Mestrezat1 in his monograph described the cerebrospinal fluid as a dialysate in osmotic equilibrium with the blood plasma. The most important evidence favoring this conception has been reviewed in some detail in several papers by one of us.2 The quantitative description of the equilibrium between the blood plasma and such a protein-free dialysate as the cerebrospinal fluid is important not only in furthering the understanding of the cerebrospinal fluid itself, but also because the cerebrospinal fluid represents the only protein-free dialysate in the organism present in sufficiently large quantities for adequate analysis.
Mestrezat1 called attention to the close analogy between the cerebrospinal fluid and the aqueous humor of the eye. He,3 and independently, two of us (F. F.-S. and M. E. D.4) suggested an analogy between the cerebrospinal fluid and the intercellular fluids. One of us (F. F.-S.)2 also noted that the glomerular filtrate