spinal cord, the part of the nervous system occupying the hollow interior (vertebral canal) of the series of vertebrae that form the spinal column, technically known as the vertebral column. Extending from the first lumbar vertebra to the medulla at the base of the brain, the spinal cord of a human adult is about 18 in. (45 cm) long. Structurally, the cord is a double-layered tube, roughly cylindrical in cross section. The outer layer consists of white matter, i.e., myelin-sheathed nerve fibers. These are bundled into specialized tracts that conduct impulses triggered by pressure, pain, heat, and other sensory stimuli or conduct motor impulses activating muscles and glands. The inner layer, or gray matter, is primarily composed of nerve cell bodies. Within the gray matter, running the length of the cord and extending into the brain, lies the central canal through which circulates the cerebrospinal fluid. Three protective membranes, the meninges, wrap the spinal cord and cover the brain—the pia mater is the innermost layer, the arachnoid lies in the middle, and the dura mater is the outside layer, to which the spinal nerves are attached. Connecting with the cord are 31 pairs of these spinal nerves, which feed sensory impulses into the spinal cord, which in turn relays them to the brain. Conversely, motor impulses generated in the brain are relayed by the spinal cord to the spinal nerves, which pass the impulses to muscles and glands. The spinal cord mediates the reflex responses to some sensory impulses directly, i.e., without recourse to the brain, as when a person's leg is tapped producing the knee jerk reflex. Nerve fibers in the spinal cord usually do not regenerate if injured by accident or disease.