Cardiac muscle forms atria and the heart ventricles. Although this muscle tissue is the same between the cardiac chambers, a few differences exist. The myocardium that is found in the ventricles is thick to allow forceful contraction of the heart, whereas the myocardium in the atria is thinner. The individual myocytes, which make up the myocardium, also vary between the cardiac chambers. Ventricular cardiomyocytes are wider and longer, having a denser T-tubule network.