The most interesting discovery of a new sense organ concerns the labyrinth of the ear. It was made quite unexpectedly. The labyrinth consists of the inner ear proper, or the cochlea, the system of three semicircular canals, and between these two organs a pair of small sacs, each containing a little stone or otolith, built of microscopic lime crystals. All these organs, being all of the nature of cavities filled with fluid and communicating, were originally regarded as serving the sense of hearing, although no one was able to say how. It was observed, however, that stimulation or lesion of the semicircular canals and of the sacs did not affect hearing, but resulted in disturbances of the coördination of the muscular activities in locomotion and normal position. For more than fifty years these observations remained unexplained; and even then their explanation was but slowly accepted. It is now recognized that the semicircular canals and the sacs are not organs of hearing, but organs informing the organism about the movements or position of the head, and indirectly of the body as a whole. The sensations coming from these organs are usually so closely bound up with kinesthetic and tactual sensations that we have not learned to become conscious of them as a separate kind. Nevertheless we may perceive them separately under favorable circumstances. If we close our eyes, turn quickly a few times on our heel, and suddenly stop, we are vividly conscious of being turned in the opposite direction. This is a perception mediated by the semicircular canals. The fluid ring in the horizontal canal gradually assumes the motion of the body, in consequence of its friction against the walls; and when the body suddenly stops moving, the fluid ring continues to move and to stimulate the sensory neurons for some time. If the body moves in a larger circle, for example on a merry-go-round or on a street car passing around a curve, the mind perceives an inclination of the body towards the convex side of the curve. If we go up in an elevator, we have the impression, just after the elevator has stopped, of moving a short distance down. These are sensations of the otolith organs. The otoliths are slightly movable, one in the horizontal, the other in the vertical direction. If the body moves through a curve, the otolith which by centrifugal force is driven outwards stimulates the sensory neurons in the same manner in which it stimulates them when the body is inclined. The perception of the body’s position is therefore the same. If the body is quickly moved up or down, the vertical otolith at first lags behind, and at the stop, through its inertia, continues to move a little in the same direction. The result is a brief perception of the body moving in the opposite direction.