HEARING 1033 



from each other, the components of a complex sound. When a 

 number of notes of different pitch are sounded together at the 

 same distance from the ear the disturbance which reaches the mem- 

 brana tympani is the physical resultant of all the disturbances pro- 

 duced by the individual notes, and it strikes upon the membrane as 

 a single wave. ' A single curve describes all that the ear can possibly 

 hear as the result of the most complicated musical performance. 

 ... In the complicated sound the variations of the pressure of the 

 air are more abrupt, more sudden, less smooth, and less distinctly 

 periodic than they are in softer, purer, and simpler sound. But the 

 superposition of the different effects is really a marvel of marvels ' 

 (Kelvin). The ear or brain must, therefore, possess the power of 

 resolving the complex vibrations into their constituents, else we 

 should have a mixed or blended sensation, and not a sensation in 

 which it is possible to distinguish the constituents of which it is 

 made up. Several hypotheses have been proposed to explain this 

 physiological analysis of sound, on the assumption that the analysis 

 takes place in the labyrinth. The most important, in spite of certain 

 defects, is still that of Helmholtz. 



Helmholtz attempted to explain the perception of pitch on the 

 assumption that in the internal ear there exists a series of resonators, 

 each of which is fitted to respond by sympathetic vibration to a 

 particular note, while the others are unaffected; just as when a note 

 is sung before an open piano it is taken up by the string which is 

 attuned to the same pitch and ignored by the rest. Let us sup- 

 pose that a given fibre of the auditory nerve ends in an organ which 

 is only set vibrating by waves impinging on it at the rate of 100 a 

 second, and that the end-organ of another fibre is only influenced 

 by waves with a frequency of 200 a second. Then, on the doctrine 

 of ' specific energy ' (according to which the sensation caused by 

 stimulation of a nerve depends not on the particular kind of stimu- 

 lus but on the anatomical connection of the nerve with certain 

 nerve centres), in whatever way the first fibre is excited, a sensation 

 corresponding to a note with a pitch of 100 a second will be per- 

 ceived. Whenever the second fibre is excited, the sensation will be 

 that of a note of 200 a second, or the octave of the first. If both 

 fibres are excited at the same time the two notes will be heard 

 together. Now, Hensen actually observed that in the auditory 

 organs of some crustaceans, the hair-like processes of certain 

 epithelial cells can be set swinging by waves of sound, and, further, 

 that they do not all vibrate to the same note unless the sound is 

 very loud. In the lobster there are between four and five hundred 

 of these hairs, varying in length from 14 //, to 740 //; and in some 

 insects, such as the locust, similar hairs, also graduated in length, 

 exist. 



To gain an anatomical basis for his theory, Helmholtz supposed 



