That portion of the nervous system composed of the brain and spinal cord. The brain is enclosed in the skull, and the spinal cord within the spinal canal of the vertebral column. The brain and spinal cord are intimately covered by membranes called meninges and bathed in an extracellular fluid called cerebrospinal fluid. Approximately 90% of the cells of the central nervous sytem are glial cells which support, both physically and metabolically, the other 10% of the cells, which are the nerve cells or neurons. Although the glial cells are much more numerous than neurons, the glial cells are also much smaller and do not directly participate in the propagation and integration of information.