In addition to these familiar internal senses we have some that are less well known. They are for the purpose of what may be described as the routine guidance of the muscles. The act of walking, as we well know, is made up of a series of muscular movements which are both accurately timed and accurately graded. We obtain startling realization of this when we come to the bottom step on our way down stairs without noticing that we have arrived there. This timing and grading are done for us by our bodies without our having to attend to it. The amount of labor that is saved is shown by walking upon railroad ties. These are irregularly spaced, and on that account it is necessary for us to pay attention to every step. There is no comparison between the fatigue of this kind of walking and ordinary progress along a smooth path. The senses that keep track of the position of the body and of individual muscles are known respectively as the equilibrium sense and the muscle-and-joint sense. The equilibrium sense has as its organ a part of the internal ear. Deeply imbedded in the bone is a series of chambers and canals lined with a delicate membrane and filled with liquid. The canals, which are three in number in each ear, are semicircular in shape, and accordingly have been named the _semicircular canals_. One of them is horizontal; the other two are vertical, and the two vertical canals lie at right angles to one another. This arrangement makes it inevitable that any movement of the head, in any direction whatsoever, will register differently on the canal system than any other movement, which is exactly what is required to make the apparatus efficient as an organ by which motions of the body are kept track of and guided. Along with the semicircular canals is a structure known as the _vestibule_ which registers the position of the head, and so indirectly of the body, when no movements are being made. We are not ordinarily conscious of the working of these senses; they carry on their guidance of muscular movement without our attention. We can, however, pay attention to what they show if we wish. For example, one who is swimming under water is never in doubt as to whether his head is turned up or down, even though his eyes may be shut. His knowledge of position in such a place is obtained from his equilibrium organ, even though he may not be aware of the fact. Sometimes the organ becomes diseased. The results, so far as the victim is concerned, are highly distressing. He usually has to stay in bed because he cannot balance himself well enough to get about.