v THE SENSE OF HEAEING 213 



the fenestra ovalis by the stapes ; but the latter have so much 

 the greater force in correspondence with their smaller amplitude 

 that they may render those of the fenestra rotunda ineffective. 



It is therefore not proved that under normal conditions the 

 transmission of the acoustic stimulus to the labyrinth occurs by 

 way of the fenestra rotunda, still less that this is the only path 

 that tones take from the external air to the labyrinth. The most 

 that can be assumed (and is held by certain physiologists) is that 

 the vibrations of the air in the cavity may co-operate with 

 those transmitted by the chain of bones. Many otologists (e.g. 

 Gradenigo) hold that both the fenestra ovalis and the fenestra 

 rotunda, which are closely connected, may serve in given cases 

 for the transmission of tones; that in very high tones which 

 are beyond the upper limit of the perceptible scale transmission 

 through the air of the cavity and the fenestra rotunda may be 

 regarded as predominant, while for low tones, on the contrary, 

 transmission must occur mainly by way of the chain of ossicles. 

 This theory is supported by clinical observations, which show 

 that in pathological processes of various kinds in which there is 

 reason to suspect an accumulation of exudates or a thickening 

 of the membrane of the fenestra rotunda the main functional 

 disturbance is defective auditory perception of the highest notes 

 of the scale, which may amount to an octave or more. On the 

 other hand, according to Helmholtz' theory, it is precisely at the 

 commencement of the first convolution of the cochlea (near the 

 fenestra ovalis) that perception of the highest tones is located. 



Under certain abnormal conditions the fenestra rotunda is 

 undoubtedly the only path of aerial transmission of the sound- 

 waves to the labyrinth. When the tympanum is perforated there 

 is invariably restriction of the perception of deep tones and a 

 marked diminution of the maximal distance at which accurately 

 measured tones and sounds are appreciable. In fixation or 

 anchylosis of the ossicles, and in immobilisation of the stapes 

 due to any cause without perforation of the tympanum, otologists 

 affirm that hearing is fairly good for high tones, but perception 

 for low tones is reduced. In both cases it is plain that the chain 

 of ossicles cannot function as a special contrivance for the con- 

 duction of sound-waves to the labyrinth. Under such abnormal 

 conditions the transmission of air- waves by the fenestra rotunda 

 obviously explains the alterations and limitations of hearing- 

 better than Bezold's hypothesis of osteotympanic conduction. 



VI. To understand the functions of the Cochlea it is necessary 

 clearly to understand the structure of its most important part, the 

 cochlear canal or scala media, which contains the very delicate 

 organ of Corti, to the cells of which the branches of the cochlear 

 nerve, the true auditory nerve, are distributed. 



The cochlear canal is seen in section as a triangular space 



