LECT. XIX. PROPAGATION OF SOUND. 357 



the tympanum, through the intermedium of the chain of 

 ossicles, and thus rendered susceptible of different de- 

 grees of tension. We must thereby obtain a greater compass 

 in the function of the ear. These replies to the questions 

 which we put, doubtless, are not the only ones that we may 

 one day be able to give ; but since the organ can exist 

 and perform its function without this chain of ossicles, 

 and without either the membrane or the tympanum, we 

 must admit that these parts are not essential to the ear, 

 and, that they serve only to render it more perfect, and for 

 its conservation. 



I shall make but a few remarks on the semicircular ca- 

 nals and the cochlea. It is generally supposed, that the 

 vibration excited and transmitted through the solid walls, 

 in which the ear is contained, are transmitted by these walls 

 to the acoustic nerve. 



I shall rapidly pass over the physical characters of sound, 

 the comparison of different sounds, and the limits of ap- 

 preciable sounds. Any impulse or excitation communi- 

 cated to the acoustic nerve, as mentioned at the commence- 

 ment of this lecture, produces a sonorous sensation. By 

 the word sound, we strictly understand a sensation which 

 is preserved uniform for a certain time, and which is sus- 

 ceptible of being measured and compared. Sound differs, 

 then, from a mere noise, inasmuch as the latter is the effect 

 of a single shock, or of a series of shocks which are re- 

 peated without any regularity ; whilst the sonorous sensa- 

 tion is that which we experience when the acoustic nerve 

 receives a certain number of successive shakings, sepa- 

 rated from each other by a certain and constant interval 

 of time. It is this which takes place with Savart's wheel, 

 with Cagnard Latour's sirene, or by vibrations of a stretched 

 cord, producing corresponding undulations in the air which 

 reach the ear and strike the acoustic nerve, producing a 



