Digital scan of a 35mm slide. Black-and-white comparison of four ball-and-cage style artificial heart valves, labeled A, B, C, and D. A and C have opaque plastic balls, while B has a transparent plastic ball. D has a metal ball. A has a plastic cage with a white mesh cloth-covered ring. B, and C have metal cages with white mesh cloth-covered rings. D has a metal cage entirely covered in white mesh cloth.