128 



Irving Hardesty 



7th 

 half- 

 turn 



13.4 



These averages are obtained from vertical, axial sections from each 

 of four cochleae. The thickness of the tip at the apex is computed froin 

 one measurement, for, unfortunately, the plane of only one of the 

 sections was such as to pass transversely through the tip. Further, it 

 is uncertain how far from the basal tip the measurements designated 

 "seventh half-turn" were taken. Evidently they passed varying distances 

 from it, for the measurements varied from 12.4 microns to 24.9 microns. 

 The other regions did not vary so widely. All the measurements were 

 made and recorded before the averages were computed and converted 

 into the terms of microns. 



It is seen from these' figures that the greatest thickness of the mem- 

 brane occurs in the first half-turn, and thence the thickness decreases 

 gradually and somewhat progressively toward the basal end. A com- 

 parison of Figs. 6 and 7 indicates that in this cochlea the greatest 

 thickness occurred in the third half-turn instead of in the first. The 

 shrinkage here seems to have resulted in a lateral crumpling and prob- 

 able increase of thickness in the third half-turn. The drawings of this 

 specimen were made before the average thickness was ascertained; 

 otherwise a more average specimen might have been used. This specimen 

 was among those measured. The first and second turns all showed much 

 smaller differences than any other adjacent half-turns. That the differ- 

 ences between adjacent basal half-turns are greater than those between 

 the half-turns of the apex is due to the fact that the basal turns are 

 longer, the basal coil of the cochlea being several times longer than the 

 apical, and thus longer strips of membrane intervened between the meas- 

 urements in the basal half -turns. The lack of greater uniformity in pro- 

 gressiveness of decrease is probably due both to irregularities in the 

 shrinkage effects of the reagents and to lack of identical orientation of 

 the plane of section of the different cochleae, the plane passing nearer 

 to the ends, especially the basal end, of some membranes than of others. 



Yon Ebner quotes Eetzius for the statement that the thickest part of 

 the human tectorial membrane measures from 24 to 25 microns. The 

 particular turn from which this observation was made is not stated. The 



