CRANIAL NERVES AND SENSE-ORGANS 249 



during auditory experiments it is utterly impossible to deter- 

 mine whether the lower animals are merely shaken by the 

 vibrations of a tone or noise, or actually hear it. Upon the 

 whole one must be prepared to assume that the same physical 

 laws which govern hearing in the higher animals and man ex- 

 tend also to the lower species. If the intensity of a tone de- 

 pends upon the extent and force of the sound waves and the 

 pitch of the tone upon their frequency, this law must hold 

 good for the auditory organ of insects as well as for the human 

 ear. Whenever sounds are emitted by an animal it is reason- 

 able to suppose that they can be heard by another animal of 

 the same species ; moreover, one is justified in assuming that 

 not only those chafers which produce " instrumental music " 

 but also others which cannot do so are provided with the means 

 of hearing. 



In the chapter on comparative anatomy we have already 

 seen that in medusa three forms of auditory organs can be 

 distinguished, namely : 



(a) Open pits lined with cells which either contain otoliths 

 or terminate in auditory bristles connected with nerves. 



(b) Auditory vesicles with one or more otoliths. 



(c) Auditory tentacles with one or more otoliths at their 

 extremities. 



These mobile otoliths and auditory bristles connected with 

 nerves are the prototype of the auditory apparatus of all In- 

 vertebrates, and in modified form participate in the structure of 

 the vertebrate ear. 



Lubbock was the first to investigate the mechanism of 

 hearing in insects, not only in those that possess a tympanum 

 with a tracheal vesicle behind, but in others also. A constant 

 feature is the presence of auditory rods connected with nerve, 

 which are in one way or another set in motion by the vibrations 

 in the air playing upon them, just as one sees the hairs on the 

 antennae of gnats set moving by sounds. 



The study of the simpler organs in Invertebrates helps to a 

 better understanding of the complex ears of Vertebrates and 

 man. Man has by no means the most perfect apparatus Hfor 

 the conduction of sound waves, since according to the laws of 

 acoustics the more at right angles the concha of the ear can be 



