A long, round mass of nervous matter situated in the cavity of the spinal column. The brain is a soft mass of gray and white nerve-matter, about three pounds in weight, which fills the interior of the skull. From it a cord of nerve substance, about as thick as the little finger, and some eighteen inches in length, passes downwards from the brain along the centre of the backbone. This is called the spinal cord. Long but very fine nerves extend from the brain and the spinal cord to all parts of the body.