Between the blood serum and the cerebrospinal fluid is interposed a membrane or membranes consisting histologically of vascular endothelium in apposition with ependymal cells (choroid plexuses), or in apposition with pia-arachnoid (meninges). Stern and Gautier1 have been pleased to call these membranes the hemato-encephalic barrier—for many purposes an excellent descriptive name, because in a measure it helps one to visualize some of the physiologic and clinical factors which involve these membranes. This partition acts in many ways, perhaps wholly, as a semipermeable membrane, permitting the passage of inorganic salts and other readily diffusable substances, such as dextrose and urea, into the spinal fluid, and excluding many blood serum constituents, notably colloids.
The arguments centering about the mechanism of the production of cerebrospinal fluid—whether or not it is formed by an active secretory process or is simply a dialysate—have been presented by numerous workers. The dialysate or physicochemical hypothesis,
GORDY ST, SMITH SM. THE PERMEABILITY OF THE HEMATO-ENCEPHALIC BARRIER AS DETERMINED BY THE BROMIDE METHOD. Arch NeurPsych. 1930;24(4):727-734. doi:10.1001/archneurpsyc.1930.02220160063005