Final answer:
The myocardium is the middle and thickest layer of the heart wall, consisting of cardiac muscle cells. It contracts to pump blood, and its contractions are involuntary, controlled by electrical impulses.
Step-by-step explanation:
The myocardium is the middle and thickest layer of the heart wall, made largely of cardiac muscle cells. When cardiac muscle contracts, the heart beats and pumps blood. Cardiac muscle is arranged in bundles, so it appears striated, or striped. Contractions of cardiac muscle are involuntary, like those of smooth muscle, and are controlled by electrical impulses from specialized cardiac muscle cells in the heart muscle area called the sinoatrial node.