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The application of dynamic systems analysis, essential for the design of controlled missiles and of man-made automatic control systems of all kinds, to the control systems within the human organism, is beginning to bear fruit in medical science. These organic control systems, which are part of the living process, generally far surpass any man-made systems in subtlety and ingenuity. Their exploration in terms of feedback control theory is highly instructive both to the medical scientist and to the control engineer, since these systems generally display a quality and perfection of control design several orders of magnitude greater than the human mind can now conceive. Some aspects of the control of the human heart rate are presented here. Normal and irregular respiration widely changes the rate of the heart from beat to beat. The mathematical laws describing this behavior were derived from analog computer simulation. On the analog computer a mathematical model of the dynamics of the processes involved was created. This was achieved through the use of dynamic data obtained from experiments designed by the help of control theory. These mathematical laws are useful for understanding the operation of the system concerned, and especially valuable for predicting the existence of hitherto unknown structural and organizational relationships, which can be confirmed experimentally. The electronic analog computer, being fed information about the respiration of the subject, in the form of an electric signal proportional to chest circumference, is able to calculate the timing of the heart beat from beat to beat.