MECHANICS OF THE INNER EAR 83 



Two consequences of the particular shape of the partition 



which we have just discussed in as much detail as anatomical 



knowledge permits should be emphasized. 



„ . The first of these is of the greatest bioloe- 



Two important . , . .^ ^ . • , • 



consequences of '"^^ significance. It is certainly important 

 the partition's foi" the animal to be very sensitive to 



shape. sound, that is, to be able to hear sounds 



Sensitiveness which are very weak and cause only a 



01 the ear minute movement of the stirrup. Now, 



the initial part of the partition being ex- 

 ceedingly narrow, even the minutest quantity of fluid dis- 

 placed by the stirrup must spread considerably lengthwise 

 over the partition and thus stimulate quite a number of nerve 

 ends. But it would not be advantageous to have the partition 

 equally narrow all along. In that case comparatively weak 

 objective sounds would cause the whole partition to move up 

 and down and the displaced fluid for which no room can be 

 made by the partition, to flow back and forth through the 

 "safety valve." Strong objective sounds would then make 

 the same impression upon the animal as sounds of medium 

 physical intensity. This disadvantage is overcome by the 

 partition's tapering, by its being narrow at the beginning, 

 but wide farther on, so that even sounds of considerable 

 strength do not involve the whole partition. But again, there 

 would be a disadvantage if the partition's width increased uni- 

 formly : for then the relative intensities of simultaneous tones 

 — as we have seen — would not be even approximately inde- 

 pendent of the absolute intensity of the total sound. This 

 disadvantage might be avoided by the width increasing first 

 rapidly, then more and more slowly. If it is thus avoided, 

 either partially or totally, we do not exactly know because of lack 

 of exact anatomical data. 



