The concentration of the normal constituents of the cerebrospinal fluid is a reflection of their concentration in the blood, changes in the latter concentration leading to corresponding changes in the spinal fluid. On the other hand, when foreign substances are introduced into the blood stream, they either fail to reach the spinal fluid or reach it in small amounts. This apparently does not depend on diffusibility, since some substances, such as iodides, ferrocyanides and phenosulfonphthalein, cannot be detected in the spinal fluid even when large amounts are administered intravenously,1 whereas certain other substances, such as salicylates and bromides, are easily demonstrable in the spinal fluid shortly after enteric or parenteral administration.
Any substance, however, which alters the composition of the cerebrospinal fluid is rapidly eliminated when introduced into the subarachnoid space. Hence, while the passage of substances from the cerebrospinal canal into the general circulation is apparently unhindered, passage