Final answer:
The thick middle, muscular layer of the heart is known as the myocardium, which is primarily composed of cardiac muscle and involved in heart contractions. It is the correct answer among the given choices and is vital for the heart's pumping action.
Step-by-step explanation:
The correct answer to your question is b. myocardium. The myocardium is the middle and thickest layer of the heart wall, primarily composed of cardiac muscle cells and supported by a framework of collagen. It is crucial in heart function, as it contains both cardiomyocytes, which account for approximately 99% of the cardiac muscle cells, and pacemaker cells, which make up the remaining 1% and help regulate the heart's rhythm. The myocardium is vital for the contraction of the heart, pumping blood effectively throughout the body. Blood vessels and nerves are woven throughout the myocardium, providing it with oxygen and allowing it to communicate with the rest of the body. The pericardium, including its two layers (the visceral pericardium or epicardium and the parietal pericardium), surrounds the myocardium, encasing the heart in a protective sac filled with fluid to reduce friction and allow the heart to pump efficiently.