Glia form the supportive and nutritive tissue of the vertebrate central nervous system. Glia, or neuroglia, consists mainly of cells derived from embryonic neutral tissue, some with long fibrous processes, others forming myelin by wrapping flattened processes in a spiral around nerve fibers. Glia also contain mesodermal cells (microglia) closely similar to macrophages. Glial cells outnumber neurons by between five and ten to one, and make up some 40% of the volume of the brain and spinal cord.