LECT. XIX. PROPAGATION OF SOUND. 349 



posed of several membranes, of osseous parts, of air, and 

 of a liquid. The vibrations are propagated through these 

 bodies, all of which transmit sound in consequence of the 

 vibratory state excited in them by that of sonorous bodies. 

 Each portion of the ear thus takes part in the function which 

 this organ has to fulfil. But in what manner do the diffe- 

 rent parts act ? Acoustics cannot give a completely satis- 

 factory reply to this question ; but by the aid of compara- 

 tive anatomy, of experimental physiology, and of pathology, 

 we may succeed in determining the different degrees of im- 

 portance possessed by each part in the function of hearing, 

 and, r hence, can determine in what degree they respectively 

 contribute to the perfection and delicacy of the ear. The 

 external and middle parts of the ear are wanting in a great 

 number of animals, which, nevertheless are considered to 

 be endowed with a perfect sense of hearing. Thus, in 

 birds there is no vestige of the pinna; in reptiles the ex- 

 ternal auditory passage is wanting; and in fishes the ear is 

 reduced to the internal part alone. But the part which is 

 present in every case, in which hearing can be effected, is 

 the vestibule ; in other words, a membranous sac, filled 

 with a liquor in which the extremities or ramifications of 

 the auditory nerve are contained. Any other mode of ter- 

 mination of this nerve would have been less advantageous 

 for the exercise of this faculty. The nerve being reduced 

 to some very small filaments, diffused in a liquid, the points 

 of contact are in this way multiplied as much as possible. 

 The structure of the nerve thus approaches more to that of 

 the liquid in which it is placed. These ramifications, scat- 

 tered in the fluid mass, are expanded in every direction, 

 and are thus directed normally with their extremities to- 

 wards the vibratory movements, which are propagated to 

 the fluid by the walls of the spherical cavity in which it is 

 contained. We know that vibrations are propagated in 



