THE EAR. 515 



Let us imagine the possibility of an object moving around 

 in the perilymph. Such an object starting in the vestibule 

 might move backwards and pass through any one of the 

 three semicircular canals, going up one and down the other. 

 Afterwards it might move back through the perilymph of 

 the vestibule and go into the scala vestibuli. This it might 

 ascend, by taking the two and one-half turns of the cochlea, 

 to the very summit. The continuity of the partition all 

 along the cochlea formed by the lamina spiralis and the 

 basilar membrane would make it impossible to pass into the 

 scala tympani. But at the very top this partition is absent, 

 and the scala vestibuli connects with the scala tympani. 

 The imaginary object might, therefore, pass into the scala 

 tympani, descend through two and one-half turns the scala 

 tympani, and finally reach the round foramen which leads 

 into the middle ear. There is, of course, stretched across 

 this foramen a delicate membrane which prevents the peri- 

 lymph from flowing into the middle ear. If the imaginary 

 object at the round foramen were to return to the vestibule 

 it could do so only by re-ascending the scala tympani to 

 the top of the cochlea, passing into the scala vestibuli and 

 descending the scala vestibuli to the vestibule. 



3. The Path of a Sound Wave Through the Inner Ear. 

 Perhaps it will be helpful in understanding the reason for 

 this arrangement to follow a sound wave after it has reached 

 the oval foramen by way of the stapes. 



The wave is (1) transmitted by the movements of the 

 stapes to the perilymph of the vestibule. In the perilymph 

 the vibrations run in every direction, but, as we shall find 

 that the perception of sound occurs in the cochlea, we are 

 interested only with those vibrations which reach it. The 

 vibrations which reach the semicircular canals do not, as 

 far as we know, figure in any way in hearing and so may 

 be disregarded. From the vestibule the sound goes, (2) 

 up the continuation of the perilymph through the scala 

 vestibuli of the cochlea to the top of same. (3) At the top 



