The walls of the heart are made up of three layers:

• The pericardium is the outermost layer. It consists of two membranes, one of which covers the heart and one which lines the cavity in which the heart lies.
• The myocardium, the middle layer, is composed of a thick layer of specialised muscular tissue. The beating of the heart is due to contractions of this cardiac muscle.
• The endocardium is a thin smooth lining of the myocardium that is continuous with the linings of the blood vessels.

You can see the thick myocardium on the diagram.