The muscle in the heart is called the cardiac muscle. It is an extremely specialized form of muscle tissue that has evolved to pump blood throughout the body. In fact, cardiac muscle is only found in the heart and makes up the bulk of the heart's mass.
The cardiac muscle is one of three types of vertebrate muscles. It is an involuntary, muscle - i.e. it does not need some external stimulus to prompt its action. It is striated, organised into sarcomeres. Highly coordinated contractions of cardiac muscle pump blood into the vessels of the circulatory system. The cardiac muscle constitutes the main tissue of the walls of the heart.