Answer:
Myocardium
Step-by-step explanation:
myo means "of muscle" or "relating to muscle"
cardium denotes tissues or cells that are associated with the heart.
The myocardium is an involuntary (we don't have to actively contract it), smooth muscle. The cells that make up the muscle are called cardiomyocytes.
These cells/tissues are hugely important for the normal functioning of the heart, and disruption is associated with diseases such as ischemic heart disease.