CoachThe nervous system’s working cells are the neurons (Remark 45.6 listed them): each has a cell body and a long thin fiber — some fibers reach from spine to toe, the body’s longest cells. A nerve is a cable bundling thousands of such fibers. Along each fiber travel nerve messages: brief electrical signals, racing at up to (Definition 33.1’s figure, now with its carrier).
Examples
Example 60.3 (Three lines, one language)
The loop’s cast (Proposition 33.2), rewired: sensory neurons carry the reports — skin to spinal cord, eye to brain; motor neurons carry the orders — cord to muscle, ending where the fiber meets the muscle and its drumroll sets the contraction’s strength (Definition 17.6); and between them, in brain and cord, uncounted connecting neurons do the deciding. Reflex or symphony, the traffic is the same signals at different rates on different lines.
Example 60.10 (The method on three cases)
The blink at a puff of air: milliseconds, one muscle ring, wired path, gone at once — nervous. The growth burst: years, whole-body, bloodstream, running for seasons — hormonal. The racing heart of the sprint (Proposition 49.4): both in relay — nerve signals set the pace up within a beat, and a bloodstream messenger (the famous surge you feel as a fright’s hot wave) holds the whole body’s alert for minutes. Telegraph first, post to follow.