Abstract
Evidence is presented for the existence of arborizing cytoplasmic processes extending from the surface of the cerebral cortex of human brain into the surrounding fluid medium. These originate from subpial fibrous astrocytes and contain the usual cytoplasmic organelles of those cells. They are bordered by basement membrane. Their occurrence is localized and variable over the cortical surface. They are more prevalent in pathological human material than in "normal" human brain and somewhat more prevalent in the latter than in normal rat cortex. Some additional information is presented regarding the relationship of leptomeninges to the cortical surface. The pia mater does not invariably adhere inseparably to the subjacent layer of fibrous astrocytes as generally assumed at present, nor does it always form a continuous layer over the surface of the brain in the material under study. Both collagen and cytoplasmic extensions of astrocytes intervene between these layers. These findings imply that glial elements of the cortex have direct access to the cerebrospinal fluid.