The jSTatiire of the Tectorial Membrane 121 



reports that for the cat, guinea pig, dog and rabbit the membrane 

 extends throughout the cochlea. Measurements of the lengths of nine 

 tectorial membranes, seven from the cochlea of pig foetuses at about 

 term and two from pigs about two weeks old, gave an average length 

 for the membrane of 25.5 millimeters. 



In both breadth and thickness, the membrane gradually decreases 

 from the apex of the cochlea toward the basal end (Fig. 1), and both 

 the end at the apex and the basal end terminate bluntly, slightly 

 tapering. The upper surface of the membrane is convex throughout its 

 length (Figs. 2, 3, 4, 6 and 7), the height of the curvature being in 

 the region which, in the natural condition, overlies the interclasped 

 phalanges of the rods of Corti's organ. The under surface, or the 

 surface next to the organ of Corti, is concave, but with a concavity 

 of several curvatures. In the region of the apex of the cochlea, the 

 outer edge of the membrane projects beyond the confines of the organ 

 of Corti and here this edge bends downward slightly, resulting in an 

 evident concavity of the under surface. The portion immediately im- 

 posed upon the surface of the organ of Corti is parallel with the surface 

 of that organ and thus is usually plane; it may slant upward or down- 

 ward from the horizontal plane of the cochlea or it may be slightly 

 concave, depending upon the direction and the shape of the upper 

 surface of the organ of Corti, which varies somewhat in the different 

 regions. In the decrease in width of the tectorial membrane toward 

 and in the basal coil, it gradually ceases to project beyond the organ 

 of Corti, and thus finally there is no separate concavity of the outer 

 edge to be considered (Figs. 4, 5 and 8). That zone of the under 

 surface beginning with the inner border of the organ of Corti, spanning 

 the spiral sulcus and including the inner, thin edge of the membrane, 

 is, throughout, more deeply and regularly concave than any other 

 portion. The greater concavity here is but an expression of the rapid 

 decrease in thickness which is attained by the inner edge as it becomes 

 attached upon the vestibular lip of the spiral limbus. 



All the investigators mentioning it describe the inner edge as ex- 

 tending to the insertion of the vestibular (Eeissner's) membrane. In 

 the main, this has been found to be true in my preparations of the pig. 

 Occasionally, observation and comparative measurements from the free 

 membrane and of the width of the labium vestibulare have indicated 

 that the inner edge does not necessarily in all regions extend quite 

 to the vestibular membrane. Hensen, '63 ; Eetzius, '84 : Barth, '89 ; 



