myocardium

from WordNet (r) 3.0 (2006)

myocardium
    n 1: the middle muscular layer of the heart wall

from 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]