608 PHYSIOLOGY 



The circular window is also closed by its membrane, in order that leakage of 

 perilymph may be prevented. When the plunger of the incus is moved 

 inwards under the influence of sound waves on the drum, the perilymph 

 being incompressible is driven inwards into the vestibule. From the vestibule 

 a corresponding volume of liquid is displaced into the scala vestibuli ot the 

 cochlea. This drives Reissner's membrane downwards, and thus increases 

 the pressure on the endolymph in the scala media. The basilar membrane 

 therefore moves downwards and the hair- cells are drawn further away from 

 the membrana tectoria. But this movement of the basilar membrane presses 

 on the flui'd contents of the scala tympani which communicates with the 

 round window, and therefore causes bulging of the membrane closing that 

 aperture. Movement inwards of the plunger is therefore accompanied by 

 movement downwards of the basilar membrane and movement outwards of 

 the membrane of the round window and vice versa. Vibrations of the ossicles 

 are in this way communicated to the basilar membrane and to the hair- 

 cells, causing stimulation of the auditory nerve. The basilar membrane is 

 composed principally of radial fibres, and varies in width from end to end 

 of the cochlea. The way in which these fibres vibrate under the influence of 

 sound vibrations will be considered in, the next section, but one further point 

 remains for consideration here, namely the compensation of the cochlea 

 against the effects of gravity and acceleration. This may be explained by 

 considering a cochlea formed of a straight tube having the f enestra rotunda 

 at one end and the f enestra ovale at the other, the basilar membrane forming 

 a diaphragm in the middle. If this tubular cochlea were so placed in the 

 skull that the two windows were at the same level, gravity acting on the 

 contained liquid would not cause flow to occur to either end. But if the head 

 were tilted so that one window was below the other, the fluid would tend 

 to run towards the lower window. This would draw the basilar mem- 

 brane over to that side, causing either the distance between the 

 hair-cells to decrease so that they would come into contact with the 

 tectorial membrane and thus cause stimulation of the auditory nerve with 

 a sensation of sound, or such an increase in the distance that response to feeble 

 sounds would be impaired. It will be seen at once that such an effect 

 of gravity would seriously reduce the efficiency of the organ of hearing. 

 Compensation could be to a considerable extent effected by bending the tube 

 so that the two windows were as close together as possible. The hydrostatic 

 pressure would thus be reduced to its smallest amount in the event of the 

 head being tilted so as to bring one window vertically over the other. 

 Such a bent tube would be compensated to a considerable extent against 

 gravity and linear accelerations in different directions, but not against angu- 

 lar acceleration, for on turning the head quickly from side to side a sound 

 would be generated in the ears. In order to compensate for this effect it is 

 necessary to bring the two limits of the tube in close apposition so that they 

 form a U. A still higher degree of correction would be obtained if the U 

 tube were now wound into a close spiral. But if the two limits of the U 

 tube were joined with the basilar membrane forming the diaphragm between 



