HEARING. 509 



both closed, and the sound is much louder with the ears closed 

 than open. 



These are the facts which have led to the belief that, in such 

 instances, the sound was communicated directly through the bones 

 of the head, vibrating in consonance with the sounding body. But 

 a little examination will show that such is not the case. When we 

 hold the end of a vibrating tuning-fork between the teeth, we no 

 longer hear the sound in the vibrating extremity of the instrument 

 or its neighborhood, but in the mouth and the nasal fossce. It is the 

 vibration of the air in these passages which produces the sound; 

 and this vibration is communicated to the cavity of the tympanum 

 through the Eustachian tube. The apparent increase of sound, also, 

 on closing the ears, which could not be explained on the supposition 

 that it was conducted directly through the bones of the cranium, 

 is due to the same cause. For it can easily be seen, on trying the 

 experiment, either with a tuning-fork held between the teeth or 

 simply with our own voices, that this apparent increase of sound 

 takes place only when the ears are closed by gentle pressure. If the 

 pressure be excessive, so that the integument is forced inward into 

 the meatus and the air in the meatus subjected to undue compres- 

 sion, the sound no longer appears louder in the corresponding ear, 

 and may even be lost altogether. 



The apparent increase of sound, therefore, in such cases, when 

 the ear is gently closed, is due to the fact that the meatus is thus 

 converted into a reverberatory cavity, by which the vibrations of 

 the tympanum are increased in intensity. But if the air in the 

 meatus be too much compressed by forcible closure, the vibrations 

 of the tympanum are then interfered with and the sound is dimi- 

 nished or destroyed. 



In all cases, then, it is the sonorous vibrations of the air which 

 produce the sound, and these vibrations are received invariably by 

 the membrane of the tvmpanum, and thence transmitted to the 

 internal ear by the chain of bones. The cranial bones are incapable 

 of communicating these vibrations to the labyrinth and its contents, 

 except very faintly and imperfectly. For common experience shows 

 that even the loudest and sharpest sounds, coming from without, 

 are almost entirely lost on closing the external ears; and our own 

 respiratory and cardiac sounds, which are so easily heard as soon 

 as the chest is connected with the ear by a flexible stethoscope, are 

 entirely inaudible to us in the usual condition. 



The exact function of the different parts of the internal ear is 



