THE FACULTY OF HEARING. ^6y 



disposes, however, of any doubt as to the mode of conduction of 

 sound in this instance, but there can be but httle variation of 

 intensity except in so far as the excursion of the tympanal mem- 

 brane may be affected by the neighbouring tracheal expansion. 

 In the apparatus of the locust and cricket the nerve scala is placed 

 on the expanded trachea of the limb separated a little from the 

 tympana on each side. Greater variation of conduction and 

 intensity of sound is therefore probable in this type of organ. 



There can be no doubt whatever, that the nerve apparatus 

 stands in the closest relation to the function of the tympanum, 

 and there are fair grounds to infer a near analogy between its 

 action and that of the cochlear apparatus of the mammalian ear : 

 viz., that its function may consist in analysing and singling out the 

 separate tones which the insect may be capable of perceiving 

 when distinguished from the aggregate mass of sounds or 

 vibrations to which the tympanum responds. 



At present we have only hypothesis regarding the special 

 functions of the several parts of the internal ear of mammals. 

 The excitation of the auditory nerve ends would seem to consist 

 in mechanical irritation, the coarsest kind being caused by the 

 contact of otoliths with the nerve ends. While this view is 

 supported by the constant occurrence throughout the animal 

 kingdom of otolith sacs, it is worthy of note that the tympanal 

 organ of insects contains nothing resembling the otolith sac found 

 in crabs and other articulata. So much more striking on the 

 other hand is the analogy of the insect tympanal organ with the 

 drum and cochlear orgpn of the higher vertebrata. But when the 

 instrument of an auditory function is reduced to a single tympanal 

 membrane and a nerve apparatus, the terminal elements of which 

 though differentiated so as to indicate a highly specialised function, 

 have yet all the same anatomical characters and presumably the 

 same kind of action, the conclusion is irresistible that the acoustic 

 function varies according as one or another part of the more 

 complete apparatus is wanting. Looking at the separation of the 



