COURSE OF THE SONOROUS WAVES IN THE EAR. 199

of the labyrinth turned towards the tympanum, and containing the cavities of communication above described. The semicircular canals are three bony tubes, situate above and behind the vestibule, into which they open by five apertures; each tube being bent so as to form the greater part of a circle. The cochlea is a blunt cone, having its surface marked by a spiral groove, which gives to this part of the labyrinth somewhat of the appearance of a spiral shell-whence its name.' Its interior is a spiral canal divided into two by a thin partition, deficient at the apex of the cochlea. The canal opens freely into the cavity of the vestibule.

'Within the osseous labyrinth, and separated from its lining membrane by a liquid secretion, is a membranous structure, which serves to support the ultimate ramifications of the auditory nerve. In the vestibule and semicircular canals this membrane has the form of a rather complex sac, and encloses a fluid called the endolymph; in the cochlea the analogous structure merely completes the lamina spiralis (the partition of the cochlea), and is covered by the membrane which lines the general cavity of the osseous labyrinth.'

The labyrinth is thus to be considered as a complicated chamber full of liquid, and containing also a membranous expansion for the distribution of the nerve of hearing. Let us next advert to the action of these different parts in producing the sensations of sound.

3. The waves of sound enter the passage of the outer ear, and strike the membrane of the tympanum. The structure of the outer ear is adapted to collect and concentrate the vibrations like an ear trumpet. The form of the shell gives it a reflecting surface for directing the sound inwards; while the passage is believed to increase their intensity by resonance. Reaching the membrane of the tympanum, the beats communicate themselves to its surface and set it a vibrating, which is done all the more easily that the membrane is very thin and light in its structure. Experiments have shown that the only means of receiving with effect the vibrations of the air is to provide a thin stretched membrane such as this. The vibrations of the membrane are communicated to the chain of

bones traversing the middle ear, and connecting through the oval foramen with the enclosed liquid of the inner ear. By this means a series of beats are imparted to this liquid, which diffuse themselves in waves all through the passages of the labyrinth, and act by compressing the membranous labyrinth, and through it the imbedded fibres of the auditory nerve, which compressions are the immediate antecedent of the sensation of hearing. The character of the sensation will of course vary with the character of the waves, according as they are violent or feeble, quick or slow, simple or complex, and so forth.

There is little difference of opinion as to the general course of the action now described. The transitions have all been imitated by experiments, and it has been found that the arrangement is a good one for bringing about the ultimate effect, namely, the gentle compression of the filaments of the nerve of hearing. No other medium could serve the final contact so well as a liquid, but in order to impress the liquid itself, an intermediate apparatus between it and the air is requisite. This intermediate apparatus is solid, and composed of two parts, the first slender and expanded, so as to be susceptible to the beats of the air, the second dense and contracted (the chain of bones), to produce a sufficiently powerful undulation in the liquid. The membrane once affected is able to communicate vibrations to the bones, and the end of the stapes is able to impress the labyrinthine fluid. So far the process has been rendered sufficiently intelligible.

Sonorous vibrations are also communicated in a feeble way from the tympanic membrane to the inner ear through the air in the tympanic cavity. The membrane of the fenestra rotunda is acted on by these aerial pulses, while the membrane of the fenestra ovalis is impinged by the stirrup bone.

When, however, we come to inquire minutely into the uses of the different parts, and the meanings of the different complications of form and arrangement, the answer is not in all cases to be had. The inner ear especially, with its labyrinthine windings, is a subject of great perplexity. It can

ACTION OF THE MUSCLES OF THE EAR.

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easily be understood that those windings, like the cavities of the nose, give a greater expansion to the membrane supporting the auditory nerve, and thus increase the effect of the vibrations; there may also be a multiplication of effect by resonance in the canals. Whether any other more special purpose is served by them has not as yet been made out.

What would be most interesting from the point of view of this treatise, would be to ascertain the precise uses of the muscles of the ear. But this subject is unfortunately very obscure. There are, I am informed, three muscles whose existence cannot be questioned: the tensor tympani, whose purpose undoubtedly is, as the name signifies, to tighten the membrane of the tympanum; the stapedius inserted into the stirrup bone, and considered by Mr. Toynbee to be antagonistic to the other; and the laxator tympani minor, inserted into the handle of the malleus, whose action cannot be guessed with any probability.

It has not been well ascertained on what occasions and with what effect the tensor tympani is brought into play. The only distinct observation on the matter is that made by Dr. Wollaston, namely, that when the membrane of the tympanum is stretched, the ear is rendered less sensible to grave sounds, such as the deep notes of the organ, or the sounds of thunder and cannon. When therefore the ear is exposed to very intense sounds of the deep kind, such as the firing of artillery, the tensor tympani coming into play would in some measure deaden the effect. The action would make little or no difference to the hearing of acute sounds, such as the sharp notes of a call whistle. Probably these muscles are excited by the reflex action of the sounds; possibly, also, they may be of the voluntary class, that is, they may come into play in the voluntary acts of listening and of preparing the ear to resist loud sounds. The only circumstance I can assign as determining the reflex action of the tensor tympani is simply the intensity of the sound. We may suppose that every sound whatever brings on a reflex action to stretch the membrane, and the stronger the sound the greater the action. When sounds

are too loud, and of the grave kind, this tension mitigates them; when too loud and acute, it either has no effect, or makes the evil worse.

Dr. Wollaston performed many experiments upon the effects of tension of the membrana tympani, and he found that deafness to grave notes was always induced, which, as most ordinary sounds are of a low pitch, is tantamount to a general deafness. Shrill sounds, however, are best heard when the tympanic membrane is tense. Müller remarks, and we have frequently made the same observation, that the dull rumbling sound of carriages passing over a bridge, or of the firing of cannon, or of the beating of drums at a distance, ceases to be heard immediately on the membrana tympani becoming tense; while the treading of horses upon stone pavement, the more shrill creaking of carriages and the rattling of paper, may be distinctly heard.'-TODD and BOWMAN, vol. ii. p. 95.

4 Passing now to Sounds considered as Sensations, I might distinguish these into three classes; the first would comprise the general effects of sound as determined by Quality, Intensity, and Volume or Quantity, to which all ears are sensitive. The second class would include Musical sounds, for which a susceptibility to Pitch is requisite. Lastly, there is the sensibility to the Direction and Articulateness of sounds; on these properties depend much of the intellectual uses of the sense of hearing.

5. Quality. (Emotional). This regards sounds as in themselves agreeable or disagreeable, apart from the intensity or quantity of the sonorous influence. Like sweet and bitter in Taste, there is a qualitative distinction of sounds into such as intrinsically possess the power of gratifying the sense of hearing, and those that give in an unmingled form the peculiar pain that we are capable of deriving through this organ. The terms, sweet, rich, mellow, are applied to the pleasing effects of pure sound. Instruments and voices are distinguished by the sweetness of their individual tones; there is something in the material and mechanism of an instrument that gives a sweet and rich effect, apart altogether from the music of the airs

performed upon it. Other instruments and sounds have a grating, harsh, unpleasant tone, like bitterness in taste, or a stink in the nostrils. We cannot explain the cause of this difference in the material of sounding bodies; we only know that some substances, by their texture, have a greater sweetness of note than others. Thus silver is distinguished among the metals; and glass is also remarkable for pure rich tones. The hard woods are usually better than the soft for the construction of instruments. It is hopeless to attempt to explain those astonishing differences that occur between one make of instrument and another, or to probe the mysterious structure of a Cremona violin.

This sensation of the sweet in sound I have characterised as the simple, pure, and proper pleasure of hearing; a pleasure of great acuteness but of little massiveness. The acuteness of it is proportioned to the rank of the ear as a sensitive organ, or to the susceptibility of the mind to be stirred and moved through the channel of hearing. Now in the generality of mankind the ear is extremely sensitive; perhaps in none of the senses are we more keenly alive to pleasure and pain than in this, although we do not obtain from it that bulky enjoyment that comes through the organic feelings.

There is however a great superiority in the endurableness of sweet sounds over the sweets of the inferior senses. In touch this distinction exists in the comparison with Taste and Smell; in hearing there is a farther progress, and we shall have to note the crowning pitch of this important property when we come to the sense of sight. By virtue of this fact we can obtain from sight and hearing a larger amount of enjoyment within the same degree of fatigue or exhaustion, or before reaching the point of satiety. Hence one reason for terming these the 'higher senses.'

I may remark that great sweetness of tone is not a usual property of the sounds about us, nor are we often exposed to very harsh or repulsive effects on the ear. The majority of actual sounds are indifferent in their emotional influence.

Sweetness of sound, as of taste and smell, is a pure and