Vitreous humor

Vitreous humor, also called the vitreous body, is the clear jelly that fills the largest cavity in the eye: the one that runs from the lens, the eye's internal focusing element, back to the retina, the layer at the back that detects light. It is not water, it is a gel, and it is made of collagen, the same protein that gives structure to many tissues in the body.

Its first job is support. By filling that whole space it pushes the retina against the choroid, the layer of blood vessels underneath, and holds the retina firmly in place.

The vitreous differs from aqueous humor, the fluid at the front of the eye, in one important way: the aqueous is renewed over and over, while the vitreous does not turn over quickly. It also carries its own phagocytic cells, cells that eat up debris to keep the path of light clear, like the cleaning crew of a closed building.

Sometimes a piece of debris keeps floating and casts a shadow on the field of vision. That is what people see as floaters, those threads or specks that drift as the eyes move, and they are much more common in older people.