The stimulation of the nerve of hearing by sound vibrations of the air is less difficult to understand than the excitation of the optic nerve by light waves which are conveyed by an imponderable medium. The motions of the membrane of the drum, being conveyed in the manner already indicated to the liquids within the internal ear, pass over and under the cells connected with the nerve terminals, which are placed on the elastic basilar membrane. The transverse fibres are set in motion by the waves in the fluid, and as they vibrate they communicate the motion to the organ of Corti. The bases of the inner rods, being fixed at the inner margin of the basilar membrane, can move but little, and the bases of the outer rods being placed near the middle of the fibres of the membrane, where the motion of the vibrations is most extensive, a slight change in their relative positions, and a consequent movement of the apex of the bow takes place. This movement at the apex of the bow, where the rods join, is communicated by the medium of the reticular membrane to the hairs in the special auditory cells, thence to the nerves, where an excitation is produced giving rise to the transmission of an impulse to the brain.
We can distinguish differences of (1) loudness, (2) pitch and (3) quality in sounds.
Since the loudness depends simply on the amplitude of the vibration, we have no difficulty in understanding how variations in it can be appreciated, since the more ample the vibration the more marked the motion, and, therefore, the more intense the stimulation of the nerve terminals. What we call the loudness of a sound simply means greater or less intensity of stimulation of the nerve.
The perception of difference oipitch presents greater difficulty. As already mentioned, this depends on the rate of vibrations. We know that most bodies capable of producing sound vibrations have a proper tone, i. e., that which they produce when struck. When the tone proper to a body capable of vibrating is sounded in its immediate neighborhood, it also is set vibrating through the medium of the air. If a clear tone be sung loudly over the strings of a piano a kind of sympathetic echo will be heard to come from the strings corresponding to the notes sounded. In the basilar membrane we have practically a series of strings of different length - since the membrane gets wider as it passes from below upward to the summit of the cochlea - and therefore a great variety of proper tones. With a high note a fibre of one part of the membrane will readily fall into vibration, and with a low note a fibre of another part. Different nerve fibrils are in relation to these different parts, and we may conclude that tones of different pitch stimulate distinct nerve terminals, and are conveyed to the brain by separate nerve channels. Impulses arriving at certain brain cells give rise to the idea of high tones, and impulses coming to others cause the impression of low tones. There are about a sufficient number of fibres in the basilar membrane for all the notes we can hear, viz., from about 33 to 38,000 waves in the second.
The rods of Corti cannot be the vibrating agents, because they are absent in birds which appreciate and reproduce various notes; and they are too few for the notes we hear. Further, the rods are not elastic, and not well suited for vibration. It may, therefore, be concluded that they only act as levers which convey the vibrations of the fibres of the basilar membrane to the nerve endings in the auditory cells.
The explanation of our wonderful appreciation of the delicate shades of quality of tone is still more difficult. Even persons with indifferently good ears, as musicians say, and no special musical education, can at once distinguish between the quality of the same note when sounded on a violin, a piano and a flute. When a note is sung against the strings of a piano, however pure its tone, a great number of strings are set vibrating. Not only does the string of that note vibrate, but also all those that have a certain simple numerical relation to its vibrations. In fact, all its over-tones resound. In the cochlea we suppose the same to take place with the fibres of the basilar membrane. Not only does the one fibre whose proper tone is sounded vibrate in response, but also all those which represent the varied over-tones or harmonics. It has already been pointed out that the quality of a tone depends on the relative number, force and arrangement of harmonics, which invariably accompany any musical note that possesses a definite character.
When a note arrives at the auditory nerve terminals, one of these is strongly stimulated by the wave of the fundamental tone, and many others by the different over-tones, for every prominent over-tone stimulates the cochlea. The complexity of the impression increases with the impurity of the tone, and so we appreciate the quality of a note. Thus, a compound of impulses, corresponding to a mixture of tones of varying intricacy, is transmitted to the brain cells, where it gives rise to the impression of the quality which we by experience associate with that of a violin, flute or piano, as the case may be.
With regard to the judgment of the distance of sound, it need only be remarked that it chiefly depends on former experience of the habitual quality and intensity of sound. A faint sound with the same quality that we familiarly attribute to loud sound seems to us to be far away. Thus, sounds reaching our labyrinths by the cranial bones appear distant, and ventriloquists deceive us by imitating the character of distant sounds.
The direction from which sound comes is chiefly judged by the difference of intensity with which it is heard by one or other ear. When we cannot form any idea of whence a sound comes we turn our heads one way or the other in order to present one ear more directly to the origin of the sound. When a sound is either directly behind or before us we cannot judge from which position it really comes, unless the head be slightly turned to one side or the other before the vibrations have ceased to be audible.