v THE SENSE OF HEAKING 219 



tectorial membrane, and that it is these impacts which cause the 

 active stimulation of the auditory nerve. 



E. ter-Kuile (1900) pronounced against this hypothesis. On 

 the strength of anatomical and physical considerations he main- 

 tained that the membrana tectoria and the underlying organ of 

 Corti cannot vibrate in opposite directions, but must necessarily 

 vibrate together in the same direction. He assumed that in 

 consequence of the vibrations of the basilar membrane the 

 arches of Corti rotated round the foot of the inner rods, which 

 form the centre of rotation, causing oscillations from above down- 

 wards and from below upwards of the entire organ of Corti, 

 including the membrana tectoria (Fig. 88). These oscillations 

 produce a slight flexion of the filaments of the hair-cells, which 

 in their turn act as a stimulus upon the nerve-endings. 



To this hypothesis of ter-Kuile the objection can be raised 

 that the basilar membrane is ill -adapted to oscillate from above 



PIG. 88. Diagram of organ of Corti, and the changes produced in it by the vibrations of the 

 basilar membrane. (After ter-Kuile.) 



downwards on gentle impacts of the endolymph. As shown by 

 Eetzius (Fig. 8 5) i the basilar membrane is lined on the side of 

 the scala tympani by a dense layer of connective tissue, and on the 

 side towards the scala vestibuli it supports almost the whole organ 

 of Corti, viz. the outer rods, the layer of Deiters' and that of 

 Hensen's cells, and finally the three layers of the external hair- 

 cells or cells of Corti. It cannot therefore be the most mobile 

 part of the auditory apparatus. Siebenmann (1900) and Kishi 

 (1907) concluded from these anatomical considerations that the 

 true vibrating portion is the membrana tectoria, the physical 

 and structural characters of which are such that it can be 

 readily influenced by slight alterations of pressure transmitted 

 by the stapes to the endolymph. It is highly elastic ; according 

 to Kishi it is under normal conditions tightly stretched over 

 the organ of Corti; it is surrounded by endolymph, and is in 

 contact solely with the filaments of the hair-cells (Fig. 87). The 

 membrana tectoria is thus eminently suited for transmitting the 

 mechanical stimulus necessary for exciting the hair -cells by 

 means of the slight deformations of the hairlets. 



