DISPLACEMENT THEORY 213 



helicotrema. He believes that the hair cells and not the basilar 

 membrane are the sensitive organ. They pass through the meshes 

 of the reticulate membrane, and, according to Wright son 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 succes- 

 sion of bendings of the innumerable hairlets can be obtained 

 to convey to the auditory nerve every impulse required to pro- 

 duce 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 

 membrane, 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 reaching it simultaneously, and it 

 does not give a very clear explanation of " tone-gaps." 



To conclude in the words of Helmholtz : " On reviewing 

 the whole arrangement there can be no doubt that Corti's organ 

 is an apparatus for receiving the vibrations of the basilar mem- 

 brane and for vibrating of itself, but our present knowledge is 

 not sufficient to determine with accuracy the manner in which 

 these vibrations take place." 



There remains one very important matter which should be 

 considered because of its diagnostic value to the physician, viz. 

 conduction of sound waves by the bones of the head. It is common 

 knowledge that sound vibrations travel more readily through 

 a solid than through a liquid or a gaseous medium. A watch, 

 placed sufficiently far away to be inaudible, can be heard ticking 

 if touched by a lath held between the teeth. If something goes 

 wrong with the mechanism of the ear, one wants in the first 

 place to locate the fault. Is the external ear, the middle ear or 

 the internal ear the seat of the trouble ? The test is usually made 

 by placing a vibrating body, such as a tuning fork, on one of the 

 cranial bones. If the sound is not appreciated, then the fault 

 lies within the internal ear. Either the organ of Corti (or 



