THE SENSE OF HEARING 145 



cipipiit apparaHis of the ear Air-wavos may communicate their impulse to all 

 bodies, whether solid or fluid, on which they strike, produc ng in these bodies 

 vibratory movements mon- or less intense according to the degree of their elas- 

 ticity tind other properties. They communicate themselves therefore to the 

 external skin of the body, and might be tranr^mitted from this to the underlying 

 soft parts; thence again to the bones, and through these b;^ finally propagated 

 to the extremities of the nerves o!' hearing But this mode is so impi'rfect that, 

 were the air waves to reach their destination only in this way, our perceptions 

 of sound would be limited to cases of violent atmospheric concussion. These 

 waves are not only transmitted with difficulty and greatly enfeebled in solid 

 bodies not rendered elastic through tension, but, if not of such excessive in- 

 tensity as to overcome all hindi'ances, their progression, in such bad conductors 

 as the soft parts of the body, would gradually fail, like that of water in sand, 

 before reaching the hidden recesses of the nerves. It would be otherwise, in- 

 deed, if we lived in water instead of air. The waves of sound propagated in 

 water are much more easily transmitted to solid bodies, and must, in fact, pass 

 through the corporeil integument of all aquatic animals endowed with the sense 

 of hearing. It would also be quite otherwise if the vibrations of sound-producing 

 bodies were conveyed to us through the medium of solid substances instead of 

 the air. If we stop our eai'S closely, so as to obstruct iu a great degree the 

 passage, and thus oppose to the air-waves a resistance like that which would 

 proceed from a corporeal surface similarly interposed, we shall hear no sound 

 from a tuning-fork thrown into vibration at some little distance, even when the 

 force of the undulations is increased by placing it on a resonant support. On 

 the other hand, if we apply the vibrating tuning-fork to one of our teeth, we 

 hear the sound in its full intensity, as probably most of our readers have ex- 

 perienced. The reason of this is clear : while in the former case the vibrations 

 communicated by the tuning-fork to the air lose so much of their force by after- 

 wards sti'iking against a badly conducting surface as to be unable to reach the 

 sensorium, in the latter, being communicated immediately to the tooth, they 

 are readily transmitted by this to the surrounding elastic solids and bones of the 

 head, by which last they are directly conveyed to the nerves of hearing. This 

 transmission of sound through the. solids and bones of the head even surpasses, 

 under certain conditions, that which takes place in the ordinary manner by air- 

 waves entering the ear, but only in the case when the distance between the ear 

 and the vibrating object exceeds a certain magnitude, not, as some physiologists 

 assert, under all circumstances. We hear a vibrating tuning-fork more forcibly 

 if applied to the teeth than if held in the air at the distance of a yard from the 

 ear, but the result is different if the instrument be held immediately before the 

 entrance of the ear, as is strikingly shown by the following very simple and 

 easily repeated experiment: If, after striking the tuning-f.>rk, we bring it into 

 contact with one of the incisors and wait until the sound, growing constantly 

 weaker, is no longer perceptible, we shall hear it again if we remove the instru- 

 ment from the tooth and hold it close beside the ear. But, apart from this, it is 

 apparent that the transmission of vibrations through the bones of the head to 

 the auditory nerve, would, of necessity, labor under so many embarrassments 

 and be so limited in its employment, that, were this the only instrumentality 

 through which we received the sensations of sound, our sense of hearing would 

 lose ail its importance as a means of intercourse between the mind and the out- 

 ward world. Let us reflect how seldom it would be practicable, and in these 

 rare cases how awkward, if we were obliged to place ourselves in immediate 

 connection with any vibrating body by means of some solid conductor, a metallic 

 rod or other ; if, in order to convey the sounds of a violin, or the humm voice, 

 to our auditory nerve, a conducting apparatus must first be interposed between, 

 the sounding-board of the one or organs of the other and the solid parts of our 

 own cranium. That we are almost exclusively lunited to the air waves as a 

 10 s G6 



