MECHANICS OF THE INNER EAR 



135 



teresting to compare this with the organ of hearing of the 

 lower vertebrates. Figure 42 indicates the 

 Comparative manner of evolution of the cochlea. An 



anatomy of the original pit (Fig. 42 a) as found in a frog is 

 auditory organ gradually lengthened and assumes in the birds 

 a banana-like shape (Fig. 42 b), showing a 

 distinct tendency to coil. In mammals the process of lengthening 

 and coiling has proceeded so far that the organ (Fig. 42 c), if it 

 were transparent, would appear as a spiral. It is clear that the 

 coiling can have little influence on the mechanical function 

 of the organ. The lengthening of the organ, however, is of 

 the utmost functional importance. The original pit does not 

 differ materially from the other cavities which we find within 



X^ 



Fig. 42. Evolution of the auditory 

 organ 



the labyrinth, communicating with the semicircular canals. 

 In this pit movements of the fluid caused by movements of 

 the stirrup — or rather columella plate, since the lower verte- 

 brates have a much simpler connection of tympanum and 

 oval window — produce, probably by mere friction, stimulation 

 of the endings of the auditory nerve. The organ of the birds 

 must function more nearly like the human organ, excepting 

 the difference of function resulting from the fact that the endings 

 of the auditory nerve are spread out over a small linear extent, 

 whereas in the mammals they are distributed over a long distance. 



