LECT. XIX. HEARING. 345 



LECTURE XIX. 



HEARING. 



ARGUMENT. Modes of exciting the sensation of sound. Structure of the 

 ear. Propagation of sonorous waves through the organ of hearing. 

 Uses of the various parts of the ear. Physical properties of sound. 



A LUMINOUS sensation is the invariable effect of any 

 excitation of the retina or optic nerve ; and, in like manner 

 excitation of the acoustic nerve, however it may be effected, 

 always gives rise to the sensation proper to this nerve. 

 Thus, when speaking of electricity, I told you that a pecu- 

 liar sound was heard when one of the poles of a voltaic 

 pile was applied to the ear, the circuit being closed. Some 

 substances, principally narcotics, when introduced into the 

 organism, also produce sensations of sound. 



In most cases, however, sound is produced by vibratory 

 movements, communicated to elastic bodies, and propa- 

 gated, by means of the air or other media, to the acoustic 

 nerve. In this way the function of hearing is usually 

 exercised. 



In special treatises on Acoustics, experiments are de- 

 scribed which tend to show that the cause of sound, the 

 difference between grave and acute tones, and the intensity 

 and timbre of sound, depend on the velocity and amplitude 

 of these vibratory movements. In that department of physi- 

 cal science, you will also find laid down the laws of the 

 propagation of vibrations in air, liquids, and solids. We 



