480 IRVING HARDESTY 



membrane of the hog (AZ, figs. 1 and 2) show that it varies very 

 little in width throughout its entire length. It is slightly nar- 

 rower in the basal turn. The free edge of the outspanning zone 

 extends well beyond the hair cells in the apical coil but barely 

 over the outermost of the outer hair cells in the basal coil. Sec- 

 tion A of figure 2 passes some distance from the actual basal end 

 of the cochlea. 



Measurements of the thickness of the tectorial membrane, 

 necessarily made from sections of dehydrated and stained speci- 

 mens in which the membrane was no doubt somewhat shrunken, 

 show that its outspanning zone likewise increases gradually and 

 regularly in thickness in passing from the basal to the apical end 

 and that at the apical end it is 3 times as thick as at the basal 

 end. There is practically no variation in the thickness of the 

 adult basilar membrane; an}' little variation shown is never 

 regular. The usual variations in the thickness of the membranes 

 are indicated in figure 2, which represents transverse sections of 

 the 1st, 3rd, 5th and 7th half turns in a section of one of the 

 cochleae of the adult hog used for the measurements recorded in 

 the previous paper. 



Considering that the usual vibrations are imparted at the basal 

 end by the basis of the stapes, then these vibrations in passing 

 toward the apical end must, according to their vibration fre- 

 quencies and amplitudes, be damped out in overcoming the 

 inertia of the membrane itself as well as the resistance offered 

 by the walls of the labyrinth and the fluid contained. Thus, it 

 may be argued that the tectorial membrane, varying far more 

 in width and thickness than does the basilar membrane in passing 

 from the basal to the apical end, is adapted for being affected by 

 a far greater variety of vibratory activity, has a much greater 

 possible scale of activity, than the basilar membrane. 



Variations in width and thickness are but indications of vari- 

 ations in volume and variations in the volume of the membrane, 

 or the load it carries, are the most important factors to be con- 

 sidered determining the extent to which it may be thrown into 

 vibrations by given vibratory disturbances. Not only must a 

 more voluminous segment absorb a greater amount of energy, 



