Passageways and organic metaphors have natural affinities. From earliest times the human body has been thought of as a container made up of conduits for conveying blood and humors. Egyptian medicine understood the working of the body by means of the metaphor of the Nile's flooding and receding.' One branch of physiological studies in early eighteenth century Britain and on the Continent also defined the body as a structure of tubes and vessels and understood health to be the maintenance of a proper velocity of blood-flow through them.' This line of inquiry strongly impressed itself on medical amateurs and remained alive in metaphor after the main current of physiological studies was directed elsewhere.