DISPLACEMENT THEOR V 



263 



hdicolrcma. He believes thaL the luiir eells and not the basilar 

 niembraiic are the sensitive organ. They pass throngii the meshes 

 of the reticulate membrane, and, according to Wrightson and 

 Keith, have their upper ends fixed in the tectoria. The tectorial 

 membrane, it will be remembered, is attached to the spiral lamina, 

 as a nail grows out of the finger. Now, displacement of the 

 fluid causes movement of the whole reticulate membrane. This 

 latter produces a to-and-fro movement of the base of all the hairs, 

 and, as these hairs are fixed in the tectorial membrane, they will 

 bend. " It would only be the simple pure tones which would give 

 to the hairlet a pure symmetrical harmonic motion, but by the 

 displacement of liquid under pressure, every conceivable succession 

 of bendings of the innumerable hairlets can be obtained to convey 

 to the auditory nerve every impulse required to produce the 

 pitch of each resultant and component tone " (Wrightson). 



Thus the cubic displacement of fluid is converted, by means 

 of the arch of Corti, into a linear 

 movement of the reticulate mem- 

 l)rane, etc. By means of the 

 resistance of the tectoria, the 

 linear movement is converted 

 into the bending of the hairlets. 



Such an explanation fails to 

 account for the ability of the 

 trained musical ear to perceive 

 as separate entities the different 

 sounds from an orchestra reach- 

 ing it simultaneously, and it does not give a very clear explana- 

 tion of " tone-gaps," nor does it explain why fatigue to one note 

 leaves the response to all other notes apparently unaffected in 

 intensity. 



Pressure-Pattern Theory. 



This theory, originated by Waller and elaborated by Ewald, is 

 based on the sand and powder patterns produced on vibrating 

 plates (Fig. 67) (see Chladni's plates. Chap. XVII.). The pattern 

 traced by the sand depends on three main factors : (1) the part of 

 the plate where the vibrations start, (2) the parts of the plate that 

 are fixed or damped, and (3) the frequency of the induced vibra- 

 tions. Ewald showed that the membrane in his " acoustic 

 camera " vibrated like Chladni's plates, and that the distance 

 between the nodes varied with the pitch of the note entering the 

 camera. The sand comes to rest on the nodal lines, or lines of rest, 

 and makes a pattern, so translating a sound figure into a light 



Fig. 67. — Chladni's plate. 



