80 THE GENESIS AND STRUCTURE OF THE MEMBRANA TECTORIA 



Prentiss (1913) subdivides the membrana tectoria into the following zones: 



1. A thin, structureless zone of the inner portion of the labium vestibulare. 



2. A second thicker zone of flattened horizontal chambers over the outer portion 



of the labium vestibulare. 



3. A still thicker zone of chambers curving downward and outward, unattached, 



over the sulcus spiralis. 



4. An outer zone thickest in the upper turn, with chambers trending downward, 



outward then inward, largely attached to the cells of the spiral organ and 

 probably normally and wholly attached in this manner. 



Many investigators describe a superficial layer in some regions of the membrane, 

 the superficial network of filaments, Fadennetz of Lowenberg, who mentions its 

 existence throughout the two-thirds of the outer segment. Its existence is con- 

 firmed by Hensen (1871), Boettcher (1872), Retzius (1884), Dupuis (1894), Spee 

 (1901), and Held (1909) as extending farther towards the inner segment. Spee and 

 Held recognize that it reaches the attachment of Reissner's membrane and forms 

 the inner zone of the tectorial membrane. 



According to the process of histogenesis, the adult membrane is formed 

 throughout its entire thickness and breadth by a system of cylinders or prisms 

 separated by a pale and homogeneous fluid which seems to be lacking in the most 

 superficial and densest layer. Figure 35, from an adult rat, represents a tangential 

 section of the outer segment (mil) and the greater part of the middle segment (mig) . 

 At first sight the figure shows a system of double lines, the longitudinal axial 

 section of the cylinders (cy). The intraprismatic liquid seems to be a little darker 

 than the more abundant interprismatic fluid. One sees, moreover, other single, 

 often darker lines, tangential sections of cylinders (cy"). At the ends of some 

 obliquely cut cylinders the double lines join together and are in continuity with 

 a single line. On the left of the outer segment (mil) and in the middle segment (cy') 

 the prisms are pressed together and their transverse section is visible in the form 

 of small polygons, between which the' interprismatic substance seems to be lacking. 



In my preparation the segment of the membrana tectoria photographed (fig. 

 35) is in continuity with its inner segment on the surface of the crista spiralis. 

 This axial portion exhibits the same structures, but the prisms are in closer contact, 

 while the interprismatic fluid is less abundant. 



The superficial network of Lowenberg and similar structures of the outer and in- 

 ner segments are represented, according to my preparations, by more compact parts 

 of the membrane, the interprismatic substance of which is less abundant or lacking, 

 so that a real chambered structure (Coyne and Cannieu, Prentiss), or, in tangential 

 sections, a kind of network appears; but the walls of the chambers or the filaments 

 of the reticulum represent the transverse section of the membranes of neighboring 

 cylinders. Figure 35 displays these longitudinal prisms on the right side of the 

 outer segment, while their apices are cut transversely, pressed together, on the left. 



Figure 23, from an adult bat, exhibits a small portion of the middle segment of 

 the tectorial membrane within which at least three similar longitudinal sections 

 (cy) of the cylinders and some obliquely tangential (cy") can be noticed. 



