Porous Nature of the Absorptive Mechanism

Abstract

The physiologist’s approach to the estimation of the porosity of the drainage channels for cerebrospinal fluid (CSF) consists essentially in introducing a variety of substances into the ventricular or subarachnoid space and estimating the rate of loss. This latter may be determined by examining the change in concentration of the CSF after a known time or by measuring the change in concentration in the blood; the blood sampled may be that from, say, the femoral artery or, when practicable, from the torcular; in the former case the picture is complicated by renal excretion etc. whilst a less equivocal picture is undoubtedly obtained by examination of torcular blood since the drainage of CSF occurs primarily if not exclusively into the superior sagittal sinus. Examination of the CSF poses problems as to the site of withdrawal in a large animal, since there is little doubt that, as drainage proceeds, inhomogeneties in concentration will develop, so that if the material is introduced into a ventricle, the concentration in this will tend to fall whilst that in the subarachnoid spaces will rise as the flow proceeds.