360 . THE FACULTY OF HEARING. 



The results obtained prove indeed most important. For the fact 

 has been established that a sensory function may be capable of 

 such simplification, and the instrument of that function reduced to 

 so few elements that its type is adequately expressed by these 

 absolutely essential elements. But, on the other hand, where 

 these elements, or their homological equivalents, are not found, the 

 presence of the organ in question is not established. Thus 

 comparative anatomy supplies us with a way-book for reference in 

 our further research where every known type becomes a sign post 

 on the way, a formula for use in testing the nature and quality of 

 any freshly discovered organ. Again, comparative Physiology tracing 

 the steady alliance between differentiation of organ and function 

 back to their starting point of original indifference as regards 

 essence and form of the first protoplasm, prepares for a ready 

 conception of possible divergence of type in accordance with 

 external surrounding. 



In forming an idea of what an insect organ of hearing might or 

 should be, it may assist us to recall the typical characteristics of 

 known organs in other animals. And if we exclude all considera- 

 tion of outward form (which mostly depends upon the modifications 

 of supplementary organs needed for the exercise of an auditory 

 function under particular circumstances) we may enumerate the 

 following as essential f actors :—( 1 ) A material susceptible of 

 vibratory movement, i.e., whose particles possess elasticity under 

 tension, allowing free play of molecular motion for transmission of 

 sound. (2) A link (or several links) of connection between the 

 vibrating body and the sensory nerv^e, which link must be capable 

 of transmitting the respective vibrations. (3) Nerve of special 

 sense (so-called), In the lower types the middle factor may 

 disappear 3 in the higher types, on the contrary, the apparatus of 

 transmission becomes more com.plicated. In the highly-developed 

 organ which we ourselves possess, nerves of hearing are distributed 

 on several specifically distinct structures : the 'Vestibular sac, and 

 ampullos of the semi-circular canals, and again on the winding 

 incline of the tnemhrana spiralis of the cochlea. In this latter organ 



