Final answer:
the myocardium is the thickest layer of the heart, composed predominantly of cardiac muscle cells, which facilitates the pumping of blood by contracting rhythmically.
Step-by-step explanation:
The statement that the major portion of the heart is the myocardium, made up mainly of cardiac muscle, is true. The myocardium is indeed the thickest and middle layer of the heart's wall and consists largely of cardiac muscle cells. This layer is responsible for the heart's contraction, pumping blood throughout the body. The myocardium is supported by a framework of collagenous fibers, along with blood vessels that supply oxygen to the cardiac muscle and nerve fibers that help regulate the heart's function. The muscular pattern of the myocardium is intricate, involving muscle cells that swirl and spiral around the heart's chambers; this complex arrangement contributes to the heart's efficiency in pumping blood. The heart consists of four chambers: two upper atria and two lower ventricles, with one-way valves that direct the flow of blood in a single direction through the heart.