SINCE the eighteenth century it has been customary to speak with awe of the facility and speed with which the human circulatory system can adapt to changing needs. Indeed, Hales,1that indefatigable English parson and natural philosopher, used changing experimental condition in a most ingenious way in order to increase his knowledge of circulatory physiology. Although we are inclined, perhaps, to worship too much and too often at the shrines of our profession's intellectual father figures, no serious student of the circulatory system can afford to ignore Hales. We usually damn him with faint praise by according him credit for being the first to measure blood pressure, which is fair enough as far as it goes. But he actually did a great deal more than this, and it is highly pertinent to what follows to quote one passage, up to now largely overlooked, from his book,Haemastaticks. Referring