PROPORTIONS OF THE TECTORIAL MEMBRANE 55 



measurement is only approximate. Individual pigs of a given 

 length vary in the degree of development of the cochlea, especially 

 if from different litters. 



It may be seen from table 6 that between pig fetuses of 9 and 

 15 cm. the width of the greater epithehal ridge increases appre- 

 ciably, and between 15 cm. and the adult the width of the space 

 occupied by it decreases. The increase and the decrease is 

 evidently greatest in the apical part of the cochlea. The de- 

 crease in the 1st and 3rd half turns may be as much as one-third 

 of the width of the greater ridge when at its maximum size and 

 activity. During the development of the spiral organ from the 

 lesser epithehal ridge, the apical ends of the inner hair cells are 

 in Une along the outer edge of the growing tectorial membrane. 

 In the 3rd (and no doubt the 1st) half turn of the adult organ, 

 these ends of the hair cells are situated along about the middle 

 of the width of the outspanning zone of the membrane (fig. 2). 

 In other words, it is suggested that in the apical coil of the 

 cochlea, after the tectorial membrane is about completely pro- 

 duced and while the spiral organ is enlarging, the inner hair cells, 

 and therefore the organ, may be moved axisward a distance of 

 about half the maximum width of the greater epithelial ridge, 

 the maximum width of the ridge representing approximately 

 the width of ,the outspanning zone of the membrane produced 

 upon it. 



It is suggested that a movement axisward of the organ of about 

 one-third the width of the outspanning zone may be induced 

 by the pressure of the growing cells of Claudius and the en- 

 larging supporting cells on the outer side of the organ, and be 

 allowed by the retrogression and decrease in number of the 

 cells of the greater ridge on its axial side. The remaining one- 

 sixth of the distance estimated for the apical coil may be accom- 

 plished by the other but less effective processes mentioned, name- 

 ly, the inclination axisward of the apical surface of the organ. Ta- 

 ble 6 shows that the movement is not so much in the basal part 

 of the cochlea and figures 3 and 4, the 5th and 7th half turn of 

 the adult, show that the distance of the inner hair cell from the 

 outer edge of the membrane is less in the basal coils than in the 



