The Nature of the Tectorial Membrane 167 



of the cochlea, and because the broader and thicker portions of the 

 membrane probably can be thrown into vibrations by the waves of 

 lower frequenc3\ In other words, the vibrations of higher frequency 

 are more rapidly absorbed and dwindle away in passing along the mem- 

 brane, while waves of greater length or lower frequency may throw the 

 thicker portions of the membrane into undulations of sufficient amplitude 

 for impingement upon the auditory hairs. Thus the first half-turn of 

 the cochlea would be a portion of the peripheral organ of the auditory 

 apparatus which is thrown into activity only by sound waves of the 

 lowest frequency to which the mechanism is capable of reacting. 



Aside from the possibility that the different widths of the tectorial ■ 

 membrane possess different natural vibration periods and therefore may 

 exercise a sort of synchronous selection or resonance with reference to 

 the waves passing in the endolymph, there is nothing in the above sug- 

 gestions to claim it improbable that the thin, basal coil is not made to 

 undulate by waves of low frequency also. It very probably is, and 

 under the following conditions : 



There is little information available as to the properties of sound 

 waves traveling in a substance whose elasticity is not the same in all 

 directions. It is known that in strips of wood the waves travel more 

 rapidly, are less impeded, in the direction of the fibers than across them. 

 The tectorial membrane is most elastic in the direction of its fibers, 

 that is, it is more resistant to stress applied transversely to them. It 

 is exceedingly flexible and sensitive to motion applied transversely to 

 its long axis. Such motions are, in the main, parallel to the direction 

 of its fibers and thus can meet less resistance from them. Since, as 

 shown- above, the fibers are not so arranged nor so attached as to act as 

 a system of resonators, and since the membrane is attached only along 

 one side instead of at each end or along both sides, it is very probable 

 that it is largely a passive structure. Attached along its inner edge 

 and held in its position over the organ of Corti by its elasticity, it 

 extends, suspended in the endolymph and subject to be acted upon by 

 whatever motion may be imparted to the latter. Sound waves trans- 

 ferred to the endolymph by the stapes at the fenestra vestibuli must 

 travel along the membrane from basal end to apex with excursions of 

 amplitiide transverse to its long axis and thus in the direction which 

 may bring into play its remarkable flexibility. Waves of a given fre- 

 quency of vibration will affect corresponding vibrations in the mem- 

 brane to just that extent and amplitude to which they are not damped 



