epi: above, myo: middle, endo: inside, peri: outside/around
The endocardium, from endo-, meaning inner or within, and cardi(o)-, meaning heart, is the thin inner layer that lines each chamber. The myocardium, from myo-, meaning muscle, and cardi(o)-, meaning heart, is the middle, muscular layer that contracts to pump blood. The epicardium, from epi-, meaning upon or on, and cardi(o)-, meaning heart, is the outer layer.