The Collaborative International Dictionary of English v.0.48:
Myocardium \My`o*car"di*um\, n. [NL., fr. Gr. ?, ?, muscle + ? heart.] (Anat.) The main substance of the muscular wall of the heart inclosed between the epicardium and endocardium. [1913 Webster]WordNet (r) 3.0 (2006):
myocardium n 1: the middle muscular layer of the heart wall