The spinal cord extends downwards from the brain stem through the protecting bony rings called vertebrae down the centre of the backbone, to the bottom of the back. Its core is H-shaped in cross section and is composed of several kinds of neurons. The sumding mattez is mostly long cables of axon fibres. The cord, too, is cwered like the brain with thne membranes and contains fluid between the membranes. 'there are 31 pairs of spinal nerves, that branch off from the spinal cord throughout its length, between the bones of the spine. These nerves carry sensory signals upto the brain and motor signals down from the brain to various parts of the body.