122 ARRANGEMENT AND STRUCTURE OF SUSTENTACULAR CELLS 



resting upon the membrana basilaris, and farther removed from the axis of the 

 framework than their more superficial portion in connection with the lower pole of 

 the hair-cells. Due to this pressure from the axia' framework, and especially from 

 the feet of the outer pillars, the nucleated portion of the cells of Deiters tends to 

 follow an oblique direction upward and inward. In other words, the superficial 

 part of the nucleated cell body will tend to join the contiguous spiral interstice into 

 which its phalanx process has already been projected during the first stage of devel- 

 opment (fig. 5, op, d', d n ) and the nucleated part of the cells of Deiters of the third 

 row will be pushed beneath the lower pole of the hair-cells of the third row (fig. 5, d ui ) . 

 Another mechanical factor plays an important part in this shifting of the outer 

 supporting cells. On enlarging and elongating, the outer hair-cells not only expel 

 from their spiral row the supporting elements and efface the intercellular spaces 

 left by them, but they also extend laterally so that each sensory row becomes 

 thicker, particularly at the level of the nuclei, while the diameters of the apices of 

 the hair-cells do not change during the second stage of development. This broad- 

 ening of the three outer sensory rows results in a more obvious enlargement of this 

 part of the lesser ridge, which in turn causes a bulging towards the outer wall of the 

 cochlea (figs. 5 and 15), since the presence of the tough axial framework renders 

 expansion towards the axis impossible. Hence the outer hair-cells run obliquely 

 (fig. 14), downward and outward. While the upper pole of the nucleated portion 

 of the cells of Deiters tends to turn inward, the lower pole of the sensory elements 

 tends to turn outward, so that the axis of the interstices between the sensory rows 

 gradually blends with that of the basal portion of the supporting elements (fig. 6). 

 This result is obtained by the concomitant action of a third mechanical factor, the 

 elongation of the hair-cells, and probably by the elongation of the cells of Deiters. 

 In figure 5 the nucleated colums (d', d'') do not encroach upon the more superficial 

 interstices, while in a more advanced stage (fig. 7) they obviously invade the lower 

 region of these spaces. This invasion is caused primarily by the elongation of the 

 hair-cells between the subjacent cell bodies of the supporting elements. No true 

 intercellular spaces exist at the level of the latter; nevertheless between them are 

 found spiral nerve bundles (N m , N iv , N v ) connected with the lower pole of the hair- 

 cells by ascending branches. Such a disposition is, of course, apt to facilitate the 

 invasion of these intercellular nerve spaces by the elongating sensory elements. 

 The explanation of the shifting of the phalanx processes of the cells of Deiters in a 

 spiral direction and toward the apex of the cochlea appears to be more difficult. 

 Before attempting any interpretation it must be borne in mind that the extremities 

 of these elements, the basal resting upon the membrana basilaris, and the superficial 

 interpolated in the membrana recticularis, always remain fixed. Of course these 

 membranes may increase in size, regularly or irregularly, so that a point A upon the 

 former, just beneath a point A 1 upon the latter, may, by a process of unequal devel- 

 opment, become situated just below a point B 1 or D\ the point A 1 being removed 

 farther from basal part of the cochlea; but point A 1 will never take the place of 

 point B { , such as has already been proved is the case with portions of the cell bodies 



