The term "heart muscle" refers to the cardiac muscle tissue that makes up the walls of the heart. This specialized type of muscle is responsible for pumping blood throughout the body, and it contracts rhythmically and continuously without fatigue. The heart contains two types of muscles: the myocardium (the thickest layer) which forms the bulk of the heart wall, and the endocardium (the innermost layer), which lines the chambers of the heart. Both layers work together to ensure proper blood flow and maintain a healthy circulatory system.