350 HEARING. LECT. XIX. 



liquids, as in membranes and in all elastic bodies, which 

 divide into vibrating parts separated by nodal lines. In 

 liquids, numerous vibratory movements can likewise be 

 propagated, and co-exist without interfering with one 

 another. Some experiments of Cagnard-Latour, seem also 

 to prove that vibrations are propagated more readily from 

 the solid walls of a cavity to the liquid contained in it, if 

 small solid bodies are dispersed through the latter, either 

 floating or fixed to the walls. The points of contact be- 

 tween the solid and the liquid are thus rendered more nu- 

 merous, and the directions in which the vibrations can be 

 propagated, in a direct line from one to the other of these 

 media, are multiplied by the inequalities presented by these 

 surfaces. These small solid bodies are met with in the li- 

 quid which fills the labyrinth of the ears of certain fishes. 

 We may, therefore, conclude, that the vestibule, or more 

 particularly the mode in which the acoustic nerve termi- 

 nates in all animals, is of great importance in the function 

 of hearing, and is explicable by the laws of acoustics. 



The. undulations of the air, produced by a sonorous body, 

 may excite the auditory nerve, by being transmitted either 

 through the osseous parts of the skull in which the air is con- 

 tained, or through the column of air contained in the exter- 

 nal auditory passage. If we carefully close the external 

 auditory passages, we can still hear very well the tick- 

 ing of a watch, held between the teeth, or applied to any 

 part of the head. Some persons afflicted with hardness of 

 hearing, manage to hear distinctly by applying to the audi- 

 tory passage, or by holding between the teeth, a wooden 

 rod fixed to the centre of a reservoir of air, placed in front 

 of the sounding body. The stethoscope employed by phy- 

 sicians acts principally as a solid cylinder, by which a great 

 number of points of contact are established between the 

 sounding body and the ear applied to it. 



The column of air contained in the external auditory pas- 



