The brain is not an entirely solid organ. There are chambers within the brain, referred to as ventricles or, more correctly, cerebral ventricles, to distinguish them from the ventricles of the heart. They represent the remnants of the central canalThe central nervous system develops, in the womb, from a strip of tissue, passing along the back of the embryo, and which then rolls itself into a tube before sinking below the skin surface. This primitive neural tube then develops into the central nervous system, made up of the brain and the spinal cord. In the fully developed adult (i.e. by the age of about 18) the original tubular configuration of the embryonic structure is represented by chambers in the centre of the brain, known as ventricles. In the spine, the tubular structure is all-but obliterated, as the spinal cord develops, being represented by just a narrow tube in the centre of the cord, known as the central canal. of the developing central nervous systemThe brain and spinal cord together make up the central nervous system. Nerves issue from both structures, passing out of the head and spinal canal, to supply all parts of the body, thereby allowing for voluntary movements to take place, various senses to be perceived and for the function of many internal organs to be regulated. Collectively, these nerves make up the peripheral nervous system., although they have taken on a much more complex shape than the original embryonic structure. There are four in total, one each side of the midline (referred to as the lateral ventricles) and two more, in the midline, referred to as the third and fourth ventricles, albeit often using Roman numerals.