Like a camera has a case, the eye is surrounded by tissues and are filled with fluid called "humours" to keep it from collapsing.
Aqueous humor
The uterus keeps the trachea from collapsing in a fetal pig.
Its cytoplasm.
cytoplasm
The sphere of the eye has the lowest internal stress energy of geometric structure and even if a vitrectomy has been performed removing all the fluid the eye maintains most of it's original shape
hydrostatic
Transpulmonary pressure
Dynamic equilibrium.
gravity maybe?
cytoskeleton
The trachea has a series of cartilage rings that prevent it from collapsing.
The cytoskeleton keeps the membrane from collapsing. A cytoskeleton is a skeleton and a muscle. The cytoskeleton function in a plant and animals.
Surfactant