NERVE 175 



subject with alterations in pitch perception, give support to 

 this theory. 



But it has recently been pointed out by Wrightson that 

 there is no real proof of this theory, and that it assumes that 

 the air waves are propagated up the scala vestibuli, through 

 the helicotrema, and down the scala tympani. 



It has further been urged that the small size of the 

 opening at the helicotrema and the large extent of scala 

 media exposed to the perilymph must tend to favour a trans- 

 mission of pressure across this scala from the scala vestibuli 

 to the scala tympani, and so to the round window. 



Now this would cause a displacement of the basilar 

 membrane' — 



1st. Downwards. 



2nd. Back to the horizontal. 



3rd. Upwards. 



4th. Back to the horizontal, as in fig. 87. 

 This would result in a lateral displacement of Cortis' arch 

 round the base of the internal rods as a hinge, and this 

 would lead to a pull first in one direction then in the other 

 of the reticular membrane, which would lead to a bending of 

 the cilia embedded in the tectorial membrane, which is firmly 

 attached to the denticulate lamina, and this might stimulate 

 the cells, thus stimulating the nerve endings and so leading 

 to stimulation of the auditory centre. 



This view of the action of the ear brings hearing into 

 accord with the sense of touch, and it is of interest that the 

 otic vesicle is developed as an invagination of the skin. The 

 mechanism is a most delicate one for "weighing" the small 

 variations of pressure which constitute sound waves. 



Such a theory serves to explain the possibility of notes 

 of different pitch producing different eftects in the short 

 stumpy cochlea of the bird, which is difficult to understand 

 on Helmholtz's theory. 



