ON THE BEST MEANS OF TESTING THE HEARING.

BY

CHARLES H. BURNETT, M.D.,

AURAL SURGEON TO THE PRESBYTERIAN HOSPITAL, PHILADELPHIA.

IN the following paper I shall endeavor to give a sketch of the known relation of hearing to the watch, the tuning-fork, and speech, the generally accepted tests. After a consideration of normal hearing, it shall be my aim to show how the diseased ear is affected by these tests, and wherein they give information as to its state, and wherein they fail to do so. Afterwards, a conclusion may be better arrived at as to the best form of test or tests for the hearing. It will be necessary to review the writings, discoveries, and opinions of leading physiologists and aurists of the last two or three decades, and by this means the ground of the best labors will be fully gone over. Nothing is promised as original on my part; rather shall it be my aim to bring together, at such a time as this, the doings of others, as impartially and completely as lies in my

power.

No precise standard of normal hearing has been defined. The normal ear hears all sounds which fall upon it; but it cannot be said, à priori, where good hearing ceases and defective hearing begins, for, in many senses, hearing is a relative term. Just as there is an average standard for the amount of food, excretion, etc., there might be an average standard for hearing. The sense of hearing must be regarded as composite, consisting as it does of the ability to perceive not only a great variety of sounds, musical or periodic, but also those which are irregular or noisy. All such sounds can be heard singly or together. Hence, the sense of hearing may be said to lie in a collection of elements, largely nervous, which can be aroused singly or together. The mere fact that more than one sound can be heard at the same time, shows that the organ of hearing is composed of a number of structures arranged so that each can receive its definite impression.

THE WATCH.

Some form of watch-work, or ticking apparatus, has been used a long time as a ready means of testing the hearing. In this way the pocketwatch, mantel-clock, metronome, or an especially contrived, ticking machine, has been called into requisition. Such a test has usually been all that has been deemed necessary, and in many cases has seemed to satisfy both patient and physician as to the amount or state of hearing. Gradually, as the anatomy and physiology of the internal ear became better known, the idea arose of assigning to the cochlea the hearing of music or musical notes, and to the rest of the labyrinth, more especially to those parts in the vestibule, the perception of noises or irregular vibrations. When this theory obtained currency, the watch was classed

among noises and not musical sounds, though its beats were regular in their occurrence.

Be this theory of the physiology of the two parts of the internal ear false or true (it is not my province nor intention to review or criticize it here), Oscar Wolf has shown that the tick of a watch is a low form, if I may use the term, of musical note, with a not easily determinable pitch. Its impulse, therefore, cannot produce irregular waves of sound. All watches, metronomes, and the like, must therefore be classed under the head of instruments producing musical tones, and of course each watch, ticking apparatus, etc., has its own peculiar pitch. But at best these instruments give out but two notes, of short duration, and decidedly poor in harmonics.

This accounts for the inferiority of the watch, etc., as a test for hearing, and also explains why some affected ears appear to hear the watch better than others, and also why it is that there is so much difference in the relative manner of hearing the voice and watch, on the part of those hard of hearing.

According to Wolf, the tick of a watch should not be classed under noises, for it is really a musical sound with very high overtones, the short duration and intensity of which fall in a few seconds from their maximum to nothing. "With the ticking of a watch, we test the patient with but two tones out of the whole scale, and those neither very pure nor precise. The hammer or anchor strikes upon the cog-wheel, and by its impact calls forth two weak tones, which are somewhat intensified by the resonance of the watch-case. The pitch of these notes is not easily determinable; the intensity is measured by the observer according to the average distance at which persons of normal hearing are able to perceive the tick." This has proven very unsatisfactory as a test, because it is often found that a watch is heard relatively better or worse than the voice, etc. This is very easily explained when we reflect that with the watch we have only tested the power of the ear to hear two relatively weak and impure tones. Wolf therefore says: "Should it happen that the patient has a disturbance of the nerve-fibres corresponding to these two tones, it is conceivable that, while absolutely deaf to the ticking of the watch, he may hear other far weaker tones which hold a different position in the scale, and, on the other hand, that the patient may hear the ticking of the watch at a relatively greater distance, and at the same time misunderstand certain spoken words, and even loudly spoken tones."

Such a disturbance of fibres in the nervous apparatus of the ear most likely happens even with the diseases of the middle ear, for Politzer has shown that the tympanic vessels are connected most closely with the vessels of the cochlea; hence, tympanic disease is more than likely to bring about labyrinthine disease. Therefore, the above test is just as applicable in diseases of the tympanum as when the disease is primary in the labyrinth.

The Tick of the Watch or Clock.-Even a child will tell us, "The clock says tick, tock;" to its ears, therefore, there are in the sound made by the watch, vowel sounds of "i" and "o," and consonant sounds of “t” and "ck." Now, vowels and consonants are musical notes of certain, definite pitch, and therefore it may be said that even in this simple way a watchtick is musical. This view of the watch-tick, viz., that it is an alternation

Archives of Ophthalmology and Otology, vol. iv. p. 69. 2 Correspondence with the writer, 1876.

of two notes simply, would explain in a measure why one clock is heard better than another by an affected ear. For, even when the intensity is the same, one watch is apparently heard better than another. This is due to the fact that the pitch of one appeals more strongly than that of another, to the ear which is being tested.

The Stop-watch. Of all forms of ticking apparatus, the stop-watch is the most useful. Besides its power as a test, we also possess in it a means of finding out whether the patient really hears the sounds of the watch, or whether he thinks he does because he knows a watch is being held near his ear. This means has often been the first to declare that the patient's statements respecting his subjective state are not entirely reliable. Such occurrences, while testing with the watch, serve to show how unreliable this apparatus is, as must be all tests in the use of which the surgeon must accept the patient's statement alone, as to how much is perceived. Not only are adults of the greatest intelligence and fairness of intention sometimes utterly self-deceived by the watch-tests, but children as a rule give entirely unreliable statements respecting their ability to hear a watch. This, of course, is perfectly well known to all aurists, and I am sure that they have long ago concluded that accuracy in the use of any form of ticking apparatus can only be obtained by the employment of some form of stop-watch, since by this means they can find out whether a patient really is aware of the going or the stopping of the ticking.

Some observers have gained a similar end by alternately holding and removing a diaphragm of paper between the ear and the watch. In some instances, even while the ticking continues, the patient who has been hearing it will state that he now no longer hears it. This may be a perfectly true statement, and is explained by the fatigue of the ear. Every one must have noticed that in some forms of chronic aural catarrh the ear seems to lose its power of accommodation, or, as one may say, the power of continued listening. This has been specially noted by some writers (Weber-Liel and others), and is explained as being due to a failure in the muscular functions of the middle ear. It appears not only when testing with a watch, but also with speech. It is more likely to occur with a watch, however, because of the sameness of the test note, and the consequent continued use of the same parts of the hearing apparatus. Hence the watch may be a better test for this condition of the ear than the more variable vocal sounds, since the latter, playing up and down a series of notes, allow momentary rest of some parts of the ear, while others are being called into action.

Not only can the smallest amount of hearing be detected by means of a loudly ticking watch, but the same means may aid in prognosis in chronic diseases. The watch which I have found preferable, can be heard from 50 to 60 feet. If this watch is heard by a patient only a few inches, I feel obliged to give an unfavorable, or at best a guarded, prognosis as to the recovery of hearing. In such a case of excessive impairment of hearing, the loudness of the watch rather than the pitch of its note is to be considered, for if the ear cannot hear such intense ticking as that alluded to above, it certainly cannot retain much of its function. In such cases, a small pocket-watch would be useless, for not only can it not be heard by a very defective ear, but even an ear far from being very deaf cannot hear the faint sounds of a watch. Hence the intensity of the sounds of a ticking apparatus is of more importance than the pitch of its poor musical tone.

The Use of the Watch in Bone Conduction.-The watch is heard, per ossa, poorly by most persons over fifty years of age. Hence, it must be expected that the young will hear the watch much better by bone conduction than the old. Bearing this in mind, the ticking apparatus, of whatever form it may be, will generally give some clue to the state of bone conduction in our patients. But the usual weakness of its vibrations will interfere with its value as a test in this respect. The tuning-fork, however, is able to communicate its powerful vibrations through the bones of the head to the auditory nerve.

Conclusions respecting the Watch.-It may, therefore, be said that the watch, being a low form of musical apparatus, giving out one or at best two notes, poor in harmonics and lying near each other in the scale, is a test of only limited value. It may, however, be of great use, if its intensity is increased, in judging of the quantitative hearing in a given case. In very defective hearing, it is to the small amount of hearing still remaining what a lens is to a small object, for it enables the surgeon to find out if any hearing be present, and about how much. As a test for bone conduction its value is limited, both by the age of the patient and by the weakness of the notes. Although of some value in this respect, it falls far below the tuning-fork as a test for the sound-conducting power of the bones of the head.

THE TUNING-FORK.

Little, if anything, has been added to our knowledge of the use of the tuning-fork in aural diseases since Politzer wrote his papers on this subject in 1868. Dr. Urbantschitsch, of Vienna, has pointed out what he calls the deaf points (tauben Punkte) of the normal ear. These "deaf points" he discovered while experimenting with the small tuning-fork, passing the latter to and fro past the ear. Any one can satisfy himself on this matter by simply passing a small tuning-fork, with the line of the vibrating tines at right angles to the side of the head, backwards and forwards in the plane of the auditory canal. A point will be reached where suddenly the sound fades or disappears, to be heard again when the fork is moved either way from this point. This is purely physiological, and should be borne in mind in testing.

The tuning-fork, like the watch, may be used in two ways as a test: (1) By the air, aerial conduction: this is a test applied to the soundconducting apparatus and secondarily to the nerve; (2) By the bones of the head, bone conduction: a test applied primarily to the auditory nerve, and secondarily to the sound-conducting apparatus. For the fork thus used shows whether sound can be conducted away from the ear in a given case, or whether, being impeded in their escape from the soundconducting parts, the sound-waves are thrown back on the perceptive part of the organ.

If the note of the fork be not very loud, and if the tines of the instrument be properly clamped, the sound will not be heard further than a few inches from the ear. Thus one ear can be tested through the air, without risk of participation in the test on the part of the opposite organ. In addition to this advantage, a tuning-fork of low note and with well clamped tines, when used alongside the ear, cannot be heard through the

Wiener med. Wochenschrift.

bones of the head. For these reasons a tuning-fork is of value as an aerial test. But, while highly musical, it has the great disadvantage, like the watch, of giving out only one note at a time, which may or may not be heard relatively better than another note by the affected ear, and therefore, like the watch, the tuning-fork is of limited usefulness as a

test.

In order, therefore, to derive a wide advantage from the tuning-fork, one must either be at the inconvenience of shifting the clamps on a single fork, or else possess a large number, permanently graded in their pitch. Such a set of forks, to be of real value, must be most carefully graduated in order to correspond with the power of the ear to perceive the intervals between musical notes, otherwise certain fibres of the organ of hearing will be left out in testing. Such a set of tuning-forks for testing is not only difficult to obtain, but extremely costly. The tuningfork, therefore, though possessing many excellent qualities as a musical test, through the air, is limited in its practical application.

It may be stated, as an axiom, that the normal ear hears the tuningfork better through the air than through the bones of the head; but chiefly in the latter way has it been used as a test.

According to Politzer, E. H. Weber first settled the point that a vibrating tuning-fork, in contact with the bones of the head, was better heard in that ear the external auditory canal of which was stopped by the finger. This phenomenon was long unexplained, until Mach, on purely theoretical grounds, advanced the view that the reason of this lay in the hindrance offered by the finger in the auditory canal, to the escape of the sound-waves from the ear. Politzer, having in mind this phenomenon, thereupon made a series of experiments upon the human ear, and came to the conclusion that the above-named, augmented perception of sound, upon closing the external auditory canal, was due to (a) the reflection of sound-waves from the bones of the head through the air of the external auditory canal to the membrana tympani and auditory ossicles; and (b) to the hindrance which the sound-waves, passing from the bones of the head to the labyrinth and tympanic cavity, met in escaping from the ear. In the latter conclusion he was in entire agree ment with Mach.

If hearing a tuning-fork better on the deafer side is due to hindrance of the escape of sound-waves, even while the external auditory canal is open, stopping up the latter with the finger should increase the sound of the tuning-fork. Dr. Roosa' is of the opinion that if closing the auditory canal by the finger does not increase the hearing of the deaf side, it is a sign that the nerve is affected. The late Mr. Hinton, of London, was inclined very strongly to the view that, when for other reasons he could diagnose a nerve-affection on the deaf side, an increase of hearing for the tuning-fork by stopping the deaf ear, was confirmatory of the presence of disease of the auditory nerve. Politzer has shown (lor. cit.) that a nerve-affection may exist on the deaf side to such a degree that the delicate noise of a ticking watch is not heard better on the deaf side by bone conduction as long as the auditory canal is left open, but that by increasing the quantity of sound made to impinge on the nerve through the bones of the head, either by stopping up the auditory canal and forcing more sound from a given source, as when using a watch, or by increasing the original amount of sound employed in the test, as

Treatise on the Diseases of the Ear.

2 Questions of Aural Surgery.