Educational Innovations (more on them coming up in the near future) makes this heart mold to be used in constructing a gelatin heart. It includes instructions for making a hear the same size, shape, mass and color as an adult human heart.
A little disturbing, a lot cool!
PS If you like this sort of thing, they also make a brain mold...
PPS Amazon sells a similar heart mold
and brain mold
. They are less expensive, but I'm not sure how the size compares and I don't believe they include the instructions for getting the proper mass and color.