The functions of the three divisions of the labyrinth have been differently stated by different physiologists. According to Duges, the vestibule concentrates the sound, measures the intensity, and consequently judges of the distance. It has been supposed that the semicircular canals either give the idea of the direction of the sonorous waves, and of the position of the body from whence they emanate, or are simply organs for increasing the sound. De Blainville thinks that the function of the cochlea is to appreciate very acute sounds; Duges makes it the musical portion of the auditory organ, the appreciator of notes, and the special apparatus for the perception of voices and articulate sounds.
Other authors have thought that the spiral plate (lamina spiralis), which narrows regularly from the base to the summit of the cochlea, corresponds to the scale of notes, from the gravest to the most acute, and that it vibrates in unison with each one of them.
According to Muller and Longet, the object of the cochlea is to furnish a solid plate upon which to spread the nervous filaments, in contact with the bony walls of the labyrinth and of the head, as well as with the fluid of the labyrinth; thus being able to transmit to these filaments the vibrations communicated to the solid or fluid portions of the auditory apparatus. And also, the spiral form of the cochlea gives in the least possible space a relatively large extent of surface for the expansion of the nervous filaments.
This diversity of opinion is easily understood, the moment we pass from natural facts to physiological speculations.
The auditory nerve is distributed to every portion of the labyrinth; but before entering it, while in the internal auditory canal, it divides into two branches, the smaller one running to the cochlea, and the larger to the vestibule and the semicircular canals. If we admit that the two branches are homogeneous, and only constitute two divisions of the auditory nerve, we must conclude that the auditory impression is perceived all over the labyrinth, just as the visual impression is felt on every portion of the retina. The division of the nerve, and the peculiar disposition of the ramifications in each of the labyrinthine cavities, seem to indicate a special function for each of these cavities. It seems natural to suppose that apparatus so different in form, and so distinct in the different parts of the organ, should have a special object, and that they combine their functions to produce the complex sensation of hearing. Muller has demonstrated that the same aerial vibrations act with much more intensity on the fluid of the labyrinth, after having traversed the chain of small bones and the fenestra ovalis, than they do after traversing the air in the cavity of the tympanum, and the membrane of the fenestra rotunda; he thinks that the waves of the same sound transmitted through the two fenestrae differ not only in intensity, but also in their timbre up to a certain point, since those reaching the fenestra rotunda are aerial vibrations, and those reaching the fenestra ovalis, by the chain of small bones, are in the state of vibrations of solid bodies. But the cochlea also receives sonorous waves of both kinds by the scala tympani and the scala vestibuli; and farther, the cavities which form the labyrinth communicate with them, all being filled with a common fluid, and all united by their walls; they would seem therefore to be bound together up to a certain point as regards auditory impressions, and nothing demonstrates that vibrations are electively directed in their movement on leaving the vestibule, either toward the cochlea or the semicircular canals.
It must be admitted notwithstanding, that authors generally agree in placing the principal, and indeed only seat of auditory impressions in the cochlea, and this is the doctrine now professed by M. Helmholtz, to whom we owe our knowledge of the origin and mechanism of the timbre of sounds. We will briefly state his theory of hearing.
We have already seen that the terminal filaments of the acoustic nerve spread themselves regularly side by side over the lamina spiralis of the cochlea, like the cords of a keyboard; the eminent professor of Heidelberg compares these nervous filaments to the strings of a piano, and explains their functions in the following manner. If the piano be opened, and a person sings loudly above the strings any note whatever, the sonorous waves cause the strings which respond to the harmonics of the voice, to vibrate also; each one of these strings vibrates exclusively in unison with one harmonic, and the note is thus decomposed by their sympathetic vibration. The same phenomenon takes place in the internal ear. The fibres of Corti decompose the sounds, each one vibrating in unison with the harmonic with which it accords, and these vibrations transmitted collectively to the brain by the acoustic nerve give the sensation of the fundamental note, and of its timbre. But here, as in every other instance, the living organ is infinitely superior to the machine constructed by man. The fibres of Corti number upwards of three thousand, and this gives four hundred sensitive cords to each octave, of which the interval or space is one sixty-sixth of a note. It is easy to understand from this how a cultivated ear can appreciate the slightest difference in sounds, as the eye perceives the least difference in the degrees of light.
This theory explains one of the most mysterious parts of the mechanism of audition, it shows us the sonorous waves exciting the Eolian harp of the acoustic nerve, just as direct observation enables us to see the luminous image painted on the retina. Just as a mirror and the camera obscura represent the eye, an instrument of music represents the ear; and we follow the sonorous and the luminous waves to the point where all is shrouded in mystery—to sensation, to comprehend which we must as little pretend, as to penetrate the mystery of life, or of our own intelligence.
But the ingenious explanation of M. Helmholtz does not at first seem to make the phenomena of hearing as accessible as those of seeing have become by means of optical instruments. The convex mirror, and the productions of photography, show us magnificent monuments and vast landscapes reproduced in microscopic proportion; we have nothing like this for the ear, and we are involuntarily led to contrast the auditory organ and its fine canals with the grandeur of sounds, and of the bodies from which they emanate. Physicists admit that the sonorous waves cross each other in the air in nearly the same manner as in a fluid, without modifying their curves, and it is thus that the distinctness of each particular sound is perceived in an accord executed by several different instruments; but, in order that this phenomenon of hearing may be displayed, the sonorous waves must move through the windings of the labyrinth with the same facility that they traverse space; the rush of meteors, and the immeasurable voices which nature has given to the atmosphere, to the ocean, and to mountains, must be transmitted to our ears in their relative proportions, as well as the sound of a falling dew-drop. How can the ear in its infinitesimal proportions perceive with equal precision the sound of the gigantic instruments which vibrate under the hand of nature, and the feeblest noise which traverses the air?
Let us remember that if we get a glimpse of the details of natural phenomena, and of those movements which constitute life, it is not in considering them as a whole, but in analyzing them as far as our limited means will permit In the vibrations of the globe of air which surrounds our planet, as in the undulations of the ether which fills the immensity of space, it is always by molecules which are intangible for us, put in motion by nature, always by the infinitely little, that she acts in exciting the organs of sense, and she has modelled these organs in a proportion which enables them to partake in the movement which she impresses upon the universe. She can paint with equal facility on a fraction of a line of space on the retina, the grandest landscape or the nervelets of a rose-leaf; the celestial vault on which Sirius is but a luminous point, or the sparkling dust of a butterfly's wing: the roar of the tempest, the roll of thunder, the echo of an avalanche, find equal place in the labyrinth whose almost imperceptible cavities seem destined to receive only the most delicate sounds.