160 



ON THE SENSES. 



into the cavity, there occurs, at the moment when the air enters, a cracking or 

 clashing noise, which is very clearly perceptible to ourselves, and may even be 

 heard by others, if their ear be brought near to ours; during the whole time 

 that air is thus compressed in the cavity there exists, but perceptible only by 

 ourselves, a humming noise, which increases and decreases with the force of the 

 compression. The nature and causes of this singular noise in one's own ear 

 are not determined with certainty. According to an old and widely-received 

 opinion, the cracking S(jund arises from a sudden and voluntary contraction of 

 the tensor muscle and the consequent sudden stretching of the membrane of the 

 tympanum ; but as the noise does not, as the supporters of this opinion believe, 

 occur at the moment when air ceases to be driven into the cavity, but, on the 

 contrary, at the moment when it commences, this view is clearly erroneous. 

 The noise arises upon the sudden flexure outwards of the membrane, and 

 therefore with the tensor muscle relaxed. A conjecture advanced by the author 

 of these lines is to the effect that the phenomenon in question is perhaps to be 

 explained in a manner analogous to the well known cracking which ensues 

 when a finger is suddenly and forcibly pulled, and proceeds from a sudden sep- 

 aration of the closely contiguous surfaces of the articulation between the mal- 

 leus and incus ; it supposes that the abrupt outward flexure of the membrane 

 may produce an instantaneotis and audible severance of these surfaces, of which 

 the first must follow the membrane, while the last cannot, on account of the 

 unyielding ligature of the stapes. This conjecture it would be difficult to verify, 

 wherefore we shall not dwell upon it, but return to the dullness produced in the 

 perceptions of the organ by the contraction of the muscle of the malleus. 



The intention of this superinduced dullness of hearing is apparent from what 

 has been said above ; it consists in preventing a too strong and dangerous con- 

 cussion of the tender nerve fibres by unduly intense undulations of the medium 

 of sound. That this end should be attained, it is unconditionally necessary that 

 the tensor muscle should, at the right time — that is, when air-waves of a certain 

 intensity enter the organ — be brought into action, and this really takes place, 

 although the " how " has not been indubitably shown. It is thought by many 

 that the will provides for the seasonable interposition of the muscular contrac- 

 tion ; that is to say, that, if a very strong sound reaches the sensorium, the will 

 calls into service the conscious reaction of the suppressive mechanism. This is 

 neither proved nor probable, for the will would be a very precarious sentinel. 

 In sleep, or when the attention is wholly diverted from the perceptions of the 

 organ, the will would not fulfil its (iLuty, and the nerves would be abandoned to 

 the mischievous effects of immoderate sound-waves. The security would be 

 much greater if the operation of the muscle were brought about by a simple but 

 sure mechanism, which should be set a going by the air-waves in a degree cor- 

 responding to their intensity. And this is doubtless the case ; the excitation of 

 the nerves of movement takes place, in all probability, through a reflex action 

 of the nerves, just as it is by this reflex action that the pupil of the eye is nar- 

 rowed, and the dazzling effect of too intense light-waves thereby diminished. If 

 a very intense wave of sound reaches the ear, so that the auditory nerves are 

 thrown into strong excitement, these immediately transfer in the brain that ex- 

 citement to the nerve fibres of the tensor muscle with which they are in direct 

 communication by means of a cellular apparatus ; the centrifugal excitement 

 thus propagated, (the nervous current), on arriving at the muscle, determines a 

 contraction of its fibres^ — a contraction which will be more considerable, the 

 stronger the excitement and more forcible the air- wave. In this way, it is true, 

 the tension of the tympanum, which serves for deadening the -sound, comes 

 somewhat j^ost festum, since already a part of the wave has been conveyed with 

 undimiuished intensity to the fluid of the labyrinth and the nerves of hearing ; 

 that part, namely, which calls the reflex action of the nervous excitation into 

 existence. But it suffices probably for the protection of the nerves of hearing, 

 if only a long continuance of the violent concussion is spared them ; but, from 



