The manifold functions of the blood described in some of the other chapters (cf. Chaps. 88, 90–99) can only be fulfilled if the blood circulates continuously through the body. The main driving force for the movement of blood is provided by the mechanical activity of the heart. Additional forces, such as skeletal muscular contractions and respiratory movements, can support the pumping function of the heart to variable extents. The cardiac contraction is initiated within the heart itself by specialized cells of cardiac musculature; unlike skeletal muscle, external nerves are not essential for the initiation of the heart beat or for the sequences of events which form the cardiac cycle.