DEVELOPMENT OF THE MEMBRANA TECTORIA 453 



5. It is narrow and thin in the basal coil and becomes wider and 

 thicker as it approaches the apex. Measurements of the func- 

 tional zone of the membrana taken from its outer border to the 

 labial teeth, show that the sectional area of the membrana in the 

 apical turn is from thirty to forty times that in the basal turn. 

 Owing to the shrinkage of the membrane such measurements 

 can be only approximate. Figure 12 shows the relative size of 

 the membrana as seen in sections of the first, second, third and 

 fourth turns. 



Fig. 12 Sections through the membrana tectoria of a full term fetus showing 

 its relative size in the four turns. 



From Hardesty's measurements of fresh and fixed preparations 

 he found that the greatest width (first or apical turn) was about 

 five times that of the minimal width (tip of basal turn) while the 

 thickness in these same turns was as 6 : 1. 



The membrana tectoria is thus to be regarded as a spiral 

 cuticular band of dehcate chambered structure, which becomes 

 gradually thicker and wider from base to apex. This band is 

 attached by its flattened inner edge to the labium vestibulare, 

 spans the spiral sulcus and its outer portion is so attached between 

 the cells of the spiral organ that the ends of the cells close the 

 chambers and the auditory hairs project into them. It may be 



