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Spinal cord. Coloured scanning electron micrograph (SEM) of the white matter of the spinal cord. The white matter (green/brown) consists of tracts of parallel myelinated nerve fibres. Myelin is a fatty insulating layer that sheaths some nerve fibres. Those which are myelinated are able to transmit electrical nerve signals faster than those without. The spinal cord, housed within the vertebrae of the backbone, links the brain to the rest of the body. Magnification unknown. (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE GSCHMEISSNER)