Abstract
1. Experiments were performed on six dogs to determine the rate of penetration of Cl33 and Na24 across the blood-aqueous humor and blood-cerebrospinal fluid barriers after intravenous injection of the radioactive ions. The radioactivity measurements were made with an immersion type of Geiger-Müller counter. 2. The concentrations of the labelled ions in the anterior chamber and the cisterna magna increase slowly to approach that of plasma. The rate of penetration k is calculated from a simple exponential equation with the half-value interval t 0.5 or the time required for the labelled-ion concentration in the fluid to reach 50 per cent of that of plasma. The average t 0.5 for Cl38 and Na24 in aqueous humor are 34.3 ± 9 and 27.3 ± 9 minutes, respectively, while those for cerebrospinal fluid are 90 ± 6 and 95 ± 6 minutes, respectively. 3. A study of the radioactivity in plasma was made to determine the per cent remaining after a steady state was reached. By means of this determination the sodium and chloride space was calculated to be 33 ± 5 per cent.
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