"The brain contains an exact configuration of about ten billion neurons, or cellular units, each of which makes hundreds or even thousands of contacts with other neurons. Vast numbers of nerve fibers pass down from the brain through the spinal cord, where they connect with still other nerves that relay information and instructions back and forth to the remaining organs of the body. The central nervous system, comprising the brain and spinal cord in tandem, receives electrical signals from no fewer than a billion sensory elements, from the visual rods of the retina to the pressure-sensitive corpuscles of the skin."