482 IRVING HARDESTY 



offer greater resistance to being thrown into vibration sufficiently 

 for the necessary impingement upon the hairs of the hair cells, 

 than the less voluminous segment, but it must possess a different 

 natural vibration frequency. The natural vibration frequencies 

 of strips of material, or the vibration frequencies with which they 

 may act in resonance, vary according to their volume or load as 

 well as according to their length. Computations, given in the 

 previous paper, based upon the areas obtained of its transverse 

 sections, suggest that the volume of a given very short length of 

 the basal end of the outspanning zone of the tectorial membrane 

 may increase as much as 40 times in grading to the volume 

 of the same length of the apical end. Calculated in the same 

 way, the volume of the assumed vibratory portion of the basilar 

 membrane at its basal end increases only about 1.4 times in pass- 

 ing to its apical end. The average thickness of the basilar mem- 

 brane of the adult hog, measured under the spiral tunnel, 

 was found to be 2.8/x. It varies irregularly from 1.8m to 3.7^. 

 Usually it is found to be thicker in its basal instead of its apical 

 end. 



The length of the tectorial membrane of the adult hog is about 

 26 mm. It comprises nearly 4 turns. 



(7) The shape and attachment of the tectorial membrane 

 may suggest that it is of other use than that of a mere foreign 

 body spanning over the spiral organ for impingement of the 

 hairs against it upon vibration of the basilar membrane. Its 

 outer edge is free in the developed mammalian cochlea; its most 

 voluminous and thus most varying portion is over the organ; 

 it suddenly thins toward its attached zone as though the thinner 

 part of this side of the outspanning zone may serve somewhat as 

 a hinge ; and the contour of the basal surface over the organ is 

 alwaj^s parallel with the apical (peripheral) surface of the organ 

 while other parts of the basal surface are not. 



(8) The structure and consistency of the tectorial membrane 

 suggest that it may be especially sensitive to vibrations in the 

 fluid in which it lies and that it may express such disturbances 

 almost wholly in motions vertical to the surface of the spiral 

 organ. It is composed of fibrils imbedded in a gelatin-like matrix 



