The central nervous system (CNS) consists of neurons and glial cells. Glial cells are astrocytes, oligodendroglia, ependymal cells, and microglia. With H&E stains, the CNS resembles mesenchymal tissues in which cells are set in an extracellular matrix. This is a wrong impression. The fibrillary "matrix" of the cerebral gray matter, the neuropil, is formed by the cellular extensions (processes) of the neurons and glial cells. These processes fit together tightly, leaving a minimal extracellular space. The neuropil is traversed by blood vessels. As leptomeningeal vessels penetrate the brain, the subarachnoid space dips into CNS tissue around them, creating a perivascular (Virchow-Robin) space. This is accentuated in paraffin-embedded CNS tissue and appears as an empty space. The Virchow-Robin space extends down to the level of arterioles and venules. In brain capillaries, astrocytes are apposed to the vessel wall.