 The spinal cord is a long, thin, tubular structure made up of nervous tissue, that extends from the medulla oblongata in the brain stem to the lumbar region of the vertebral column. It encloses the central canal of the spinal cord that contains cerebrospinal fluid. The brain and spinal cord together make up the central nervous system CNS. In humans, the spinal cord begins at the occipital bone where it passes through the ferritment magnum, and meets and enters the spinal canal at the beginning of the cervical vertebrae. The spinal cord extends down to between the first and second lumbar vertebrae where it ends. The enclosing bony vertebral column protects the relatively shorter spinal cord. It is around 45 cm 18 in in man and around 43 cm 17 in long in women. Also, the spinal cord has a varying width, ranging from 13 mm 1.5 in thick in the cervical and lumbar regions to 6.4 mm 1.4 in thick in the thoracic area. The spinal cord functions primarily in the transmission of nerve signals from the motor cortex to the body, and from the afferent fibers of the sensory neurons to the sensory cortex. It is also a center for coordinating many reflexes and contains reflex arcs that can independently control reflexes. It is also the location of groups of spinal interneurons that make up the neural circuits known as central pattern generators. These circuits are responsible for controlling motor instructions for rhythmic movements such as walking.