Myocardium is the thick middle layer made of muscle.

• A. Inner lining of the heart
• B. Outer membrane sac around the heart
• C. Thick middle layer made of muscle
• D. The heart's chambers

Answer: (C)

The heart wall has three layers: an inner lining called the endocardium, a thick middle muscular layer called the myocardium, and an outer layer called the epicardium (with the pericardial sac surrounding the whole heart). The myocardium is thick because it is made of cardiac muscle that contracts powerfully to pump blood. Its thickness isn’t the same in all chambers—the left ventricle has the thickest muscle to generate the high pressure needed to reach the entire body, while the right ventricle and the atria have thinner muscle. This is why a statement describing the myocardium as the thick middle layer made of muscle is the accurate description. The inner lining is not muscle, the outer membrane around the heart is the pericardium/epicardium region, and the heart’s chambers are the atria and ventricles.