Final answer:
The myocardium is the middle and thickest layer of the heart wall, primarily composed of cardiac muscle cells. These cells, supported by collagenous fibers and supplied by blood vessels and nerve fibers, are crucial for the heart's pumping action. The muscle patterns of the myocardium enable efficient circulation of blood.
Step-by-step explanation:
The middle and thickest layer of the heart wall is known as the myocardium. This layer is predominantly made up of cardiac muscle cells, which are responsible for the heart's contractile function. The myocardium is constructed upon a framework of collagenous fibers, and includes blood vessels and nerve fibers which supply and regulate the heart, respectively.
The intricate pattern of muscle cells in the myocardium, including a swirling and spiraling arrangement around the heart chambers, enables effective blood pumping. The two types of cardiac muscle cells are cardiomyocytes, which make up about 99% of the cells and are the main contractile cells, and pacemaker cells, which are responsible for conducting electrical impulses.
The heart is also covered by the epicardium, which is part of the protective pericardium, and the innermost layer is called the endocardium. The pericardium also includes a fluid-filled cavity known as the pericardial cavity, which reduces friction between the heart and surrounding tissues.