The circulation of blood occurs under the regulation of definite physical laws. Their clear statement has proved difficult, however, owing to the facts that the vessels divide, that the elasticity and resistance vary in different parts of the circuit, and that a pulsating and not a constant flow exists in the distributing portions. Nevertheless, a number of fundamental physical laws can be derived from simple hydraulic experiments, in which the forces are kept in dynamic equilibrium. These are useful in crystallizing preliminary conceptions, and in understanding the principles utilized in designing apparatus for investigating the circulation. It must be kept in mind, however, that many are only approximations not to be applied to the circulatory system without reservations, since a state of dynamic equilibrium never exists more than momentarily (Wiggers). When estimating the velocity and circulation time, it must also be kept in mind that the blood is composed of a liquid part, the plasma, and the corpuscular elements, the red and white corpuscles and the blood platelets. For estimating the velocity in the arteries, many methods have been used, and by these the mean velocity has been calculated in the carotid artery and the aorta of various experimental animals. The velocity has been found to vary between 10 and 75 cm. per second. According to Tigerstedt, all these methods are not suitable for following, satisfactorily, the variations of the blood current. Numerous studies have been made to determine how long a period is required for the blood to make a complete circuit. The concept of circulation time is a figment of the imagination. The term has been used, however, to denote the time required for a substance injected into the heart, or in man into a brachial vein, to reach some artery or branch where it can be detected. Various substances have been thus injected: decholine, histamine, saccharin, fluorescein, radium emanation, etc., and appropriate means for their detection have been devised. The “circulation time” has been found to vary between ten and twenty-six seconds, with an average of eighteen seconds. Earlier investigations have established that the heart during its action performs an exceedingly complicated movement. By observing the position and form of the heart in the cadaver, with or without rigor mortis, and also from observations made with Ludwig, during physiological investigations on animals, Henke found the position of the atrio-ventricular ostia to be different during systole and diastole. Thus, the base of the heart seems to be nearer the apex during systole and farther away during diastole. Henke assumed, therefore, that the cardiac base makes a downward movement toward the apex during systole, and a regressive movement during diastole. That such a movement does occur during systole, with a return to the original position during diastole, has been shown by Filehne and Penzoldt, in experiments on rabbits and dogs.