372 INTERFERENCE OF SOUNDS. 



distinctly, though not so plainly, when the external canal is closed by 

 some obstruction nay, even when the sound-giving object, as a watch, 

 is put into the mouth. So it would appear that the function of the coch- 

 lea is, in a certain sense, independent of the drum, though we have to 

 admit that, for the precision and perfection of hearing, the latter is nec- 

 essary. 



In the view here presented, I consider that each external musical note 

 The cochlea causes a special portion of the spiral lamina to vibrate, and 

 HEKfiL that the particular nerve fibril supplying that portion is af- 

 tion of sounds, fected thereby, and thus a distinct sensation is communi- 

 cated to the brain, the nerve fibrils to the right and left of the one af- 

 fected lying at rest. It may probably be that the denticulate structure 

 -described by Drs. Todd and Bowman has for its duty the more perfect 

 production of this isolated effect, or that the teeth thereof act like the 

 dampers of a musical instrument, and restrain the vibration. Notes the 

 wave length of which is great, or, what is the same thing, the times of 

 the vibrations of which are long, affect those portions of the spiral lamina 

 which are broad and near to the base of the cochlea, but notes whose wave 

 lengths are short, and times of vibration correspondingly brief, affect 

 those portions near to the apex. But probably the scale is changed, as 

 before said, by the tension of the cochlearis muscle, and thus the same 

 part of the lamina can take charge of a range of many octaves. 



It may be inquired how it is that a sound passing through the audi- 

 tory canal, the bones of the tympanum, the membrane of the 

 an interference fenestra ovalis, and thus affecting its destined portion of the 

 mechanism. l am i naj does not give rise to an idea in the mind of repeti- 

 tion or reverberation by moving back and forth through the two scala3, 

 and affecting its proper nerve fibril at each passage. Is there not a ne- 

 cessity for the existence of some mechanism of interference which shall 

 destroy the wave after it has once done its work ? Admitting the force 

 of such inquiries, we can not avoid being impressed with the fact 'that 

 the two scala3 into which the cochlear tube is divided present all the as- 

 pects of a mechanism constructed for the discharge of such a duty. For 

 interference to take place among undulations of any kind, waves upon 

 water, sounds in the air, or the ethereal undulations which constitute light, 

 the essential condition is that they shall run through paths of unequal 

 length, the inequality being one of a series of numbers. They must also 

 be brought, for a full practical effect, to their common point of encounter 

 under a very acute angle, and these conditions are represented in the 

 scala vestibuli and the scala tympani, which are of unequal length, placed 

 at such an acute angle to one another that they might almost be said to 

 be parallel, occupied by a fluid of the same density, and through both at 

 the same moment are passing the undulations which constitute the same 



