886 PHYSIOLOGY OP THE DOMESTIC ANIMALS. 



and the first process, therefore, to be considered in the study of the 

 function of hearing is the means by which sound-waves reach the laby- 

 rinth, and then the way in which they excite the nerve of hearing. 



It has been seen that every sonorous body is in a state of vibration 

 and that it- transmits these vibrations to the surrounding atmosphere, 

 resulting in waves of condensation and rarefaction traveling in radii from 

 the locality in which the sonorous body is located. 



It has been further stated that sound-waves may be conducted through 

 any elastic medium, but in, the appreciation of sound by the ear evidently 

 the greatest importance is to be placed on the conduction of sound- 

 waves through the air, since the conduction of sounds through solids 

 can but infrequently be of any importance. It has been stated that 

 sound-waves can be conducted through the bones of the head, and 

 although this must be exceptional in the case of man, yet in fishes, where 

 no external ear, auditory canal, or ear-bones are present, it plays an 

 important part. So, in this case, the sound-waves of the water are 

 directly transferred to the labyrinth. 



We have now to trace the path of the sound-waves from the sonorous 

 body, through the external ear and auditory canal, through the tj'mpa- 

 num to their termination in the labyrinth, with the operation of the 

 different apparatuses by which this transfer is facilitated. 



The external ear evidently fulfills the part of an ear-trumpet, and 

 the great improvement in hearing produced by artificial addition to the 

 auricle serves to emphasize this point. The external ear is evidently of 

 a certain amount of assistance in recognizing the direction of sound, 

 but, as is well known, we are liable in this respect to make errors of 

 judgment. The origin of sound-waves is determined simply by the fact 

 that the sound is heard most distinctly when the auditory canal is in the 

 line of propagation of the sound-waves, and, therefore, in order to 

 determine the direction of a sound we turn the head from one side to the 

 other until the sound appears to be the loudest. In the lower animals 

 this is, to a certain extent, facilitated by the exceptional degree of mobility 

 possessed by the auricle, where we must assume that the recognition of 

 the direction of a sound is a matter of greater importance and accom- 

 plished with a greater degree of facility and perhaps exactness. 



Having reached the auricle, sound-waves enter through the external 

 auditory canal and strike against the tympanic membrane. It has been 

 stated that if a tightly stretched membrane be set into vibration it will 

 produce a sound, the lowest note being termed the fundamental tone ; 

 and, further, that if a sound which corresponds with the fundamental 

 tone of such a membrane be sounded in its proximity the membrane will 

 be set in vibration by sympathy. It is evident, if such a fact were ap- 

 plicable to the tympanic membrane, the greatest confusion would result 



