In the walls of the heart, what is the thick layer of muscle called?
The heart walls consist of of three tunics or layers : the internal or endocardium, the middle or myocardium and the external or pericardium.
The endocardium is homologous with the intima of the blood vessels. It consists of a single layer of squamous endothelial cells resting on a thin subendothelial layer of loose connective tissue.
The myocardium is the thickest of the three tunics. It consists of cardiac muscle tissue.
The pericardium is a serous membrane in which the heart lies. The visceral layer of the pericardium corresponds to epicardium, which covers the heart externally.
This diagram shows the three layers of the walls of the heart: