804 HEARING, SMELL, AND TASTE. 



action has been suggested for the membrane of Corti (membrana tectoria) 

 overhanging the fibres and hair-cells ; and some writers have supposed that 

 muscular fibres present in the planum semilunare may by tightening the 

 basilar membrane serve as a sort of accommodation mechanism. 



It must, however, be borne in mind that even making the fullest allow- 

 ance for the assistance afforded us by the organ of Corti, the appreciation 

 of any sound is ultimately a mental act. The analysis of the vibrations by 

 the fibres of Corti or the basilar membrane is simply preliminary to a 

 synthesis of the sensory impulses so generated into a complex sensation. 

 We do not receive a distinct series of specific auditory impulses resulting in 

 a specific sensation for every possible variation in the wave-length of sonor- 

 ous vibrations any more than we receive a distinct series of specific visual 

 impulses for every possible wave-length of luminous vibrations. In each 

 case we have probably a number of primary sensations, from the various 

 mingling of which, in different proportions, our varied complex sensations 

 arise ; the difference between the eye and the ear being that whereas in the 

 former the number of primary sensations appears to be limited to three or 

 at most to six, in the latter, thanks to the organ of Corti, the number is 

 very large ; what the exact number is we cannot at present tell. Our ap- 

 preciation for a sound is at bottom an appreciation of the combined effect 

 produced by the relative intensities to which the primary auditory sensations 

 are, with the help of the organ of Corti, excited by the sound. 



718. Whatever be the explanation of the manner in which our distinct 

 auditory sensations arise, the range and precision of our appreciation of 

 musical sounds is very great. Vibrations with a recurrence below 30 a sec- 

 ond l are unable to produce a sensation of sound ; if the waves are powerful 

 enough we may feel them, but we do not hear them if the vibrations are sim- 

 ple, and such as would give rise to a pure tone ; if the fundamental tone is 

 accompanied by overtones we may hear these, and are thus apt to say we 

 hear the former when in reality we only hear the latter. The note of the 

 16-feet organ pipe, 33 vibrations a second, gives us the sensation of a droning 

 sound. A tone of forty vibrations is, however, quite distinct. In the other 

 direction it is possible to hear a note caused by 38,000 vibrations a second, 

 though the limit for most persons is far lower about 16,000. Some persons 

 hear grave sounds more easily than high ones and vice versa. This may be 

 so pronounced as to justify the subjects being spoken of as deaf to grave or 

 high tones respectively. The range in different animals is very different. 



The power of distinguishing one note from another varies, as is well 

 known, in different individuals, according as they have or have not a " mu- 

 sical ear." A well-trained ear can distinguish the difference of a single or 

 even of a half vibration a second, and that through a long range of notes. 

 The range of an ordinary appreciation of tones lies between 40 and 4000 

 vibrations a second, i. e., between the lowest bass C (Cj 32 vibrations) and 

 the highest treble C (C 5 4224 vibrations) of the piano ; tones above and 

 below these, even when audible, being distinguished from each other with 

 great difficulty. 



719. When two consecutive sounds follow each other at a sufficiently 

 short interval the sensations are fused into one. In this respect auditory 

 sensations are of shorter duration than ocular sensations. When ocular 

 sensations are repeated ten times in a second they become fused (p. 766), 

 whereas the ticks of a pendulum beating 100 in a second are readily audible 

 as distinct sounds. When two tuning-forks not quite in tune are struck to- 

 gether the interference of the vibrations gives rise to an alternating rise and 



1 By some authors the limit is placed as low as 24 or even 15 a second. 



