FUNCTION OF THE COCHLEA. 371 



canal ; q, branch of sacculus ; 7 1 , branch of utricle ; s, branch of horizon- 

 tal canal ; t, t, branch of superior vertical canal. 



We proceed now to the consideration of the functions of the cochlea. 



The principles of acoustics would lead us to infer that sounds entering 

 the cochlea throw into vibration its spiral lamina, an inference Functions of 

 which is supported by anatomical considerations in regard to the cochlea, 

 the position and function of the cochlearis muscle in keeping the mem- 

 branous portion of the lamina at a due degree of tension. We should 

 also infer that each external sound does not throw the lamina into vi- 

 bration throughout its whole length, but only on a special and corre- 

 sponding point, and thereby affects solely the filament of the auditory 

 nerve in connection with that point ; that sounds which are low will act 

 upon the broader portions of the membrane, near the mouth of the coch- 

 lea, and those which are high, the narrower portions near the apex. In 

 this respect, therefore, the function of hearing should have two limits, 

 one for low and the other for high notes, as experience proves to us is 

 actually the case ; but possibly the scale is, so to speak, enlarged through 

 the various degrees of tenseness which may be given by the contractions 

 of the cochlearis muscle. A general idea of the nature of this limited 

 vibration may be obtained by recalling the effect which is Physical iiius- 

 produced when one musical instrument is played in the vi- ^J^"^^^ 16 

 cinity of another, as when, for example, a flute is played cochlea. 

 near to a piano-forte, the strings of the latter are thrown into sympathetic 

 vibration, and the piano emits a note answering to each note of the flute. 

 All the strings are not thrown into vibration at once, but for each note 

 of the flute that string of the piano vibrates, the length and tension of 

 which are duly adjusted. The same thing, again, may be seen when 

 musical sounds are originated near a stretched membrane, the surface of 

 which has been dusted over with grains of dry sand. The whole sheet 

 of the membrane is not cast into vibration at once, but some parts move 

 and some remain at rest, and so the sand-grains dance up and down on 

 the vibrating parts, and soon, being cast therefrom, accumulate on the 

 parts that are still, and mark out what are termed nodal lines. These 

 nodal lines, or places which are motionless, are frequently of remarkable 

 complexity and symmetry, as may be seen from the figures of them given 

 in any of the books on natural philosophy. m 



It is immaterial in what manner the sound has reached the cochlea, 

 whether through the auditory canal or through the bones of Course of 

 the skull generally; the effect, as far as the spiral lamina is sounds to the 

 concerned, will be the same in both cases. That sounds can cochlea - 

 efficiently reach the auditory nerve, and produce thereupon their proper 

 effect, without ever having passed through the auditory canal or the drum, 

 is manifested by a great many familiar facts. We still continue to hear 



