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XXXIV. The Analysis of Sound Waves by the Cochlea. By 

 R. E. Roaf. (From the Department of Physiology, 

 London Hospital Medical College?) * 



fl^HE variations of air pressure which produce the 

 L sensation of sound are conveyed to the fenestra oralis 

 through the foot plate of the stapes. The mechanics of this 

 conveyance is well described by Wrightson, who points out 

 that the liquid in the internal ear is practically incompressible, 

 therefore movements of the stapes can take place only by 

 mass movements oi: liquid and by yielding of the membrane 

 closing the fenestra rotunda f. 



Mass movement of the liquid can take place in one of two 

 ways, • Liquid may pass up the scala ve'stibuli through the 

 helicotrema and down the scala tympani, or the scala media 

 may be pushed towards the scala tympani. The resistance 

 to these movements is in the former case the inertia of 

 the mass of liquid to be moved, and the friction of the 

 liquid against the walls of its containing tube, and in the 

 hitter case the tension of the basilar membrane (Reissners 

 membrane is usually represented as being flaccid). The 

 relations of these two movements are shown in figs. 1 and 2. 

 Fig. 1 represents the cochlea uncoiled, and arrows indicate 

 the direction of movement of liquid if it were to take place 

 through the helicotrema. Fig. 2 represents a cross section 

 through one part of the coiled tube, and an arrow indicates 

 movement of liquid when the basilar membrane is pushed 

 towards the scala tympani. 



The relation of movement of liquid along the sealce to 

 deformation of the basilar membrane is difficult to assess 

 because the viscosity of the liquid and the tension of the 

 basilar membrane are unknown, but the following factors are 

 in operation. The resistance to movement is high in such 

 small tubes as it is inversely proportional to the fourth power 

 of the radius in circular tubes, and the shape of the sealce is 

 such that there is a relatively larger surface to area than ii 

 they were circular tubes. Such high resistance will damp 

 any movement, so that the movements of the liquid will be 

 t; dead bent." such as are those associated with the recognition 

 of tone 1. The width of the basilar membrane is greater the 

 further up the cochlea it is measured, therefore it will be 



* Communicated by the Author. 



f Sir Thomas Wrightson, 'The Analytical Mechanism of the Internal 

 Ear." Macmillan & Co. (1918). 



X II. llartrioVe. J. Physiol, vol. liv. Proc. p. vii (1920). 



