AUDITORY SENSATIONS 



575 



there is a single row, on the outer side three rows of hair-cells. Between 

 the hair-cells are the sustentacular cells, or cells of Deiters, the peri- 

 pheral processes from which join together so as to form a reticulate 

 membrane over the hair-cells, the hairs themselves projecting through 

 orifices in the membrane. Resting on the upper surface of the mem- 

 brana reticularis is the membrana tectoria. To this membrane is often 

 ascribed a damping effect on the vibrations of the structures below. 

 Any movement of the basilar membrane would be transmitted to the 

 rods of Corti, and by these to the overlying hair-cells. With every 

 vibration these would move in the line of their long axis so that their 

 hairs would move up and down in the membrana reticularis and 

 possibly strike against the under surface of the membrana tectoria. 



m.t 



B.M 



FIG. 250. Section through the end-organ of the auditory nerve in the 



cochlea (organ of Corti). 



BM, basilar membrane ; c, canal of Corti ; RC, rods of Corti ; IH and 

 OH, inner and outer hair-cells ; so, sustentacular cells ; An, auditory nerve ; 

 rat, membrana tectoria. 



The fibres of the auditory nerve pass up through the column of the 

 cochlea, through the bipolar ganglion-cells which form the spiral 

 ganglion, and then out along grooves in the spiral lamina to end in 

 arborisations, partly in the inner hair-cells and partly among the outer 

 hair-cells. 



The complexity of the structure above described suggests that a 

 large amount of discriminating and analysing power possessed by the 

 ear for sounds of different qualities is determined by the differentia- 

 tion of the end-organ itself. Not only are we able to appreciate 

 differences in amplitude and pitch of the sound waves which arrive 

 at the ear, but we are also capable of analysing the compound sounds 

 and determining the simple tones out of which they have been com- 

 posed. This power of analysis must be due either to the presence of 

 some battery of resonators in the end-organs of the auditory nerve, 

 or to the existence of a large number of different nerve fibres, each of 

 which is excited only by a distinct number of vibrations per second, or, 

 finally, we must assume that the end-organ of hearing is affected as a 

 whole and that the nerve fibres transmit to the brain the different 

 forms of wave caused by various complex sounds, the analysis being 



