CHAPTER 



OF THE SENSE OF HEARING. 



BETWEEN the three preceding senses, which may be called tactile 

 senses, and the senses of hearing and of sight, there is an impor- 

 tant difference in the mode of generation of the sensations. For 

 the tactile senses, the direct contact of the bodies capable of 

 exciting the sensation is indispensable. On the other hand, for 

 hearing and sight, the sensitive organ is agitated indirectly by 

 simple vibrations transmitted to fluid or solid media. As I have 

 not here to write a treatise on acoustics, it is enough for me to 

 recall that any sonorous body is only a body of which the 

 molecules vibrate more or less regularly, and communicate to 

 the ambient media movements which from point to point go to 

 agitate the auditory organs. 



The physiology of the invertebrates is still so imperfectly 

 known that we are unable to say whether in a number of 

 them the sense of hearing exists. Guided by anatomical facts 

 and analogies, men of science have thought that they recognised 

 auditory organs in all the groups of the invertebrates. In 

 worms the auditory organ seems to be a vesiculiform capsule, 

 full of liquid, and containing solid concretions, analogous to 

 those which we meet with in the internal ear of the verte- 

 brates. These ototitks are put into vibratory movement by 

 cilia lining the wall of the auditory sac. 1 



Certain echinoderms seem to be provided with auditive 

 1 Gegenbaur, loc. tit., p. 197. 



