This Sushumna is in ordinary persons closed up at the lower extremity; no action comes through it. The Yogi proposes a practice by which it can be opened, and the nerve currents made to travel through. When a sensation is carried to a centre, the centre reacts. This reaction, in the case of automatic centres, is followed by motion; in the case of conscious centres it is followed first by perception, and secondly by motion. All perception is the reaction to action from outside. How, then, do perceptions in dreams arise? There is then no action from outside. The sensory motions, therefore, are coiled up somewhere. For instance, I see a city; the perception of that city is from the reaction to the sensations brought from outside objects comprising that city. That is to say, a certain motion in the brain molecules has been set up by the motion in the incarrying nerves, which again are set in motion by external objects in the city. Now, even after a long time I can remember the city. This memory is exactly the same phenomenon, only it is in a milder form. But whence is the action that sets up even the milder form of similar vibrations in the brain? Not certainly from the primary sensations. Therefore it must be that the sensations are coiled up somewhere, and they, by their acting, bring out the mild reaction which we call dream perception.