GROWTH OF THE INNER EAR OF ALBINO RAT 117 



We have already noted that at birth the greater epithelial 

 ridge constitutes the main part of the tympanic wall, and the 

 lesser epithelial ridge, from which arises later the most important 

 organ, is represented by a small and undeveloped prominence. 

 With age this greater ridge disappears gradually and is trans- 

 formed into a furrow lined with low epithelial cells, the sulcus 

 spiralis internus (Waldeyer). These changes appear first at the 

 base and then pass gradually to the upper turns. In the lesser 

 ridge also there are important developmental changes. At first 

 the hair cells and pillar cells grow, and just before the special 

 function appears, striking changes are seen in Deiters' and 

 Hensen's cells. These increase, especially in their length, very 

 rapidly. 



Thus the papilla spiralis, which hitherto had its highest 

 point at the summit of the arch of Corti, shows a remarkable 

 change of form, as the outer part of the papilla increases its 

 height, so that finally Hensen's cells mark the highest point 

 in the papilla. The surface then ceases to be parallel to the 

 basilar membrane, and slopes inward, making with the basilar 

 membrane an acute angle opening outward. At the same time 

 the papilla spiralis appears to be shifted inward i.e., towards 

 the axis. 



Kolliker has described how the cells, from which the pillars or 

 rods of Corti arise, at first stand nearly parallel, but later separate 

 at their base. He thought that this "von einem Langen- 

 wachstum (?) der Zellen selbst oder ihrer Grundlage, der Mem- 

 brana basilaris, abhiingen kann. " 



Hensen ('63) first studied this interesting problem in the ox 

 and found it to depend on a peculiar process. He regarded the 

 inward migration as taking place chiefly in the inner pillar cell. 

 The outer pillar cell in the upper turn moves somewhat outward ; 

 in the base, however, inward. Moreover, the outer pillar cell 

 increases its length during the development of the papilla much 

 more than the inner does. Thus the summit of the arch of Corti 

 and therefore the papilla spiralis shifts inward on the basilar 

 membrane. 



