THE SENSE OF HEARING 159 



tension of the tympanum, but leaves the incus in its natural position, because it 

 turns the malleus less upon the axis common to the two ossicles than upon a 

 vertical axis which passes through the point at Avhich its long projection is at- 

 tached to the wall of the cavity. Both malleus and incus acquire, through the 

 resulting pressure of their articulating surfaces against each other, a new rel- 

 ative position, but act in this new position, towards the waves of sound arriving 

 from without, as a single lever which propagates their impressions by means of 

 the stapes to the fluid of the labyrinth. 



If our readers should perhaps think this explanation of a small and incon- 

 spicuous mechanism tiresome and superfluous, while we can take no exception 

 to the first of these epithets, we must protest against the application oF the 

 second ; the influence of that mechanism on the hearing is, in fact, of the highest 

 importance. In what follows we shall simply regard, as the efi'ect of the muscle 

 in question, an increase of the tension of the membrane of the tympanum, and 

 shall inquire in what consists the utility of that efi'ect. There can be no doubt 

 that it consists in deadening the effect of too intense an action of the waves of 

 sound. It is a physical fact that strongly stretched membranes respond to these 

 waves with less intensity, are thrown by them into weaker vibrations, than 

 membranes in a state of less tension ; a fact which has been shown by multi- 

 plied experiments and may be here taken for granted. As the skin of a drum 

 is pressed inwards by the hand with more difiiculty when it is tightly stretched 

 than when relaxed, so the membrane of our ear opposes the greater resistance 

 to the force of the impinging undulations of the air the greater the tension of 

 the membrane. It is a matter of indifference whether the stretching force is 

 exerted around the rim in the plane of the membrane, oi', as is the case with 

 the action of the tensor tympani, at the centime of the membrane and perpen- 

 dicularly to its plane. It has already been seen that the resistance opposed by 

 the membrane to the air-wave increases in proportion to its convex curvature 

 from the increasing force of the elastic reaction ; the tensor muscle, which acts 

 like a wave of condensation in producing inward flexion of the membrane and 

 thereby increasing its tension, must in like manner intensify the elastic forces 

 and increase the resistance to the air-waves. Now, if the membrane, in conse- 

 quence of the increased tension, executes vibrations of less magnitude, in the 

 same degree will the oscillations of the small bones which are moved by it, and 

 finally the intensity of the undulations in the fluid of the labyrinth, be abated; 

 in a word, a given air-wave excites a weaker sensation of sound under strong 

 tension of the tympanum than under slight tension. 



This result may at any time be verified in the simplest manner — not, indeed, 

 by a voluntary contraction of the tensor tympani, but by other means which we 

 possess of increasing the tension of the membrane The cavity of the tympa- 

 num is not altogether closed up, for the Eustachian tube penetrates into it with 

 a rather long channel, which, at its anterior extremity, opens into the hinder 

 part of the throat near the palate ; by this channel the air within the cavity is 

 placed in communication with the outer atmosphere, which, in the act of breath- 

 ing, passes by the orifice of the tube. By a certain effort, during that act, it is 

 in our power to press the air through this tube into the cavity of the tympanum, 

 and, by another sort of efi"ort, to withdraw it therefrom ; the first takes place if, 

 in the act of ordinary expiration, we bar the egress of the air through the mouth 

 and nose, and thus compel its passage into the Eustachian tube ; the second, if, 

 with closed mouth and nose, we make an effort at inhalation, and, by thus en- 

 larging the space occupied by air, produce its rarefaction in all parts of the re- 

 spiratory appg,ratus, and consequently in the cavity of the tympanum. In both 

 cases the tympanic membrane is more tightly braced ; in the former the inhaled 

 air presses the yielding membrane outwards ; in the latter the membrane is 

 drawn more strongly inwards in consequence of the rarefaction of the internal 

 air, and the same effect is produced as by a contraction of the tensor muscle. 

 In both cases obtuseness of hearing is the result. If we press air with force 



