The eyes are mostly filled with fluids, but not the tears we are familiar with. Tears are an external fluid that bathes the cornea and sclera (white part) to keep them moist and remove dust or foreign bodies. Internal fluids are not visible when you look at a person’s eyes.
Starting at the front of the eye and working back towards the retina, we have:
Aqueous fluid moves through the posterior chamber and into the anterior chamber, moistening the lens and back of the cornea. It is 99 percent water, but slightly thick and totally clear so that light can pass through it towards the retina.
Besides allowing light through, the aqueous has other functions:
All aqueous functions are not fully understood. It may have a role in the eye’s immunity, defending it against pathogens; and it may act as an anti-oxidant, reducing the number of free radicals in the eye.
Aqueous is produced by the ciliary body, a circular structure around the lens. The ciliary body secretes it into the posterior chamber and from there it flows forward:
In this way, structures in the front third of the eye are moistened, supported, nourished, and drained of waste products. When the drainage system is blocked, fluid cannot drain out at a speed to match its production and pressure builds up inside the eye. This is called Glaucoma. The increased intraocular pressure eventually damages the optic nerve and if no treatment is done, blindness occurs.
The vitreous gel is firmer than the aqueous and although the aqueous touches against it, the two do not mix. The vitreous is produced by some of the retinal cells (not light-sensitive cells) and is 98 percent water. One percent is salts, sugars and collagen fibers, and the remaining one percent is hyaluronic acid, a substance that strongly bonds with water. There are no blood vessels.
The vitreous pushes against the retina and helps hold it in place, but is only connected to the retina in three places:
In older eyes, the vitreous can shrink slightly and pull away from the retina. Where it is connected to the retina, it can pull the top retinal layer with it. This is called retinal detachment and needs immediate medical attention if the person’s vision is to be preserved.
Vitreous is not continually replenished like aqueous humor, but is static. That means that if anything gets into it, it will remain there unless surgically removed. An object floating in the vitreous will cast a shadow on the retina that you will be able to see. This is called a floater. Floaters are the shadows of tiny clumps of collagen.
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