Each step forward in science represents a simplification. Complex and hitherto apparently unrelated phenomena begin to fall into place after the development of an idea or hypothesis. The nearer a causal chain approaches to the final event, the more influential do individual factors seem to be until, when we reach the terminal stage, the causal agent seems to be in unique relationship with the ultimate effect. G. W. Pickering "High Blood Pressure" Ever since Johnson (1868) advanced the conception that hypertension results from the contraction of the arterioles throughout the body there has been ceaseless clinical and experimental investigation aimed at the detection of the factors responsible for arteriolar constriction in hypertension. Although Johnson's teleological explanation is untenable nowadays, the conception that hypertension is produced by functional vasoconstriction has steadily gained ground and is now universally accepted. Two main factors have been considered in the mechanism of arteriolar constriction for