HEAEING. 



As the end-organ oi vision is protected both without and 

 within, so is the still more complicated end-organ of the sense 

 of hearing more perfectly guarded against injury, heing in- 

 closed within a membranous as well as bony covering and sur- 

 rounded by fluid, which must shield it from stimulation, except 

 through this medium. 



Hearing proper, as distinguished from the mere recognition 

 of jars to the tissues, can, in fact, only be attained through the 

 impulses conveyed to the auditory brain-centers, as originated 

 in the end-organ by the vibrations of the fluid with which it is 

 bathed. 



It will be assumed that the student has made himself famil- 

 iar with the general anatomy of the ear. The essential points 

 in regard to sound are considered in the chapter on The 

 Voice. It will be remembered that what we term a musical 

 tone, as distinguished from a noise, is characterized by the 

 regularity of vibrations of the air that reach the ear; and that 

 just as ethereal vibrations of a certain wave-length give rise to 

 the sensation of a particular color, so do aerial vibrations of a 

 definite wave-length originate a certain tone. In each case 

 must we take into account a physical cause for the physiological 

 effect, and these bear a very exact relationship to one another. 



As will be seen later, whUe in all animals that have a well- 

 defined sense of hearing the process is essentially such as we 

 have indicated above, the means leading up to the final stimu- 

 lation of the end-organ are very various. At present we shall 

 consider the acoustic mechanism in mammals, with special ref- 

 erence to man. There are in fact three sets of apparatus : (1) 

 one for collecting the aerial vibrations; (3) one for transmit- 

 ting them ; and (3) one for receiving the impression through a 

 fluid medium; in other, words, an external, middle, and in- 

 ternal ear. 



