Final answer:
The substance that helps maintain the structure of the eye is called vitreous humor, a gel-like fluid found in the posterior cavity behind the lens.
Step-by-step explanation:
The gel-like substance that maintains the structure of the eye is the vitreous humor. The posterior cavity of the eye, which is the space behind the lens and extends to the retina, is filled with this more viscous fluid. Unlike the aqueous humor, which is a watery fluid filling the anterior chamber including the cornea, iris, ciliary body, and lens, the vitreous humor fills the larger posterior chamber and plays a key role in supporting the shape of the eyeball.