LOCALIZATION OF SOUNDS.
They are the most familiar things which in our science become the strangest. Not to know where you are when seasick, still less where your mind is, is common enough. Our next experiment will trace our power to know where sounds are to the same origin as seasickness.
Seasickness starts in the ear. In its cavity are three small tubes, each bent in a circle, and filled with fluid. The three sit at right angles to each other, like the three sides at the corner of a room or a box. Consequently, in whatever direction the head is moved, the fluid in some one of the tubes is given 403 a circular motion. Hanging out into the tubes, from their sides, are hairs or cilia, which connect with nerve cells and fibres that branch off from the auditory nerve. When the head moves the fluid moves, the hairs move, the cells are “fired off,” a nervous current is sent up to the brain, and a feeling of the head’s peculiar motion is consequent.
As for seasickness: this nerve current, on its way to the brain, at one point runs beside the spot or “centre” where the nerve governing the stomach has its origin. When the rocking of the head is abnormally violent and prolonged, the stimulus is so great that the current leaks over into this adjoining “centre,” and so excites the nerve running to the stomach as to cause wretchedness and retching. Deaf mutes, whose ear “canals” are affected, are never seasick.
But normally the amount of ear-feeling which we get by reason of moving our head in a particular direction comes in a curious way to be a measure of the direction of sound. The feelings we get from our skin and muscles in turning the head play a similar rôle. We turn our ear to catch a sound. We do this so frequently for every point, that in time we learn to judge the direction of the sound by the way we would have to turn the head in order to hear the sound best. Thereafter we do not have to turn the head to get the direction, for we now remember the proper feeling and know it. This memory of the old feeling is our idea of the present direction. If we never moved our heads we never could have any such notion of the location of sounds as at present—perhaps none whatever.