on the Membrana Tympani. iy 



ruption, from the liquid in which they live, to the organ of 

 hearing. 



Man is capable of hearing in a similar manner to fishes, when 

 a communication of solid parts is kept up between the sounding 

 body and the bones of the skull : experiments of this kind must 

 have been made by many members of this learned Society. 



One of the most common is, applying a watch to the fore- 

 head, and stopping the ears, which does not prevent the ticking 

 from being heard : the sound is still more distinct when the 

 watch is applied to the mastoid process. Here, as the sound 

 can neither pass through the meatus externus, nor by the Eus- 

 tachian tube, while the mouth is kept shut, it evidently must 

 be conducted through the bones of the skull. 



When the sound produced by boiling water is brought to the 

 ear, by one end of an iron rod resting upon the side of the kettle 

 and the other kept in contact with the teeth, the sound is con- 

 ducted in the same way, although in this case it has by some 

 been supposed to pass through the Eustachian tube. 



In this mode of hearing, the vestibulum and semicircular 

 canals are probably the only parts of the organ which are ne- 

 cessary to convey the impression to the expansion of the audi- 

 tory nerve. 



In hearing in air, the use of the membrana tympani in man 

 and quadrupeds has already been explained. Its office in 

 birds is precisely the same ; but as in birds this membrane has 

 no tensor muscle to vary its adjustment, but is always kept 

 tense by the pressure of the end of the slender bone, the scale 

 in birds cannot descend so low as in the human ear ; and the 

 intervals in their scale will be more minute, in consequence of 

 the slightest tremor communicated by the action of the radiated 



mdccc. D 



