.')88 " LECTURE XXXII. 



ing the functions of every part of the ear, since its structure varies so much 

 in different animals, that we cannot pronounce with certainty respecting the 

 indispensable necessity of any one arrangement for the perfection of the sense. 

 And even in the case of the human ear, many of these parts may be spared 

 withoyt great inconvenience ; thus, we hear very perfectly, by means of im- 

 pressions communicated to the teeth, and through them to the large bones, 

 of the head; and even when the membrane of the tympanum, and all the 

 small bones of the ear have been destroyed by disease, the undulations 

 of the air still continue to affect the organ in the usual manner.. (Plate 

 XXV. Fig. 349. . 351.) 



Such is t^e delicacy of the organs of hearing in their perfect state, that 

 we readily distinguish not only the frequency of the vibrations of a sound, 

 whether constant or variable, and its loudness or softness, but also the 

 quality of tone, depending on the law which governs each separate vi- 

 bration, and which constitutes the difference between instruments of differ- 

 ent kinds, or different instruments of the same kind, or even the same instrument 

 differently employed. Thus, we can distinguish very accurately the voices of 

 our friends, even Avhen they whisper, and those modifications of the same voice 

 which constitute the various vowels and semivowels, and which, with the 

 initial and final noises denominated consonants, compose the words of a 

 language. We judge also, without an error of many degrees, of the exact 

 direction in which the sound approaches us; but respecting the manner in 

 Vhich the ear is enabled to make this discrimination, we cannot reason upon 

 any satisfactory grounds. 



