The action and absorption of subarachnoid blood. I. Experimental hydrocephalus. II. Absorption of intact red blood cells from the subarachnoid space into the blood.

(I) Experimental Hydrocephaius: Repeated injections of autogenous blood components into the cisterna magna were given to adult rabbits. Only the whole blood, if injected more than three times, could cause the ventricular dilatation. Hydrocephalus was not produced in animals which received repeated injections of washed red blood cells, plasma or hemolysed red blood cells. However, the breakdown product of red blood cells was able to induce meningeal react ion and fibrosis, and acted as a noxious agent on the CNS. It is concluded that an or· ganization of the clot in the subarachnoid space results in pia-arachnoid adhesion and blockage of the CSF circulation. (II) Absorption of Intact Red Blood Cells from the Subarachnoid Space into the Blcod: Absorption rate of intact red blood cells from the subarachnoid space into the blood was measured after the intracisternal injection of the RBC tagged with radioactive chromium. In normal rabbits, up to 16.6% of the total injected dose was recovered in the systemic blood in 72 hours, but the highest absorption was observed within 24 hours. Paradoxical enhancement of the absorption rate was observed, if the cisternal injections of whole blood had been made prior to the measurement. It is concluded that the absorption mechanism of the RBC is different in some respects from that of CSF. The concept on the valvular structure of the aracJ.noid villi, which was presented by Welch and Friedman, was taken into account and discussed. I. Experimental Hydrocephalus