What is the tough muscular wall of heart called?

The tough muscular wall of heart is called the myocardium. The myocardium is a thick, muscular tissue that makes up the bulk of the heart's structure and is responsible for pumping blood throughout the body. It is composed of specialised cardiac muscle cells called cardiomyocytes, which are organised in a complex arrangement that allows for coordinated contraction and relaxation. The myocardium is divided into three layers: the epicardium (the outermost layer), the myocardium (the middle layer), and the endocardium (the innermost layer).