The Nature of the Tectorial Membrane 143 



an average of only 75 microns, while measurements of the width of 

 the greater epithelial thickening in the 15 cm. pig gave an average of 

 174 microns, showing the space in the foetal to be more than twice as 

 wide as in the developed cochlea. (The measurements indicate, fur- 

 ther, what is shown in the specimens, that the epithelial cells of fhe 

 fcetal cochlea, though much higher, are thinner than those of the 

 developed.) 



The direction of the fibers of the inner region of the body of the 

 membrane is outward and almost parallel throughout. In this region 

 multitudes of fibers are added to the body of the membrane, contributing 

 to its sudden increase in thickness. The practically uniform outward 

 direction here is due to the fact that these fibers were produced before 

 the epithelium began to recede and the width of the thickening to 

 decrease. Further outward, as far as Hensen's stripe, the fibers are 

 directed outward, then downward, but still outward. At the region 

 of Hensen's stripe, many fibers curve downward from their outward 

 direction above and approach the under surface vertically. In fact, 

 Hensen's stripe seems to be an expression of the period at which the 

 retrogression of the epithelial thickening began. It also represents 

 the line along which the thick, outer edge of the thickening was last 

 attached and along Avhich growth was last contributed to the membrane. 



With the exhaustion and retrogression of the cells of the inner portion 

 of the thickening began the decrease in the width of the space occupied 

 by it. Consequently, the portions of fibers produced by it after the 

 decrease in width began were no longer directed outward or vertically, 

 but inward, drawn inward, as it were, by the now receding outer edge 

 of the thickening. As a result, there is in Hensen's stripe a crossing 

 of the ends of fibers approaching it from the two directions, as noted 

 above, in describing Hensen's stripe. 



iVnd also as a result, the fibers coursing in the uiembrane on the 

 outer side of Hensen's stripe are directed outward in the upper surface 

 (the portions earlier formed), then curve downward (as do those on 

 the inner side) and finally curve inward to approach the under surface 

 at acute angles or to run short distances parallel with it. The tectorial 

 membrane, firmly attached along its inner zone and sustained by its 

 semi-solid matrix and its consequent elasticity, holds the position and 

 shape in which it was molded; while the organ of Corti, during its 

 elaboration, appears to follow the receding greater epithelial thickening 

 and finallv comes to be situated under the outer zone of the tectorial 



