FUNCTION OF THE SEMICIRCULAR CANALS. 375 



INTERIOR OF LABYRINTH, SEEN ON ITS EXTERNAL OR TYMPANIC FACE. 



Fig. ic4. Fig. 194 : #, vestibule, into which open the semi- 



circular canals by five orifices, and the vestibular 

 scala of the cochlea ; b, b, section of the wall of the 

 cochlea ; <?, c, bony portion of spiral lamina, dividing 

 the conoid cavity of the cochlea into scala vestibuli 

 interior of labyrinth. and scala tympani ; d, orifice at summit of cochlea. 



INTERIOR OF LABYRINTH, SEEN ON ITS INTERNAL OR CRANIAL FACE. 



Fig. ic5. Fig. 195 : #, vestibular cavity, into which open the 



cavities of the semicircular canals and the cochlear cav- 

 ity ; b, bottom of internal auditory canal, answering to 

 base of cochlea, 'and partly to internal wall of vestibule ; 

 , simple foramen for facial nerve ; d, many openings 

 interior of labyrinth. f or auditory nerve. 



The explanation usually given of the function of the semicircular ca- 

 nals is, that they serve to determine the direction of sounds, Criticism on 

 an idea which has arisen from their remarkable rectangular the expiana- 

 position. However, this is accomplished in almost all cases ^"^life 

 by directing the external canal toward the point from which function of the 

 the sound is coming, and judging of its place by the varia- 

 tions of its intensity. Animals commonly direct the external ear toward 

 the sounding point, guided doubtless in the same way. Some physiolo- 

 gists have restricted the use of the semicircular canals to the collection 

 of those sounds which strike upon the skull, but, besides the preceding 

 considerations, there are others derived from comparative anatomy which 

 seem to indicate that this can scarcely be their duty. 



The intensity of sounds is judged of by the tympanum ; their pitch 

 or wave length is determined by the cochlea, and therefore They are for 

 there arises a strong presumption that the semicircular ca- q S u 4 a t atl "f tt 

 nals must have the function of distinguishing the third char- sounds. 

 acteristic of sounds, that is, their quality ; since, if this be not the case, 

 there seems to be no other portion of the auditory mechanism to which 

 that office could be assigned. 



The suspicion which we are thus led to entertain, that the semicircu- 

 lar canals are for appreciating the quality of sounds, is strengthened in 

 no common degree by facts of comparative physiology. Unfortunately, 

 we know so little of the mechanical peculiarity on which distinctions of 

 quality depend, that we are wholly unable to trace out the structural con- 

 ditions of an organ which should be calculated for seizing on them. We 

 know that the quality of a note emitted by a violin is different from that 

 emitted by a flute, though the intensity and pitch may be the same, but 



