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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. 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 aircontaining 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 are 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 aircontaining 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.
CONDUCTION OF SOUND VIBRATIONS THROUGH THE
EXTERNAL EAR. External Ear.-In man, the muscles are so poorly developed that he can hardly move the external ear or pinna perceptibly, and the part commonly called the ear is of little use. We know this, because the outer ear may be quite removed without materially affecting the power of hearing. The sound reflected from the pinna may be excluded, without reducing the intensity of that heard, by placing a little tube in the auditory canal. Birds hear well without any outer ear.
But the movable ears of many animals are, no doubt, useful in helping them to ascertain the direction of a sound by catching more of the vibrations coming toward their pinna. That the external ear may be of some use, even to man, one is led to believe by the natural readiness with which a person with dull hearing supplements it by means of his hand. In this act the ear is pushed away from the head to an angle of about forty-five degrees, and its projection is considerably increased.
External Auditory Meatus.—The auditory canal is a crooked and irregular passage, getting rather wider as it approaches the tympanic cavity. It is the seat of some short, stiff hairs, which help to prevent the entrance of foreign matters. It is supplied with a peculiar modification of sweat glands, which secrete a waxy material that helps to keep the walls of the canal and the outside of the membrane moist and soft. The elastic column of air in any circumscribed space
resounds more readily to some one tone, varying according to the capacity of the space; thus resonators of different pitch are formed. Different tubes have different notes when blown into, so the auditory canal has a note of its own, and if the canal be short, the note is one of a very high pitch. When a tone of the same pitch as that to which the canal is tuned strikes the ear, it is unpleasantly magnified, and such sounds are called shrill and disagreeable. Upon the more ordinary sound vibrations, however, the auditory canal has little or no effect.
CONDUCTION OF SOUND VIBRATIONS THROUGH THE
TYMPANUM. The end of the auditory canal is closed by the membrana tympani, which slopes obliquely from above downward' and inward, in which direction its size is greater than if it were straight across the canal.
This membrane is not flat, for the central point is drawn in by the handle of the malleus, which is firmly attached to it. The membrane is thus held in the shape of a very blunt cone, somewhat like a Japanese umbrella, the apex of which points inward toward the cavity of the drum. The peculiar form of the membrane of the drum is of great importance for distinct hearing.
As every confined volume of air has a certain proper tone to which it resonates readily, so a membrane of a given size and tension has a proper tone (self-tone), the vibration period of which it follows naturally. This tone varies with the tension, as may be seen in a common drum, the note of which can be changed with the tension of its parchment; the tenser the membrane, the higher the pitch. If the membrane of the drum of our ears were set to one tone, our hearing would be imperfect and unpleasant, for we should be wearied by the reiteration and