264 OUTPOSTS OF THE INTELLIGENCE SERVICE 



figure. A light powder like lycopodiuni, on the other hand, is 

 caught up in the air whirling above the moving parts of the plate 

 and comes to rest in heaps where the greatest movements of the 

 plate have been. 



Ewald used a rubber membrane stretched over a frame and 

 innnersed in fluid. Instead of producing figures in sand, he smeared 

 the surface of the rubber with oil and examined it under oblique 

 illumination by means of a very low-power microscope set at an 

 angle. The acoustic images so produced can readily be photo- 

 graphed, and are found to be characteristic for each note sounded 

 near the apparatus. These images produced on the basilar 

 membrane cause some hair cells to move and so transmit a 

 stimulus to the fibres of the cochlear nerve with which these cells 

 are richly provided. (According to some investigators, the moving 

 part is the membrana tectoria, which by coming against the hairs 

 of the cells excites the nerve fibres.) 



This theory and that of Helmholtz have this in common, that 

 the primary analysis of the sound takes place in the ear. This is 

 as one would expect from a study of all other external receptors, 

 and, as is the case with the other receptors, the ultimate analysis 

 must take place in the cortex. This final analytical act is not an 

 inborn process, but is acquired through experience. 



To conclude in the words of Helmholtz : " On reviewing the 

 whole arrangement there can be no doubt that Corti's organ 

 is an apparatus for receiving the vibrations of the basilar mem- 

 brane and for vibrating of itself, but our present knowledge is 

 not sufficient to determine with accuracy the manner in which 

 these vibrations take place." 



Intensity. Sounds differ not only in pitch, but in loudness and 

 in quality. We must go to Adrian's experiments to find out how 

 the ear differentiates between sounds differing in intensity. It 

 has been found that the frequency with which the electric impulses 

 pass towards the sensory centre when a sensory nerve fibre is 

 stimulated, varies with the intensity of the stimulation. That is, a 

 loud sound of a certain pitch would cause a rapid succession of 

 impulses to pass along certain fibres of the cochlear nerve, while 

 less intense sound of the same pitch would produce probably only 

 a single volley of impulses. Now although these sounds were 

 ])rodueed, say, by the same tuning fork, vigorously banged or 

 gently touched, and, therefore, of the same pitch, the louder sound 

 would appear to us to be of higher pitch than the less intense 

 sound. This is due to the more intense sound producing some 

 vibration in the hair-cells lying nearer the entrance of the disturb- 

 ance into the cochlea, i.e. nearer the base where the fibres are 



