A KNOWLEDGE of the volume and composition of the extracellular space in brain substance is of considerable importance in any investigation dealing with electrolyte concentrations in nerve tissue and with mechanisms of fluid shifts in cerebral edema. Although fluid partitions have been thoroughly studied in the body as a whole, and in muscle tissue in particular, there have been no well-established determinations of extracellular versus intracellular compartments in the brain.
One approach to the problem has been the use of chloride and sodium spaces. In such methods the two ions are assumed to be distributed completely in the extracellular phase; consequently, the content of chloride (or sodium) in cerebral tissue divided by its concentration in extracellular fluid results in a figure which may be called the chloride (or sodium) space. One immediate difficulty arises at this point in regard to the ionic concentration which one selects as comparable to that