THE HIGHER ORGANISMS 



163 



An advance in the development of the apparatus is 

 found among the insects whose "ears" or chordo- 

 tonal organs are provided with a tympanic membrane. 

 These organs are large, and may be situated upon the 

 sides of the abdomen or upon the anterior tibiae. Some- 

 times, however, no auditory organs can be found when 

 it is supposed that the antennae act as vibration-receiving 

 organs as well as organs of touch and smell. 



The vertebrate ears are two in number, vary in 

 elaborateness, and are situated in special cavities of the 

 cranial bones. The elaborate auditory mechanism found 

 in mammals is divisible into an external ear to receive the 

 sound waves, a middle ear or "drum" to intensify them, 

 and an internal ear containing the actual auditory nerve 

 fibres spread out in what is called 

 the "organ of Corti." 



From the simple and complex 

 ears nerve fibres pass to the brain, 

 communicating with special audi- 

 tory centres and so increasing its 

 complexity both by the addition 

 of fibres and cells. 



Smell. This sense may be re- 

 garded as an amplification of chem- 

 otropic irritability, by virtue of 

 which certain superficially situated 

 cells specialized for the purpose re- 

 ceive and transmit chemical im- 

 pulses to the central nervous 

 system. 



The primitive means by which 

 such impulses are collected are en- 

 tirely conjectural. In the absence 

 of distinct organs to which such a 

 function can be ^signed we are in gJ3,t*ilti 



doubt as tO Whether the lowly r , rod. (After Hauser.) 



organisms possess the sense of 



smell or not. Even among the insects no definite 



organs for the purpose can be found, and yet the 



n 



FIG 64 Longitudinal 

 section of antennal olfac- 

 tory organ of wasp, Vespa; 

 c, Olfactory cell; en, olfac- 



