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RECENT THEORIES ON SOUND CONDUCTION.

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BY DR. TREITEL, OF BERLin.

Translated from the German Edition, XLI., 1902,

By Dr. W. S. BRYANT, of New York.

F late the ancient and revered theory of Helmholtz has been questioned in many quarters. The criticisms, however, were either unnoticed or silenced without careful consideration, while some authors made elaborate investigations to sustain the old theory, or to establish it in a new form. The difficulty was that certain pathological changes of the ear could not be explained by Helmholtz's theory of the function of the drum membrane and ossicular chain. Many cases have been observed in which serious interference with the mobility of the sound-conducting apparatus could be expected, but where hearing was but slightly or not at all affected. Excellent hearing may exist in cases of extreme opacity and calcification or retraction and flattening of the drum membrane, and what is still more important, good hearing occurs in some cases of total loss of the drum membrane together with the ossicles. Furthermore, the theory does not explain how bone-conduction is lengthened for low forks in interference with the ossicular chain, or in its absence. Thus it is apparent that the doubt of these authors is not wholly unfounded, and it will be worth while for us to investigate their views and reasons.

Long ago the physiological functions of the auricle were denied in man, but in animals its function was evident, for its mobility served to determine the direction of sound. Experience teaches that there is no difficulty in hearing after

the auricle has been lost through frostbite or other accident. Some authors are of the opinion that sound vibrations are caught and reflected into the ear by the auricle, or, at least, a part of the waves, while the rest go direct to the drum membrane. Schneider early demonstrated that filling the concha with wax caused a reduction of hearing. Politzer (1) observed that dulness of hearing resulted when he covered the concha with a piece of stiff paper. On the other hand, the concha alone and not the other depressions of the auricle are of importance in sound conduction. Politzer also mentions the well-known fact that in hardness of hearing, and even in normal hearing, pressing the auricle forward and increasing its surface with the hand caused an improvement in hearing. Furthermore, Politzer ascribes an importance to the tragus in reflecting the sound waves, because the sound waves reflected from the concha are prevented from escaping, and are directed into the meatus by this valve-like projection. From the tragus the sound waves pass to the depression which lies immediately inside the entrance of the canal, and from there to the anterior lower meatal wall, whence they reach the obliquely-placed drum membrane. Owing to these reflections the sound waves should lose intensity, and it appears this is the fact. Zimmermann (2) found that a smooth cylindrical tube could intensify sound, but that the auditory canal, which was irregular in all dimensions, could not. The auditory canal serves also to warm the air and to protect the organ of hearing. He further believes that sound waves suffer a loss in intensity not only in the auditory meatus, but in passing the drum membrane and ossicles before they enter the cochlea.

Before entering more into this author's theory, I propose to draw attention to several of the fundamental experiments and tests of Helmholtz and of other investigators which serve as proof for the aforementioned theory. Helmholtz (3). after study of the mechanism of the ossicles, came to the conclusion that the transmission of sound through the drum membrane and the chain of ossicles took place by progressive In particular, he found the ossicles and the attachments of the malleus and the incus very well suited for a

waves.

quick transmission of sound waves. Johannes Müller believed that the chain of ossicles vibrated more like a columella or a stiff rod, or indeed, like a great molecule. Helmholtz has pointed out that the movements of the separate ossicles, produced by air pressure or by sound, could be registered by sensitive recording specimens. Politzer (4) had previously performed the same experiments and had come to the same conclusions. The excursions of the stapes in specimens appeared to be not more than 0.1mm. Helmholtz showed that the foot-plate moved like a lever on account of its peculiar attachment in the fenestra ovalis, and at the same time like a piston. Lucae (5) observed the vibrations of the drum membrane and ossicles and their excursions during bone conduction when the base of the tuning-fork was fastened in the opened mastoid process. The labyrinthine fluid is supposed to be set in motion by the piston-like movements of the stapes, and through the difference of their amplitudes different-sized waves are produced, which set the fibres of the basilar membrane in motion. Moreover, Helmholtz attributed a sound-intensifying action to the drum membrane because of its curved fibres and its umbilicated centre. He supposed the drum membrane and the auditory meatus to act as a resonator. This was supported by an experiment in which he used a glass cylinder with an opening covered with membrane and a small rod, giving it the form of a drum membrane. The little rod was in contact with a violin string, which was fastened to a wooden bridge. On touching the string with a bow the membrane sounded loud. This extended over most of the scale, especially the high notes. Helmholtz, therefore, attributed the special function to the drum membrane and the chain of ossicles of conducting sound waves to the labyrinth.

These functions of the drum membrane and ossicles have been doubted by a number of observers in recent times. Beckmann was one of the first among these, and questioned whether the fibres of the membrane of Corti could be excited by the impact of the stapes. The impact of the stapes on the scala vestibuli produces a progressive depression of the membrane of Reissner. These depressions

act on Corti's arches before they touch the basilar membrane. This could only have a dampening effect. Beckmann (6) showed further that the drum membrane was weighted far too heavily by the ossicles, especially in pathological conditions, to conduct the most delicate waves. Schmiedekam demonstrated that the drum membrane could not respond to a tone of over four hundred and eighty vibrations; and Blake, in experiments with sound complications, found that the drum membrane does not respond and consonants were not to be recognized in the curves. All authors are agreed, including Bezold (7), that for high tones conduction through the ossicles is of little importance. The conduction through the drum membrane and the chain of ossicles is still accepted for low notes. Beckmann thinks no such duality in the mode of conduction possible, and cites as proof of his opinion, the improved bone-conduction of low tones when the drum membrane and ossicles have been lost. He hopes to prove thereby that the conducting apparatus in the case of high tones is superfluous, and that the vibration of the so-called sound-conducting apparatus does not give the impression of sound. Beckmann sees only a dampening apparatus in the drum membrane and ossicles, for the movements of the very unstable labyrinthine fluid. Its equilibrium is preserved through both the fenestral membranes; in the case of the membrane of the round window by its elasticity, and in the oval window by the adjustment of the complicated governor composed of the drum membrane and ossicles. The regulating is aided by the muscles. Beckmann does not accept the possibility of sound conduction through the round window. By fixation or absence of this regulating apparatus a sympathetic vibration of the deeper strings of the basilar membrane would be impossible, because they require the balance of labyrinthine fluid on account of their great amplitude. The better hearing through the bone is owing to the stronger impact which it gives to the labyrinthine fluid. He believes that improvement in hearing with an artificial drum membrane is due to a weighting of the stapes. We will see that this explanation is not always admissible, as it is enough if the cotton pellet lies on the

promontory, where it can exercise no impeding function, but perhaps dampening the intensity of the sound.

According to the researches of Beckmann, a further explanation is given by the dulness of hearing in patients with sclerosis. Through fixation of the stapes or of the whole chain, the dampening is lost and the undamped strings cannot perceive the separate notes; therefore, conversation with many people at the same time is especially difficult. The paracusis of Willis in this form of deafness is explained by the concussion of the body during a journey, or by the effect of loud noise on the labyrinth, which causes the sensibility of the labyrinthine fluid to be in some degree re-established. As a convincing proof of his theory, Beckmann cites the condition of the sound-conducting apparatus in many cases of acute unperforating otitis media. The drum membrane and malleus are fixed in the exudate. The stapes is not fixed but only weighted so that air-conduction is only slightly diminished while the bone-conduction is lengthened.

Besides Beckmann's there are several small papers by Zimmermann, which are collected and enlarged in a monograph on Die Mechanik des Hörens und ihre Störungen. Zimmermann's fundamental idea is the same as Beckmann's in respect to the drum membrane and chain of ossicles acting as dampers, but not for the usual sound conduction but for that of unusual intensity. He assumes direct sound conduction through the promontory wall for all tones, because bone is the best conductor. The equilibration of the labyrinthine fluid is maintained normal only through the round window, which, on account of its structure, can only be displaced outward. He considers conduction of sound waves through the window as ruled out, because it is so completely hidden in the niche of the round window. Favorable to this view are cases of fixation of the stapes in which the hearing was still present and at autopsy the membrane of the round window was found mobile. Every sound wave which enters the labyrinth from the bone goes transversely through its contents and all the fibres which lie in its way, but only sets vibrating those which are equivalent to the vibrations contained in the body of the sound. Every

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