HEARING. 



575 



different individuals a similar variation in the 

 limits of sensibility to sounds, that is, we find, 

 in the words of Dr. Wollaston, that " an ear 

 which would be considered as perfect with 

 regard to the generality of sounds, may at the 

 same time be completely insensible to such as 

 are at one or the other extremity of the scale 

 of musical notes, the hearing or not hearing 

 of which seems to depend wholly on the pitch 

 or frequency of vibration constituting the note, 

 and not upon the intensity or loudness of the 

 noise."* And we owe to this distinguished 

 man the knowledge of the interesting fact, 

 that an insensibility of the ear to low sounds 

 may be artificially induced, by exhausting the 

 cavity of the tympanum to a great degree. 

 This may be effected by forcibly attempting to 

 take breath by expansion of the chest, the 

 mouth and nose being kept shut ; after one 

 or two attempts, the pressure of the external 

 air is strongly felt upon the membrana tympani, 

 which is thus from the external pressure thrown 

 into a state of considerable tension. An ear 

 in this state becomes insensible to grave tones, 

 without losing in any degree the perception of 

 more acute ones. This induced defective state 

 of the ear, from exhaustion of the tympanum, 

 may even be preserved for some time without 

 the continued effort of inspiration and without 

 even stopping the breath, and may in an in- 

 stant be removed by the act of swallowing. 

 In repeating this experiment as I sit writing at 

 my desk, I perceive that a great degree of 

 stillness ensues immediately the sensation of 

 pressure upon the tympanic membrane is felt, 

 owing no doubt to the low rumbling noise of 

 the waggons and carriages in the street being 

 imperfectly audible. A similar observation 

 was made by Dr. Wollaston : " If I strike the 

 table before me with the end of my finger," 

 he says, " the whole board sounds with a deep 

 dull note. If I strike it with my nail, there 

 is also at the same time a sharp sound pro- 

 duced by quicker vibrations of parts around 

 the point of contact. When the ear is ex- 

 hausted, it hears only the latter sound, without 

 perceiving in any degree the deeper note of the 

 whole table. In the same manner, in listening 

 to the sound of a carriage the deeper rumbling 

 noise of the body is no longer heard by an 

 exhausted ear; but the rattle of a chain or 

 screw remains at least as audible as before 

 exhaustion." Dr. Wollaston refers to the cu- 

 rious effect produced by trying this experiment 

 at a concert : " none of the sharper sounds are 

 lost, but by the suppression of a great mass of 

 louder sounds the shriller ones are so much 

 the more distinctly perceived, even to the rat- 

 tling of the keys of a bad instrument, or 

 scraping of catgut unskilfully touched." Ano- 

 ther very interesting circumstance connected 

 with this subject is the production of the same 

 condition of the tympanum by the sudden in- 

 crease of external pressure as well as by the 

 decrease of that within, as occurs in the diving- 

 bell as soon as it touches the water, the pres- 



* Wollaston on Sounds inaudible by certain 

 Ears. Phil. Trans. 1820. 



sure of which, according to Wollaston, upon 

 the included air closes the Eustachian tube, and 

 in proportion to the descent occasions a degree 

 of tension on the tympanum, that becomes 

 distressing to persons who have not learned to 

 obviate this inconvenience." 



From one opportunity which I had of de- 

 scending in the diving-bell now exhibiting at the 

 Polytechnic Institution in Regent-street, I ex- 

 perienced this sensation very strongly, and 

 exactly as Dr. Wollaston describes it. The 

 first effect of the pressure on the tympanic 

 membrane was the production of a crackling 

 noise, which was immediately succeeded by a 

 painful sense of pressure in both ears ; but this 

 is immediately relieved by the act of swallow- 

 ing ; it soon however recurs, and may be in a 

 like manner again relieved. I had no means 

 of judging exactly as to the limits of audition ; 

 but I distinctly observed in conversation with 

 those who descended with me, that grave tones 

 were those least distinctly heard; the grave 

 tones of my own voice also were less distinct as 

 well as the low notes in whistling. 



In such cases then it would appear that from 

 the strong compression exerted on the mem- 

 brana tympani, that membrane cannot vibrate 

 in unison with tones which result from a small 

 number of vibrations. On the other hand we 

 may infer, from Dr. Wollaston's observations, 

 that " human hearing, in general, is more con- 

 fined than is generally supposed with regard 

 to its perception of very acute sounds, and has 

 probably in every instance some definite limit 

 at no great distance beyond the sounds ordi- 

 narily heard." The ordinary range of human 

 hearing comprised between the lowest notes of 

 the organ and the highest known cry of insects, 

 includes, according to Wollaston," more than 

 nine octaves, the whole of which are distinctly 

 perceptible by most ears. Dr. Wollaston has, 

 however, related some cases in which the range 

 was much less, and limited as regards the per- 

 ception of high notes ; in one case, the sense 

 of hearing terminated at a note four octaves 

 above the middle E of the piano-forte; this 

 note he appeared to hear rather imperfectly, but 

 the F above it was inaudible, although his 

 hearing in other respects was as perfect as that 

 of ordinary ears ; another case was that of a 

 lady who could never hear the chirping of the 

 gryllus campestris; and in a third case the 

 limit was so low that the chirping of the com- 

 mon house-sparrow could not be heard. Dr. 

 Wollaston supposes that inability to hear the 

 piercing squeak of a bat is not very rare, as he 

 met with several instances of persons not aware 

 of such a sound. 



The opinion prevailed for a long time that 

 rupture or destruction of the membrana tym- 

 pani is necessarily followed by the loss of the 

 sense. But Sir A. Cooper proved distinctly 

 that not only was hearing not destroyed, but 

 that in some cases of deafness it might be 

 punctured with considerable benefit to the 

 patient.* The most frequent cause of destruc- 

 tion of the tympanum is otitis, and provided 



* Phil. Trans, for 1800 and 1801. 



