HEARING OR AUDITION. 119 



1st. External ear ; 2d. The middle ear, or tympanum; 3d. The 

 internal ear. 



Sounds may be propagated in three ways, by reciprocation ; by 

 resonance ; and by co7iductio7i. Vibrations of reciprocation are ex- 

 cited in a sounding body when it is capable of yielding a musical 

 tone of definite pitch, and another body of the same pitch is made to 

 sound near it. Thus, if two strings of equal tension be placed side 

 by side, and one be thrown into vibrations, the other, although un- 

 touched, will be thrown into corresponding vibrations. The same 

 is true of membranes as of strings. If a membrane and a string 

 both capable of yielding the same note, be placed side by side, and 

 one be thrown into vibrations, the other will reciprocate. But no 

 membrane or string will reciprocate any tone that is lower than its 

 oy^xi fundamental note, by which is meant, the lowest note which it 

 will yield when the whole of it is in vibration together. 



Vibrations of resona7ice occur when a sounding body, as a tuning- 

 fork, is placed in connexion with any other, of which one or more 

 parts may be thrown into reciprocal vibrations, even although the 

 tone of the whole be different, or it be not capable of producing a 

 definite tone at all. If a tuning-fork, whilst vibrating, be placed in 

 contact with a sounding-board, the board will divide itself into a 

 number of parts, each of which will reciprocate the original sound 

 so as greatly to increase its intensity. 



Vibrations of conduction are the only ones by which sounds can 

 be said to be propagated. If the ear be placed at one extremity of 

 a long board, and the other be lightly struck, the sound will be con- 

 ducted to the ear along the whole length of the board. All media 

 are capable of conducting sound, a vacuum being the only space 

 through which it will not pass. Solids are better conductors than 

 fluids, and fluids than gases. The greatest diminution in the inten- 

 sity of sound is usually perceived, when a change takes place in the 

 medium from which it is propagated, especially from the aeriform to 

 the liquid. 



The object of the external ear is to receive sonorous vibrations, 

 concentrate and conduct them inwards. The various elevations and 

 depressions of the external ear, adapt it peculiarly to catch the sono- 

 rous waves arising from opposite quarters. The auMtory canal — 

 meatus exteriius — receives the sonorous pulses immediately, and 

 conducts them to the membrana tympani. The sound is at the 

 same time strengthened by reflection from the walls of the meatus, 

 and the resonance of the mass which it incloses ; the walls of the 

 passage, moreover, are solid conductors of sound. 



The use of the membrana tyynpani is to receive the sonorous un- 

 dulations in such a manner as to be thrown into recij)rocal vibra- 

 tion, which is to be communicated to the chain of bones. It cannot, 

 however, reciprocate any sounds that are lower than its own funda- 



