Final answer:
The posterior cavity of the eyeball contains the vitreous humor, a viscous fluid that supports the eye's shape and internal structures, and aids in light transmission to the retina.
Step-by-step explanation:
The posterior cavity of the eyeball is found behind the lens and extends to the posterior side of the interior of the eyeball, reaching the retina. This cavity is filled with a substance known as vitreous humor. The vitreous humor is a viscous fluid that helps maintain the shape of the eye, provides support to the internal structures, and in conjunction with the lenses, helps to transmit light to the retina.