SUBJECT HEADINGS.(a.) The character and source of the fluid.(b.) The intraventricular and subarachnoid courses of the fluid.(c.) The basic structure of the meninges.(d.) Previous views on the drainage of cerebro-spinal fluid.(e.) Classification of the methods of studying the escape of cerebro-spinal fluid.(f;) The limitations of the use of granular suspensions in the study of absorption.(g.) The objections to the use of viscous injection fluids.(h.) The introduction of emulsions.(i.) The injection oftrue solutions.(j.) Ferrocyanide solutions for injection purposes.(k.) Technic employed in this study.(Z.) Injection pressures andthe sources of error.(m.) Application of the ferrocyanide method to the study of the escape of cerebro-spinal fluid.(n.) Conclusions.(a.) The character and source of the. fluid.-The cerebro-spinal fluid is characterized by Halliburton18 as a clear, limpid fluid with low specific gravity, containing few cells and little protein, with small salt content, and possessing dextrose in definite traces. It exists in the membranes of the cerebro-spinal axis and throughout the ventricles of the brain, in amounts of from one hundred to one hundred and fifty grams, according to Testut, although the quantities, obtained in cadavera at routine autopsy, vary between sixty and eighty cubic centimeters. This decrease in quantity is apparently attributable to the rapid absorption of the fluid after death. In Mott's laboratory, by an ingenious method, the workers are now able to procure from each case the full amount of fluid, as given by Testut's calculations.