No. I.] THE VERTEBRATE EAR. 107 



though not visible. The whole membrane is composed of such 

 fibres, and they are apparent in radial sections, in which they 

 are seen to run as arcs downward. 



It has been proposed several times to distinguish zones in the 

 membrane. In the rabbit, however, there are only two zones 

 distinguishable, — the zone of the inner portion, fixed to the 

 Hmbus spirale, and the free zone, both of which pass insensibly 

 into one another, since the boundary between the two is con- 

 fined to the stripe present on the lower surface. The two zones 

 are also characterized by their different thicknesses. 



Retzius (1884, 237) : Membraiia tectoria of the cat. This 

 structure accompanies as usual the papilla basilaris from its 

 base to the apex of the cochlea, i.e. throughout its entire length. 



One can best distinguish two zones, an inner and an outer. 

 These are bounded on the tympanal surface by the stripe, 

 which is formed along the line of attachment of the membrane 

 to the free edge of the labium vestibulare of the limbus spiralis. 

 The outer zone extends from this free edge quite to, or perhaps 

 somewhat beyond, the third row of hair cells. The inner zone 

 extends from the free edge of the limbus to near the line of 

 origin of the membrana Reissneri. The breadth of the inner 

 zone of the membrana tectoria is different in the three spirals 

 of the cochlea. It decreases in radial breadth gradually from 

 the base to the apex, thereby decreasing the breadth of the 

 whole structure. The membrane is consequently of different 

 widths in the three spiral turns of the canal. The outer zone, 

 however, gains in breadth from the base to the apex of the 

 cochlea. 



On the outer free zone of the tympanal surface of this mem- 

 brane in the cat the striae discovered by Hensen run in spiral 

 direction. In perpendicular radial section it appears refrac- 

 tive, and, as Hensen stated, it lies almost directly over the row of 

 inner hair cells. In the basal and spiral turn this Henseii s ridge 

 lies nearer the edge of the limbus spiralis and is less distinct, 

 and appears more or less serrated and broken up into papillae, 

 and is located about midway between the edge of the limbus 

 (the bounding line between the two zones) and the outer edge 

 of the membrane, consequently about the middle of the outer 

 zone. In the apical spiral turn it appears again less sharply 

 defined, and approaches nearer to the outer border of the mem- 



