The far-reaching effects of motor expressions are too infrequently emphasized, but psychology forces us to give them prime consideration. We are first apprised of their importance when we study the nervous system, and find that every incoming sensory message pushes on and on until it finds a motor pathway over which it may travel and produce movement. This is inevitable. The very structure and arrangement of the neurones is such that we are obliged to make some movement in response to objects affecting our sense organs. The extent of movement may vary from the wide-spread tremors that occur when we are frightened by a thunderstorm to the merest flicker of an eye-lash. But whatever be its extent, movement invariably occurs when we are stimulated by some object. This has been demonstrated in startling ways in the psychological laboratory, where even so simple a thing as a piece of figured wall-paper has been shown to produce measurable bodily disturbances. Ordinarily we do not notice these because they are so slight, sometimes being merely twitches of deep-seated muscles or slight enlargements or contractions of arteries which are very responsive to nerve currents. But no matter how large or how small, we may be sure that movements always occur on the excitation of a sense organ. This led us to assert in an earlier chapter that the function of the nervous system is to convert incoming sensory currents into outgoing motor currents. So ingrained is this tendency toward movement that we do not need even a sensory cue to start it off; an idea will do as well. In other words, the nervous current need not start at a sense organ, but may start in the brain and still produce movement. This fact is embodied in the law of ideo-motor action (distinguished from sensory-motor action), "every idea in the mind tends to express itself in movement." This motor character of ideas is manifested in a most thorough-going way and renders our muscular system a faithful mirror of our thoughts. We have in the psychological laboratory delicate apparatus which enables us to measure many of these slight movements. For example, we fasten a recording device to the top of a person's head, so that his slightest movements will be recorded, then we ask him while standing perfectly still to think of an object at his right side. After several moments the record shows that he involuntarily leans in the direction of the object about which he is thinking. We find further illustration of this law when we examine people as they read, for they involuntarily accompany the reading with movements of speech, measurable in the muscles of the throat, the tongue and the lips. These facts, and many others, constitute good evidence for the statement that ideas seek expression in movement.