THE SENSE OF HEARING. 



765 



A man who is absolutely deaf in one ear has great difficulty in 

 recognizing the direction of sound. 



It will be recalled by the student that all of the spaces and com- 

 partments of the internal ear, or labyrinth, are filled with perilymph, 

 and that in this fluid float saccules containing endolymph fluid. So 

 intimately are all of the parts of the labyrinth associated that any 

 vibration of its contained fluid at one part is promptly propagated 

 to every other portion. The vibrations of the fluid striking upon 

 the tiny nerve-filaments act as stimulants whose impressions are 

 carried to the center of hearing, where the impressions are recog- 

 nized as sound. 



To epitomize: The sonorous waves collected by the auricle to 

 pass through the external auditory meatus and along its canal strike 

 the surface of the membrane of the tympanum. It becomes tense, 

 vibrates in unison, and then communicates its vibrations through the 

 ossicles and contained air in the tympanum to the oval window. 



From here the vibrations are carried over the vestibule, semi- 

 circular canals, and labyrinth to the perilymph. From this the vibra- 

 tions are transmitted through the membranous walls of the sacculus 

 to the endolymph. Vibrations also pass from the vestibule to the 

 scala vestibuli of the cochlea, and, through the helicotrema, descend- 

 ing the scala tympani, end as an impulse against the membrane of 

 the round window. 



Most of the organs of special sense contain a "specially modi- 

 fied epithelium" for the reception of the particular kind of stimulus 

 peculiar to each other. Nor is the sense of hearing different from 

 the others. It also has its tissues representing "specially modified 

 epithelium" in which lie the terminal filaments of the auditory 

 nerve. These tissues are so constituted that they receive the "waves 

 of sound" which generate auditory impulses in the auditory nerve. 

 These last, when conveyed to the brain, are developed into auditory 

 sensations. 



The vibrations of elastic bodies produce condensation and rare- 

 faction of the enveloping atmosphere. That is, there are developed 

 waves whose particles vibrate longitudinally. These waves are 

 usually spoken of as sound-waves. 



Normally, then, the auditory nerve may be stimulated by sonor- 

 ous vibrations which set into motion the end-filaments of the acous- 

 tic nerve. The filaments are distributed over the inner surface of 

 the membranous labyrinth, upon the membranous expansions of the 

 cochlea, and in the semicircular canals. The excitement of the fila- 



