noun, plural: cardiac muscles
(1) The heart muscle of the vertebrates
The muscles of the vertebrates can be classified into three major types: (1) skeletal muscle, (2) smooth muscle, and (3) cardiac muscle. The skeletal muscle is the muscle attached to the skeleton and is responsible for the voluntary movement of the skeletal system. The smooth muscle is the type of muscle found in the viscera as well as blood vessels of the vertebrates. The cardiac muscle is the muscle of the heart. Both cardiac muscle and the skeletal muscle have rather conspicuous striations when viewed under the microscope (in which the smooth muscle is lacking, thus the name). Nevertheless, the cardiac muscle is similar to the smooth muscle in terms of involuntary contractions. Also, the cardiac muscle cells are joined through a junctional complex that is referred to as the intercalated disc. The cardiac muscle cells are not fused together into multinucleate structures just as in skeletal muscle cells. The cardiac muscle is also referred to as the myocardium. In particular, the myocardium is the muscle tissue of the heart and is found in between the epicardium and the endocardium. Similar to other types of muscles, the cardiac muscle is formed by the process of myogenesis and originates from its primordial cell, the cardiomyoblast. The cardiomyoblast develops into myocardiocytes (or cardiomyocytes).