Which layer of the heart is primarily composed of cardiac muscle?

The layer of the heart that is primarily composed of cardiac muscle is the myocardium. This thick middle layer is responsible for the contractile function of the heart, allowing it to pump blood effectively throughout the body. The myocardium's muscle fibers contract in a coordinated manner, which is essential for maintaining a consistent and powerful heartbeat.

The epicardium, which is the outer layer, serves as a protective layer and is not primarily muscular, while the endocardium lines the interior chambers of the heart and is made up of a thin layer of cells that provides a smooth surface for blood flow. The pericardium is a fibrous sac that encases the heart, providing additional protection and stability, but does not contribute to the muscle tissue of the heart itself.