Which layer of the heart wall is composed of cardiac muscle responsible for contraction?

• A. Epicardium
• B. Endocardium
• C. Myocardium
• D. Pericardium

Answer: (C)

The layer that contracts is the myocardium, the muscular middle layer of the heart wall. This tissue is made of cardiac muscle cells that are branched and linked by intercalated discs, allowing electrical impulses to pass quickly and the atria and ventricles to beat as a single unit. This coordinated contraction is what generates the force needed to propel blood. The outer surface, epicardium, is a protective layer; the inner lining, endocardium, lines the chambers and valves; and the pericardium surrounds the heart as a fibrous sac with lubricating fluid. The myocardium is thickest where the most powerful contractions occur, namely the ventricles.