Borderlands Borders and Global Frontiers - Frontier As Membrane

These sorts of considerations led the historian Richard W. Slatta to describe frontiers as membranes. This is a singularly appropriate metaphor for frontiers and to somewhat lesser extent for borders, borderlands, and boundaries. Membranes are differentially permeable with respect to what may pass through them and what is blocked. Their permeability often is different for opposite directions. That is, some goods are allowed to pass, say horses entering China from the central Asian steppes and silk leaving. Other things, such as armies, are not allowed to pass. Horses came into China but seldom left, unless mounted by soldiers seeking retribution for raids; silk left China but seldom came in. Membranes have thickness. When viewed from a distance they seem thin, almost like lines. When viewed up close they are zones through which objects, people, and ideas may pass.