"[T]he first important conception to be grasped by the student is that of the blood pressure, and the first experiment is designed to demonstrate its existence and to estimate its amount. ...It consists in connecting the of a dog to a long glass tube. Into this tube the blood mounts up and up to a height of five or six feet and then oscillates up or down with each contraction or relaxation of the heart. Such a tube... is called... the "Hales manometer" after its inventor... Dr. Hales."