A nervous activity which consists of the passage of a disturbance from a sense organ to a muscle is called a _reflex_. We have many examples in ourselves; if we inadvertently touch something hot, the hand is jerked away. Tickling the soles of the feet in one who is asleep will cause them to be drawn up; irritation in the throat causes us to cough, or in the nose to sneeze; the flashing of a bright light into the eye compels us to wink; all these are examples of the direct passage of nervous disturbances from sense organs to muscles. In every case the path is from the sense organ over the sensory nerve cell to some point in the central nervous system, then either by a direct nerve junction to a motor nerve cell or over one or more connecting nerve cells to a motor nerve cell and so to the appropriate muscles. These reflex actions follow the arousing of the sense organ with no more delay than is required for the passage of the disturbance over the nervous pathway. There is a delay of a small fraction of a second at every nerve junction, which makes some reflexes slower in their action than others. For example, there is a reflex known as the knee jerk; this is an outward kick which results from a sharp blow on the front of the leg just below the knee. The kick follows so closely after the blow that there cannot be more than one or at the most two nerve junctions in the pathway. The reflex of winking, on the other hand, takes several times longer, although the eye is much nearer the central nervous system than is the place on the leg which is struck in arousing the knee jerk. Since the actual length of nerve to be passed by the disturbance is much shorter in the winking than in the knee jerk, while the time for the reflex is a good deal longer, we conclude that the nerve pathway which is used in arousing winking contains a great many more nerve junctions, and therefore includes a great many more connecting cells than does the path for the knee jerk. In this particular case the reason why the pathway of winking contains so many connecting cells is that it is a brain pathway, while that for the knee jerk includes only the lower end of the spinal cord, where the arrangement of nerve cells is very much simpler. It is important for us to get clearly in mind the working of the reflexes in order to be able to understand the more complex nervous actions which will be described in the next chapter. We need to remember that the ordinary way of starting nervous disturbances is from the sense organs. With a few exceptions, to be described later, whenever a nervous action occurs anywhere in our bodies it can be traced back, although often very indirectly, to the sense organ from which the disturbance originally came. The part played in this by the brain and by what we call our mental processes will be described in the next chapter.