The wall of the heart is mainly composed of three layers: the epicardium (outer layer), the myocardium (middle layer), and the endocardium (inner layer).

• The epicardium is the outermost layer, consisting of a thin layer of connective tissue and a surface of flattened epithelial cells, and it forms the visceral layer of the pericardium.
• The myocardium is the thick middle layer made up primarily of cardiac muscle cells (cardiomyocytes), which are responsible for the contractile function of the heart.
• The endocardium is the innermost layer, made of a lining of simple squamous epithelium and connective tissue, covering the heart chambers and valves.

Thus, the heart wall's main component responsible for pumping is the myocardium composed of specialized cardiac muscle cells, with the epicardium and endocardium serving protective and lining roles, respectively.