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. Each neuron has a cell body (the perikaryon), an axon, and dendrites. The dendritic tree is the receptive part of the neuron. The axon conveys the signal to its target. The cell body contains the nucleus and most organelles that perform the synthetic and catabolic activities that keep the neuron and all its parts alive and functioning. Neurons come in all sizes. Motor neurons, which are the largest cells in the CNS, have a cell body measuring up to 135 microns. Granular neurons of the cerebellum, which are the smallest, measure 4 microns