Monkey bone marrow extract when injected intracerebrally into guinea pigs or rabbits produces a distinctive encephalopathy. The Purkinje cells are severely affected, especially those at the periphery of the cerebellum. Nuclear alterations first appear, with well marked intranuclear acidophilic inclusion bodies. Similar inclusions appear at a little later stage in the ectodermal glia. The affected cells later become necrotic and usually disappear. The reaction is essentially glial and non-inflammatory, and hence is called encephalopathy rather than encephalitis.

A hyaline necrosis of cerebral blood vessels, especially at the base of the brain, is described. Small areas of softening may appear in the cerebellum as a result, but this is considered a secondary process, independent of the Purkinje cell loss. There may also inconstantly be found a loss of cerebellar granule cells and a selective necrosis of the hippocampal pyramidal cells. Secondary and reparative reactions are described.

Similar changes are produced by extracts of lymph nodes from Hodgkin's disease, and by leucocytic cream of human blood.

A tentative explanation of the pathogenesis is suggested, and similarities to certain virus diseases are pointed out.

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