Between the superior vena cava and the right atrium is a group of specialized cardiac muscles. It is called SAN. Its length is 10-15 mm, width is 3 mm and thickness is 1 mm. The SAN gives rise to cardiac contractile stimulation or action potentials. Hence the SA node is called primary motion generator. The SAN is called a pacemaker because it acts as a stimulus for excitatory waves and waves. It makes the heart beat 100 beats per minute. Even when the heart is separated from the body, it continues to contract. If its effectiveness decreases, fatigue and shortness of breath occur. This is called ischemia. Martin Flack (1907) discovered it.