Muscle tissue, known as myocardium or cardiac muscle, wraps around a scaffolding of tough connective tissue to form the walls of the heart’s chambers. The atria, the receiving chambers of the heart, have relatively thin walls compared to the ventricles, the pumping chambers. The left ventricle has the thickest walls—nearly 1 cm (0.5 in) thick in an adult—because it must work the hardest to propel blood to the farthest reaches of the body.
What is the myocardium?
The myocardium (mi"o-KAR'de-um) is the heart's muscular wall. It contracts to pump blood out of the heart, then relaxes as the heart refills with returning blood. Its outer surface is called the epicardium (ep"ih-KAR'de-um). Its inner lining is the endocardium (en"do-KAR'de-um)