Spinal cord. Light micrograph (LM) of a cross-section through the human spinal cordin the lumbar region. The spinal cord consists of a butterfly-shaped core (yellow) known as grey matter, which consists of nerve cells. Surrounding the grey matter is a larger region of white matter, made up of myelinated nerve fibre bundles or tracts. It is the white matter which is largely responsible for transmitting impulses. The spinal cord is wrapped in three layers of membrane called the meninges (outer edge). The cord extends from the stem of the brain to vertebrae of the lower back; 31 pairs of nerves arise along its length (visible in the space between the cord and the meninges), carrying information to and from the brain and body. Magnification: x20 at 10 cm wide.