Spray the inside of your mold with vegetable cooking spray.
Place gelatin into a medium-sized bowl. Add 2 1/2 cups of boiling water and stir with a whisk until completely dissolved, about 3 minutes.
Stir in 1 cup of cold water.
Pour in skim milk and stir for about 2 minutes.
Add a few drops of green food coloring to produce the flesh color, and stir well.
Wrap the mold in a kitchen towel and place into a bowl, sitting upright. You'll want to be sure that the mold is sitting tight in the bowl, so that it won't move around.
Add several drops of red food coloring to the bottom of the mold, to create blood vessels.
Pour Jell-O mixture into mold, leaving approximately 1 inch of space from the top of the mold.
Place in refrigerator overnight, and allow to set completely.