SENSE OF HEARING. 



883 



other cellular structures to be noticed, which are, perhaps, to be regarded 

 as special cells (Fig. 408). 



Waves of sound falling upon the auditory nerve produce no sound, 

 but only when the terminal organs are stimulated. 



The Function of Hearing. To be enabled to understand the use of 

 the different portions of this complicated organ it will be necessary to 

 refer to some of the more important laws governing the propagation of 

 sound. Sound, as before stated, is the result of the vibrations of elastic 

 bodies, which result in the production of alternate condensation and 

 rarefaction of the surrounding medium. As a consequence, sound-waves 



FIG. 407. SCHEME OF THE DTJCTTTS COCHLEARIS AND THE ORGAN OF CORTI. 



(Landois.) 



N, cochl ear nerve ; K inner and P outer hair-cells ; n, nerve-fibrils terminating in P: a, a, supporting 

 cells : d, cells in the sulcus spiralis ; z, inner rod of Corti : Mb. Corti, membrane of Corti, or the niem- 

 brana tectoria ; o, the membrana reticularis ; H, G, cells filling up the space near the outer wall. 



are produced, in which the particles vibrate longitudinally, or in the 

 direction of the propagation of the sound, forming so-called waves of 

 condensation and rarefaction, occurring in concentric circles around the 

 sounding body. 



Like rays of light, sound-waves may be reflected when they impinge 

 upon an opaque solid, and the same rules as to the angle of incidence 

 and reflection prevails. It is this throwing back of sound from a resisting 

 medium that constitutes the echo. 



When transmitted through the atmosphere these waves of sound 

 are collected by the auricle, and from the auricle they are transmitted 

 through the external auditory meatus to the membrana tympani, which 

 is thus thrown into vibration. Thus, they are communicated to the 



