PROPORTIONS OF THE TECTORIAL MEMBRANE 23 



thickest part of the outspanning zone at the different intervals 

 are given in table 2. The thickest part usually appeared to be 

 in the region of the small ridge on the basal surface known as 

 'Hensen's stripe' {HS, figs. 1 to 3). 



TABLE 2 



Giving in viicra averages of the thickness of the thickest part of the tectorial mem- 

 brane of the adult hog obtained by measurements at the intervals specified in median 

 vertical sections of five cochleae 



It may be seen from table 2 that, just as the outspanning zone 

 is widest, so is it thickest in the apical end of the coil of the cochlea. 

 After whatever shrinkage that may have resulted in the prepara- 

 tion of the sections, computations from the table indicate that 

 in the apical end this zone may be 110 /i thicker or about 3 times 

 as thick as it is at the 7th half turn. Between the 1st and 3rd 

 half turns its thickness appears to decrease about 19 per cent,, 

 between the 3rd and the 5th half turns about 18 per cent, and 

 between 5th and 7th half turns, the longest interval between 

 measurements, the decrease is about 48 per cent. The smaller 

 amount of decrease between the 3rd and 5th half turns is prob- 

 ably due to irregularities of shrinkage, but this cannot be deter- 

 mined from the preparations used. The drawings, figures 1 

 to 4, made from a single cochlea show the decrease in thickness 

 between the 1st and 3rd half turn to be least and that the de- 

 crease increases progressively between the 3rd and 5th and the 

 5th and 7th half turns. One would assume from studies of the 

 teased out unshrunken membranes that this latter form of de- 

 crease in thickness is the normal one. 



The volume of the tectorial membrane may be computed ap- 

 proximately from the areas of its transverse sections. Considered 

 the chief vibratory mechanism in the auditory apparatus, varia- 

 tions in the volume of the membrane, or the 'load' it carries, in 

 different regions are the most important of the proportions. 

 Most all of its volume being carried in its outspanning zone, 



