The central nervous system is made of two types of tissue: white matter and grey matter. Grey matter contains nerve cell bodies, and white matter is made of the nerve fibres.
When a sample is freshly cut, it usually slightly pink in appearance (for both types). When preserved in formaldehyde, white matter appears to be almost white. White matter consists mostly of myelinated axons:. Myelin is a lipid tissue (a fat) with capillaries. It helps to keep the electrical in the nerve fibre. This is important for fast and accurate sending of signals. In the brain white matter is surrounded by grey matter most of the time. In the spine, white matter is outside, and grey matter inside: the myelin sourrounds the nerve cells, and helps the transmission of nerve signals.
White matter is used to connect different areas of grey matter. If a section of white matter is damaged, the brain may be able to find a different route to replace the lost connection. The brain contains a third kind of tissue, which appears darker. It is called substantia nigra, a part of the brain that is involved in pleasure & reward, addiction and movement.