170 ANATOMICAL AND PHYSIOLOGICAL STUDIES ON 



Within the membranous cochlea there occurs a wave of growth 

 passing from the axis to the periphery as shown in figures 4 to 13. 

 The crest or highest point of the tissue mass appears at birth 

 near the axis, in the greater epithelial ridge, and then progress- 

 ively shifts toward the periphery, so that at maturity it is in 

 the region of the Hensen cells. With advancing age the hair 

 cells come to lie more and more under the tectorial membrane 

 and the pillar cells seem to shift toward the axis. 



At from 9 to 12 days the tunnel of Corti appears and the rat 

 can hear. 



All of these changes occur first in the basal turn and progress 

 toward the apex. The mature relations are established at about 

 twenty days. There are thus two waves of change in the mem- 

 branous cochlea, from the axis to the periphery and the other 

 from the base to the apex. The rat can usually hear at twelve 

 days of age or about three days before the eyes open. 



The largest cells in the ganglion spirale are very immature at 

 birth, reach their maximum at twenty days, and after that di- 

 minish in size, slightly but steadily. The rat hears, therefore, 

 before these cells have reached their full size. 



The largest cells in the vestibular ganglion are precocious 

 and remarkably developed, even at birth. They cease their 

 rapid growth at about fifteen days of age, but increase very 

 slightly though steadily throughout life. 



