I. It is in the sense of hearing, above all other senses, that we may most advantageously observe the real nature of tremulation. All the organs and membranes connected with this sense are so prepared for the reception of this motion, and everything is so formed and arranged with cavities, bones, and turbinated, spiral passages, that the tremulation is excellently fitted to produce and communicate a sound or sensation in all that system which is included by the nerves and the bones.
Considering the anatomy of the organ of hearing, it will be observed, first, that nature has here created a channel through which the tremulatory air must flow, that is, in the curvatures of the external ear, which are able to gather in and concentrate a great deal of the tremulatory motion in order to bring about the better cooperation and cotremulation in the internal organ. Further in, this concentrated air is received by a bony canal, across which is placed a stiff membrane, known as the tympanum, or drum of the ear; the concentrated tremulations then strike against this membrane, effecting a vibration of the same kind as that which had taken place in the air outside the ear. Immediately within, or on the other side of this membrane, there is a cavity or meatus tympani, in which are placed the little bony bodies called the Malleus, the incus, and the stapes, each connected with one another by articulated joints, so that the motion and tremulation of the one must be communicated to the others. It is this malleus, or little hammer, which especially causes the tympanum to bend inward or outward, making the membrane stiff or slack, or more or less springy, as may be required by the necessary degree of the attention, or by the strength or softness of the sound.
The stapes, or stirrup, lies in a special space, by itself, together with two minute membranes expanded like a little drum, so that every vibration in the bones of the stapes is instantly communicated to this drum. For the further promotion of tremulatory sound there is also a labyrinth, or bony and turbinated passage, formed like a sea-shell, according to the nature of sound. The widest part of this passage, next to the base of the stapes, is called the vestibule, which also has a membrane of its own, of an oval figure, as its base. Up to this vestibule there run ten or twelve little foramina, through which run little threads or nervules, a part of which make the periosteum, the other part running on through the cochlea round about its walls; these nerves appropriate the sound and communicate it to the other periostea and membranes which are of the same root or contiguity. Behind the other part of the labyrinth, and opening into the vestibule, there are certain small, semicircular canals, bent about like horns or trumpets, which still more augment and concentrate the sound toward the interior part of the cochlea, after Which the tremulatlon flows on into other cavities and membranes.
From the whole anatomy of this origin we may thus know what kind of a mechanism the tremulation requires for its proper communication to the membranes, nerves, and bones in the body. The most important thing to observe is that the little membranes keep close to what is hard, and stand expanded over the air in the Concavities like little drums, whence the tremulation is communicated to other membranes; further, that the membranes weave themselves like the finest network about the bones, like musical strings over the bridge, and, like the bridges is fixed to the solid part of the instrument into which the tremulation must be carried, if there is to be any strength in the sound. The chief membrane in the mechanism of the ear is the one against which the tremulation strikes in the first instance, and injury to this membrane means injury to the whole organ of hearing. It will be observed that this membrane is joined to what is hard, and that it is derived from the periosteum of the skull; some suppose that one of its lamellae (for the tympanum itself is said to be double) is an extension of the dura mater itself, although, indeed, the periosteum of the skull has so close a communication with the dura mater that it may almost be counted as contiguous with it; we may thus see how this membrane connects itself with all that is hard round about. Within the cavity of the tympanum we find all the little bony bodies, the hammer, the anvil, etc., all of which appropriate the same tremulation and propel it further inward. The further it penetrates, the more little membranes does it meet; all of these are joined to their own bones as their periostea, which afterwards spread out into an expanse, and thus each membrane is endowed with its own solidity and hardness, as a bridge to which the tremulation may be carried beneath the stapes, and within the labyrinth, nay, everywhere within the petrous bone, there are wonderfully constructed cavities and cartilages, all surrounded with periostea and meninges, here and there extended over foramina, all of these being little harmonic skins or strings by which the sound is carried Into what is solid. The most subtle mechanism in the construction of the ear consists, therefore, in this, that all the membranes are periostea, so that these may be able to communicate their tremulation to the bones, and thus multiply and distribute the vibration.
2. the nature of tremulation as shown in the above, may be illustrated still further by many of the experiments which have been made in the art of music. It does, indeed, seem wonderful that so small a membrane as the one which closes the auditory meatus and which is expanded over the little pores and entrances to the bones and cartilages, is able to effect a tremulation throughout the entire solid system of the body, and that so small a fountain can produce so great a motion. But such is the nature which is peculiar to tremulation. What a commotion and distribution of sound is not. caused by the membranes of a battle-drum or kettle-drum, or by anything in which a membrane is expanded over a solid body! What a difference would there not be if the membranes were expanded over a soft substance ! They might be expanded to their utmost capacity, and still give but a dull sound, unless their edges were fixed to a hard or solid bottom. It is, therefore, the hardness alone, which, by its corresponding vibration, can contribute strength and distribution to the tremulation. Other vocal instruments show this still more plainly. The most rudimentary kind of a violin must have its bridge and must have its strings fixed at both ends to a solid body, in order that the corresponding tremulation in the body of the instrument may make the sound sharper, stronger, and more continuous.
Tremulation has this further peculiarity, that the least of its motions has the least regard for the mass or volume of the body in which it moves; it regards neither weight, nor hardness, nor grossness, but the contremiscences, as soon as they have originated, run like lightnings over the whole of that body which is subjected to them, while local motions, on the contrary, show respect and fear, as it were for heavy bodies. This is again illustrated in musical instruments. A string in a piano, when touched, at once sounds and plays its vibration over all that solid part with which it has a contiguity, thus permitting the tremulation to carry its reverberation to greater distances.
From this, also, we may see how small a cause is needed to produce too wide a distribution of the sound. Anything touching the body of an instrument, be it only by a single point of contact, is at once subjected to the tremulation in the instrument itself. If the instrument were in contact with a mast or any long pole, and if we were to put our ear to the other end, or touch it with our teeth, we would both feel and hear the tremulation which has been caused by a slight touch on the little string in the instrument. The tremulation is often endowed with a special sound from the nature of the place in which the instrument is; if there is a great rock below, one may at once notice that the sound is affected by the tremulation in the rock, something which deadens the usual sound in the bottom of the instrument; if there is a cupboard, box, or case in the room above the piano, the resonance must necessarily be increased hereby. All this shows that everything contiguous is set into a corresponding tremulation by the touch on the little string.
The increase of the sound is also affected by the quality of the material Which constitutes the bottom of the instrument; it is of one kind if the instrument is made of oak, another if made of spruce, cedar, etc.; the lighter and more porous the material, the heavier is the reverberation and the cotremulation. The tremulation is also subjected to great variations according to the varying thickness of the bottom of the instrument, or according to the hardness or softness of the wood, or its dryness and brittleness, or if it is close to some metallic object, or if the string is wound with horsehair or any other soft substance; all this a clear proof that the smallest membrane or string, Which is fixed to a solid substance by the two ends, is able to effect a corresponding tremulation in the most massive objects and to communicate the tremulatory motion to everything that is contiguous with it round about. If I should add Chapter VIII. to the present copy, you would see, from what I say there in respect to the distribution of tremulation, that the most minute vibration is able to permeate the greatest of bodies, even as the least contremiscence of a violin permeates the whole room where the music is performed. This is proved incontrovertibly by our own sensation, inasmuch as our hand can sensibly receive the tremulation by touching the wall of the room or the body of the instrument.
A few palpable illustrations may show still more plainly the reason why so small a cause can produce so great a flow of tremulations. In the shafts of mines one will see hanging great cables, weighted with hundreds of pounds; any small motion at the upper end, such as pulling it with the hand or striking it with a stick, will cause the whole cable to vibrate from one end to the others the tremulation flowing or undulating up and down with Swift serpentine motions such as may also be seen in the air or the water; the weight at the lower end is thereby lifted up and down, often with such a force as to cause danger to any one standing near, and yet the whole motion may have been produced by a rather insignificant cause. It may thus be seen that the tremulation or vibration has no regard for the weight below, and that the force increases mechanically according to the length or distance of the cable. Again, if a long rope is held by two persons and one of them pulls quickly at one ends even though with only a finger the other person will experience difficulty in retaining the other end. Or still better, a long rope, held horizontally, naturally hangs in something of a curve, similar to a parabola. Now in order to stretch this rope into a straight ones one needs a strong 'machine, and yet it will be impossible to stretch it so stiff that there will not always remain some invisible curve according to the length of the rope; a tremulation, passing along the ropes makes a continuous series of such Curves and as each one of these possesses its own mechanical force which counteracts the effort to stretch it into a straight one, and as these curves are swiftly passing on to the other end of the ropes we may not wonder that the impulse and the tremulation has increased in force when reaching its extremity. All this is one with the phenomenon which we observe in the little membranes of the body or the strings of the instrument, which when affixed to their solidities, give so great a force to the tremulation, that the hardest substance must partake of the tremulation, no matter what its size may be.
3. We may now be able to understand more clearly wherein the sensory in the ear consists, and how the tremulation is able to distribute its motion over the entire osseous and nervous systems from so small a beginning; all the membranes, which are intended for the distribution of the motion, are attuned and affixed to what is hard; the first, which is the membrane of the tympanum, is contiguous with the pericranium, so that the least tremulation in the tympanum effects a corresponding tremulation in the cranium and in the petrous bone; further in we find certain membranes which extend from the periostea over little cavities such as may be seen in the stapes in the fenestra ovalis and rotunda, and in the vestibule of the labyrinth, all of these being little tremulatory drums and cotremulating cartilages; still more interiorly we find wonderfully constructed rooms of cartilage and bone, all covered with little membranes which contribute their share to the tremulation of the entire systems; the use of the cavities is to carry the motion over to the other side and to effect a cotremulation in various places, so that the whole motion may spread with force over the membranes of the brain, the lymphatic canaliculi, the teguments of the nerves, and the bones. The sensory itself consists, therefore, probably in this, that the vibration presses in with force upon everything bony and membranous in the body, whence the communication of the tremulation throughout the system effects the sensation, and the sensation effects that which is perceived by us as a sound.
4. The same testimony is given by those sounds which enter the systems by ways different from those of the usual organs of hearing. First of all, it is to be observed that the sound itself does not reside in the membranes of the tympanum, a]one, but rather in the interior membranes, and especially in the dura mater and in the tremulation in the solid parts, and this without regard to the means or methods by which the motion has entered into the body. Any tearing of the dura mater, which may result from a blow on the head or from too great an exertion, is followed by a crashing sound or report as loud as if the head were between two cannons when being fired. When, therefore, the sound does not flow the usual way, nor touches the ordinary membranes, but originates further in the interior and thence sends a tremulation into the cartilages, bones, and nerves and membranes in the body--when we then seek to discover where the tearing or breakage has taken place, or where this interior sound has originated, we may find the cause far beyond the whole auditory canal; this has been established by the science of anatomy, from numerous cases. The sensory itself may therefore reside in whatever part of the cranium it pleases, provided only the tremulation is distributed over the same systems that are affected by the usual sound from without, through the external organ.
There are many other phenomena which show that the real sensory consists in the trermulation of the cranium. During a dream, for instance, we often carry on long conversations with imaginary persons, or we may hear whole melodies or other sounds which affect us exactly as those sounds which enter by the external way; when recollecting the dream during the following day, it seems altogether as if we had heard actual sounds. It is well known that the cerebellum (hjernan), while we are sleeping, is in a state of perpetual agitation; everything is in the effort to react and to restore itself to the proper order for the reception of a new motion and activity; an uninterrupted stream of tremulatlons is then flowing over all the systems, expanding everything and filling it anew with blood and fluids, mending, correcting, and restoring all that which has fallen into disorder by disharmonic tremulations during the preceding day. This, therefore, is a clear proof that sound is really an internal tremulation in the cranium and its membranes, and that it does not exist only in the little membranes of the cochlea and the labyrinth.
In fantastic imaginations, also, persons are able to hear various sounds and connected conversations, so that they sometimes persuade themselves that a spirit hem. I have spoken with them. I have spoken with a woman, who every day continually heard the singing of hymns within her, from the first to the last verses these hymns were often such as she herself had never heard or sung; she diligently sought help and cure from clergy men and others, but in vain, for the melodies and songs continued in the brain as if she were perpetually at tending a great concert. Does not this show that the sensory of hearing consists essentially in internal tremulations in the cranium, which are able to extend themselves to the ordinary organ of hearing if only there be a similar motion in the matres. A singing or ringing sound is also noticeable in the ear, when the matres or membranes are diffused and greatly distended with blood by the arteries, whence the tremulation is unduly hastened over everything contiguous.
The contrary takes place when the internal membranes become more thick or slack, as when the arteries are dry or when the blood has escaped from the most minute vessels, thereby causing a pallor on the surface, as takes place in a state of fear and by various accidents and diseases, when neither the external nor the internal membranes are able to receive the tremulation and still less to carry it round about. For a slackening of the membranes must necessarily cause a letting back or regurgitation in all the lymphatic vessels, so that when a tremulation then enters the most minute of the canals, it finds the membranes slack and wrinkled and thus obstructing the communication.
Other phenomena show still more plainly that the sound or hearing is caused by the tremulation in the cranium and its membranes; a vibration in any one of the bones of the cranium immediately produces a sound within which is also perceived as sound by the external ear, and this on account of the close communication that exists between the tympanum and the pericranium. If, again, the vibration enters immediately by means of the teeth, especially those of the lower jaw, it will similarly flow into the cranium and there produce the perception of sound; for the ramifications of the auditory nerve extend not only to the organ of hearing, but also to the periosteum which surrounds the roots of the teeth; one branch, which is soft, goes to the cochlea and makes the membrane of the restibulum, whence arise a number of other membranes in the semicircular canals, etc.; the other branch, Which is hard, runs to the tympanum, as also to the tongue and the teeth; branches from the first and the second cervical nerve run also to the mouth and to the teeth, so that the tremulation flows to and fro as on minute bridges, finally arriving at the cartilages and membranes in the petrous bone, and hence round about the whole cranium and all its systems.
5. If we closely examine wherein the tremulation of sounds differs from other tremulations, we will find that the former flows with greater swiftness and force over all the bones and membranes than do any other tremulations, just as when one bends a string firmly, drawing it into a curve for a broad tremulation, whence the sound becomes stronger than if the vibration is moving more close]y around its centre; if we strike anything that is expanded in the air, we will produce a stronget sound than if we strike what lies close to something solid. To effect a sound in the organ of hearing, there is needed, therefore, the following mechanism: a, the dura mater is expanded directly over the cerebrum and exposed for the reception of tremulation, whence any impulse on the dura mater immediately effects a tremulation; b, the membranes are in various places affixed to the cranium, the better to arrange for the concert and to give sufficient initiative to the sound; c, there are also a number of cavities or vacuities which permit the little membranes to play with their tremulation without the hindrance of anything that is soft; moreover, there are, in the cranium, an infinite number of porosities of such a kind as are required by the tremulation in any solid substance. The auditory part of our living organism consists, therefore, of stronger tremulations than are found in the other parts, as shall be shown further in what follows; the difference consisting not only in the swiftness of the motion, but also in the degree of the tremulation.