Just as impulses traveling along the optic nerves can only give rise, in the sensorium, to impressions of light, so impulses passing to the sensorium via the auditory part of the portio mollis of the seventh pair of cranial nerves can only excite impressions of sound, and any stimulation of that nerve gives rise to sound sensations.
The peripheral end of the special nerve of hearing is distributed to an organ of very peculiar construction situated in the internal ear, which, from its complexity, has been called the labyrinth. The nerve endings are spread out between layers of fluid, so that they must be stimulated by very gentle forms of movement; and when we consider their delicacy, we cannot be surprised that even sound vibrations suffice to stimulate these terminals and transmit nerve impulses to the brain. The organs of hearing of mammalia are so deeply placed in the petrous part of the temporal bone, that special mechanisms have to be adopted to convey the sound with sufficient intensity from the air to the fine nerve terminals. These make up a complex piece of anatomy which will be briefly referred to presently.
Before attempting to describe the complex mechanisms by which sound is conveyed from the air to the nerve endings, some notion must be formed of what sound is from a merely physical standpoint. By means of the sense of hearing we form an idea of sound, and here the knowledge of sound ends with many people, since they only think of it as something they can hear. A physicist, however, regards sound in a different way. He knows that it is produced by the vibrations of elastic bodies, such as a tense string, a metal rod, or an elastic membrane. These vibrations, being communicated to the air, are conveyed by it to our nerve endings, where they set up a nerve impulse. The impulse is transmitted along the nerve to the brain, and there gives rise to the sensation with which we are familiar as sound.
The vibrations of the air are wave-like movements depending upon a series of changes of density in the gases, the particles of which move toward or from one another, and transmit the motion to their neighbors, so as to propagate the sound wave. To demonstrate these vibrations a special apparatus must be used.
When a tuning fork is struck it is thrown into vibration, and a sound is given forth. But the vibrations are often so rapid and so small that the motion of the tuning fork cannot be appreciated by the eye. But if a fine point be attached to one prong of the tuning fork - or, indeed, any elastic body, such as a bar of metal - and this point be brought into contact with a moving smoked surface, such as has been already described for similar records, a little wavy line is drawn, showing that the vibrating fork moves up and down at an even and regular rate. Each up and down stroke indicates a vibration. The length of the wave, as drawn on the evenly-moving surface of the recorder, shows the amount of time occupied by each vibration. This is always found to be the same for a tuning fork of a given pitch, and thus the recording fork is in constant use by the physiologist as an exact measure of small intervals of time. The pitch of the note depends upon the rate or period of vibration, a tone of a certain pitch being simply a sound caused by so many vibrations per second. The quicker the vibration the higher the note, and the slower the deeper, until, at the rate of about thirty per second, no sound is audible. Whether a note be produced by a metal fork, a tense string, or any other vibrating body, if the number of vibrations per second be the same, the note must have the same pitch.
The elevation of each vibration as seen in the tracing made by a recording fork is different at different times. When the fork is first struck, the waves are high and well marked; the excursions of the recording prong become less and less extensive as the fork gradually ceases to vibrate and the sound diminishes; or in other words, as the sound produced becomes fainter, the vibrations become smaller. The amount of excursion made by the vibrating body is spoken of as the amplitude of the vibration, and upon it depends the loudness or intensity of the sound. The pitch of a tone bears no relation to the amplitude of the waves of vibration, but depends upon their rate; while its loudness is quite independent of the period occupied by the vibrations, but is in proportion to the square of the amplitude of the waves.
So far only tones or musical notes have been mentioned. They are produced by vibrations occurring at perfectly regular periods. The simpler and more regular the vibrations, the purer the tone. The great majority of the sounds we are accustomed to hear are not pure tones, but are the result of an association of vibrations bearing some relation to one another. When the variety of vibrations is very great, their intervals irregular and out of proportion, they give rise to a discordant sound called a noise. So long as such commensurability exists in the rate of the vibrations as to produce a sound not disagreeable to the sense of hearing, it may be called a note.
By the use of a series of different resonators, each of which is capable of magnifying a certain tone, it can be shown that the clearest and purest notes of our musical instruments are far from being simple tones, but are really compounds of one prominent note or fundamental tone, modified by the addition of numerous over-tones or harmonics. If one blows forcibly across an orifice leading to a space in which a small amount of air is confined, such as the barrel of a key or the mouth of a short-necked flask or bottle, either a clear shrill or dull booming sound is heard, which varies in pitch according to the proportions of the air-containing cavity. This dull note is a simple tone. It is devoid of character, and in this respect differs greatly from the notes produced by a musical instrument. The notes of every instrument have certain characters or qualities which enable even an unpracticed ear to distinguish them.
This quality, which is independent of the pitch (i. e., rate of vibration), or the intensity (i. e., amplitude of wave), is called the color or timbre of the note. It depends on the number, variety and relative intensity of the over-tones or harmonics, which accompany the notes. So that really the timbre or quality of a note, and therefore the special characters of the different musical instruments, is produced by their impurity, or the complexity of the over-tones which aid in producing them.
All elastic bodies can vibrate, and therefore are capable of conducting sounds. Sound vibrations can be transmitted from one body to another placed in contact with it. From a hard material the waves are readily communicated to the air, and this is the ordinary medium by means of which sound is transmitted to our organs of hearing. In the old experiment of placing a small bell under the glass of an air pump, and making the tongue strike after the air has been removed, the fact that no sound is produced shows that the medium of the air is essential for the transmission of sound vibrations.
The transmission of waves of sound from the air to more dense materials, such as those which surround our auditory nerve terminals, takes place with much greater difficulty than that from a solid to the air, and we find a variety of contrivances by which the gentle air waves arriving at the ear are collected and intensified on their way to the labyrinth.
The medium of the air is not necessary in order that sound may reach the internal ear. Nor is the route through the outer canal, and the drum and its membrane, the only one by which the vibrations can arrive at the cochlea. The solid bone which surrounds the labyrinth is in direct communication with all the bones of the head, and sound can travel along these bones and reach the nerve endings. This can easily be proved by placing the handle of a vibrating tuning fork against the forehead, or better still, against the incisor teeth. The sound, although previously hardly audible, at once becomes quite distinct, or even appears loud.
This direct conduction through the bones of the head is, under normal conditions, of little use to man; but attempts have been made, in cases where the ordinary auditory passages were rendered inefficient by disease, to gather the vibrations on an elastic plate, and apply this to the teeth. This direct conduction of sound is very valuable in determining the seat of disease in cases of deafness. So long as a clear sensation of sound reaches the brain through the bones of the head, we know that the important nerve endings and their central connections ate unimpaired, and conclude that the disease lies in the mechanical conducting parts of the hearing organ.
In fishes, where the labyrinth is the only existing part of the auditory apparatus, it is embedded in the cranium, and the sound waves, arriving through the medium of water, are directly conveyed to the nerve endings by the bones of the head. An air-containing tympanum would rather impede the hearing of these animals.
The parts of the ear through which sound passes before it reaches the nerve are separated into three departments, viz., (1) the auditory canal and external ear; (2) the middle ear, tympanum or drum, which is shut off from the latter by the tympanic membrane; and (3) the labyrinth.