The corpus cavernosum is a tissue that surrounds the optic nerve. It is made up of a large number of blood vessels surrounded by elastic fibers. When the blood vessels fill with blood, the corpus cavernosum enlarges and pushes the eye outward from the eyelid. When they empty, they retract it back into the eyelid. This mechanism helps focus, maintain eye temperature, and protect the optic nerve.
Scanning Electron Photograph, 1500x magnification. The outline of a large blood vessel has been textured, painted in red, surrounded by wavy elastic fibers colored green.
Next to this photograph is another, non-textured one, showing the fibers again.