

1. water (99%)
2. a network of collagen fibrils (connective proteins)
3. large molecules of hyaluronic acid (amino acid that works like a cementing substance. It also appears in the umbilical cord)
4. peripheral cells
5. inorganic salts
6. sugar
7. ascorbic acid

You know the "floaters" you sometimes get in your eye? Sometimes they're foreign substances, but often, they're pieces of your eye. As we age, the vitreous goo gradually changes from a gel to a liquid. At the same time, the vitreous mass starts to shrink and separate from the retina. Pieces of the vitreous gel and some of the cells come loose in little clumps and start floating around in your eye. Hence, floaters.

Sources:

Gray's Anatomy: The Classic Collector's Edition
http://www.tedmontgomery.com/the_eye/vitreous.html
http://www.mercksharpdohme.com/disease/glaucoma/the_eye/anatomy/sclera.html#eye
http://www.onelook.com/