536 THE SENSE OF HEARING. 



ceiving the impulses of the air, the cartilage of the external ear, 

 while it reflects a part of them, propagates within itself and 

 condenses the rest, as all other solid and elastic bodies would 

 do. Thus, the sonorous vibrations which it receives by an ex- 

 tended surface, are conducted by it to its place of attachment. 

 In consequence of the connection of the parietes of the audi- 

 tory passage with the solid parts of the whole head, some dis- 

 persion of the undulations will result ; but the points of at- 

 tachment of the membrana tympani will receive them by the 

 shortest path, and will as certainly communicate them to that 

 membrane, as the solid sides of a drum communicate sonorous 

 undulations to the parchment head, or the bridge of a musical 

 string, its vibrations to the string. 



Regarding the cartilage of the external ear, therefore, as a 

 conductor of sonorous vibrations, all its inequalities, elevations, 

 and depressions, which are useless with regard to reflection, 

 become of evident importance ; for those elevations and de- 

 pressions upon which the undulations fall perpendicularly, will 

 be affected by them in the most intense degree ; and, in conse- 

 quence of the various form and position of these inequalities, 

 sonorous undulations, in whatever direction they may come, 

 must fall perpendicularly upon the tangent of some one of 

 them. This affords an explanation of the extraordinary form 

 given to this part. 



Functions of the Middle Ear : the Tympanum, Ossicula, and 

 Fenestrce. 



In animals living in the atmosphere, the sonorous vibrations 

 are conveyed to the auditory nerve by three different media 

 in succession ; namely, the air, the solid parts of the body of 

 the animal and of the auditory apparatus, and the fluid of the 

 labyrinth. 



Sonorous vibrations are imparted too imperfectly from air 

 to solid bodies, for the propagation of sound to the internal 

 ear to be adequately effected by that means alone ; yet already 

 an instance of its being thus propagated has been mentioned. 



In passing from air directly into water, sonorous vibrations 

 suffer also a considerable diminution of their strength ; but if 

 a tense membrane exists between the air and the water, the 

 sonorous vibrations are communicated from the former to the 

 latter medium with very great intensity. This fact, of which 

 Mu'ller gives experimental proof, furnishes at once an expla- 

 nation of the use of the fenestra rotunda, and of the membrane 

 closing it. They are the means of communicating, in full in- 

 tensity, the vibrations of the air in the tympanum to the fluid 



