Experimental hydrocephalus of the rat was produced by cisternal injection of Kaolin-solution . Intraventricular injection of India ink disclosed obstruction of cerebrospinal fluid pathways at the tentorial incisura in the hydro cephalic rat, while the same injection to the normal rat showed diffuse spreading of the dye over whole subarach noid space of the brain. The injection of the dye into the cerebral subarachnoid space showed obstruction of the dye at the tentorial incisura in the

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... alic rat, while sufficient communication was observed between supratentorial and infratentorial subarachnoid spaces in the normal rat. There were however no remarkable differences between the normal and the abnormal rats about distribution of India ink over the cerebrum under macroscopic and microscopic observations. Obstruction at the tentorial incisura was caused by arachnoid fibrosis and granulation and not by tentorial herniation. Considering experimental findings of paucity of adhesive arachnoiditis over the cerebrum and clinical reports of reversibility of the cerebral subarachnoid space , much effort should be made to promote CSF absorption over the cerebrum, instead of external drainage.