endocardium

from WordNet (r) 3.0 (2006)

endocardium
    n 1: the membrane that lines the cavities of the heart and forms
         part of the heart valves

from The Collaborative International Dictionary of English v.0.48

Endocardium \En`do*car"di*um\, n. [NL., fr. Gr. 'e`ndon within +
   kardi`a heart.] (Anat.)
   The membrane lining the cavities of the heart.
   [1913 Webster]