CHAP, iv.] HEARING. 221 



nerve is fundamentally the same as that of the vestibular nerve. 

 Whether the fibrillae are actually continuous with the substance 

 of the hair-cells, or whether the material of the one is only in close 

 juxtaposition with the material of the other must be left for the 

 present uncertain. We may however conclude that it is in the 

 hair-cells, inner and outer, that auditory impulses are originated, 

 and that the other parts of the organ of Corti are subsidiary in 

 function, helping or guiding in some way the development of the 

 nervous impulses-, but not actually giving rise to them. 



The space of the tunnel between the rods of Corti appears to 

 be occupied by fluid, which gives support to the bundles of fibrillae 

 stretching across the tunnel. This space is continuous, through 

 the clefts between the limbs of the outer rods, with the space 

 between the outer rods and the first row of hair-cells, and so with 

 the narrow spaces between the several rows of hair-cells. But the 

 whole of this space is completely shut off from the endolymph space 

 of the canalis cochlearis by the reticulate membrane above, by the 

 closely packed epithelial cells on the inner side of the inner hair- 

 cells to the inside, and to the outside by the closely packed cells 

 of Hensen lying on the outside of the outermost row of outer hair- 

 cells ; it is also completely shut off from the scala tympani by the 

 basilar membrane. 



837. The organ of Corti is overhung by a peculiar structure 

 projecting from the vestibular lip of the spiral lamina, and known 

 as the factorial membrane, membrana tectoria (Fig. 179 m.t.). 

 It is in a fresh state soft and elastic, is fibrillated in a radial 

 direction, and indeed appears to be largely composed of fine fibres 

 or fibrils ; these resist the action of acetic acid. It begins on the 

 surface of the limbus of the spiral lamina not far from the attach- 

 ment of Reissner's membrane ; it is thin over the vestibular lip, 

 but beyond the free edge of the lip, overhanging the inner hair- 

 cells and rods of Corti, it becomes thick, ending gradually in a 

 thin and often ragged edge at about the zone of the outermost 

 row of outer hair-cells. It is frequently pitted or otherwise 

 sculptured on its under surface. 



838. The features and the structure of the canalis cochlearis 

 and especially of the organ of Corti differ in details in different 

 mammals ; the description given above applies to man. We said 

 above that the number of rows of outer hair-cells was in man four ; 

 but in the lowermost turn of the spiral three only are present, and 

 even in the upper turns, the number four is not constant ; in 

 places five rows are sometimes seen. In the majority of mammals 

 there are three rows of outer hair-cells, but a fourth row is some- 

 times present. 



In the same cochlea, the features differ along the length of the 

 spiral, so that a vertical section from an upper whorl presents many 

 differences when compared with a section from a lower whorl. We 

 must not dwell on these differences ; but we may call attention to 



