Now, the object of this arrangement is that the ear may be able to fulfil one of its principal duties, namely, the perception of sound. Sound, as the reader is doubtless aware, is transmitted through the air as waves of condensation and rarefaction, due to the swinging backwards and forwards of its particles; it resembles the passing on of a bump along a line of trucks on the railway when the engine runs up against the end one preparatory to coupling. The magnitude of this oscillation we perceive as the loudness, the frequency as the pitch of a note. Now, when the waves of sound strike against the drum of the ear, they cause it to vibrate backwards and forwards also. Supposing there was no middle ear, and the sound waves beat directly upon the membranous windows of the inner ear, these could not be made to vibrate, as there is liquid behind them, and liquids are incompressible; so, in order that the movements of the drum may be transmitted to the liquids of the inner ear, they are carried across the middle ear by a chain of small bones, by which their extent is curtailed, but their force increased, and brought to bear upon one only of the two openings. The consequence of this is that the membrane closing it is able to vibrate and pass on the vibrations to the liquid within, since when it is pushed in, the membrane covering the other hole is pushed out. [Illustration: DIAGRAM 59.—THE SEMICIRCULAR CANALS.] Exactly how the different parts of the membranous labyrinth contribute to our perception of sound we do not quite know. It appears as though the difference of pressure in saccule and utricle originally conveyed to the brain a sensation of noise without any idea of quality, while the cochlea was developed later to analyze sounds and give information as to pitch and tone. Whether the rest of the labyrinth has any longer a part to play in the perception of sound, we cannot say with certainty; but it seems pretty certain that the cochlea is the organ for receiving musical impressions. Here, again, though, we are at a loss, for we do not know with certainty how the cochlea acts. In shape it is a long tube, and in the head is coiled spirally—like a snail’s shell to look at. Along its whole length is a ridge of cells with short hairs projecting from their inner surface into the liquid it contains; and to the cells along this ridge a branch of the auditory nerve is distributed. But as to whether one of the cells along this keyboard responds to each of the notes we can distinguish, or whether they are affected as a whole, physiologists are not yet agreed. At least one other important duty the ear performs; it tells us in what position we are, and how our whole head moves or is moved. On the top of the saccule, in Diagram 57, Fig. 4, there are shown three little loops which are called the semicircular canals. They are shown again more clearly by themselves in Diagram 59.