826 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the vibration-rate increases the pitch is elevated, and vice versd. If some body 

 capable of producing sound should have its rate of vibration changed grad- 

 ually from 5 or 10 vibrations per second to 50,000 per second, no sensation 

 of sound would be aroused until the vibrations reached the rate of about from 

 16 to 24 per second. The droning note of the 16-foot organ-pipe and the 

 lowest bass of the piano represent a vibration-rate of 33 per second. In 

 most persons sounds cease to be audible when the air-waves have a fre- 

 quency of 16,000 per second, though to some the note produced by 40,000 

 vibrations is perceptible. It seems clear that some animals hear tones whose 

 pitch is so elevated as to make them inaudible to human ears. When a mov- 

 ing bell or whistle, as of a locomotive, rapidly approaches, its pitch seems to 

 rise, and then to fall as it recedes. The reason for this variation is that the 

 motion of the locomotive adds to or subtracts from the number of sound- 

 waves reaching the ear in a given time. In musical execution and in the 

 ordinary uses of life the limits in the pitch of sounds are much narrower. 

 Thus, as just stated, the lowest bass of the piano (C^ represents a vibration- 

 rate of 33 in a second, while the highest treble (c"" r ) has that of 4224. As 

 to the absolute number of vibrations necessary to produce the sensation of 

 sound, it has been found that 2 or 3 vibrations excite the sensation of a mere 

 stroke ; 4 or 5 vibrations are necessary to give a tone ; and some 20 or 40 are 

 required to develop the full musical qualities of a tone. 1 That is to say, when 

 a musical tone falls upon the ear its characteristics cannot be appreciated until 

 20 to 40 vibrations have been completed. 



Thus, from a physical scale representing aerial vibrations of indefinitely 

 various rapidity the mind selects and appreciates as sound a very small 

 fraction. 



Tympanic Membrane as an Organ of Pressure-sense. There is good 

 reason to suppose that variations in air-pressure succeeding one another too 

 slowly or too irregularly to produce sound-sensation are still of great import- 

 ance in the extensive realm of sensations which but obscurely excite our con- 

 sciousness. Slow inward movements of the tympanic membrane may still 

 give rise to a perception of external changes. Thus, a blind man has been 

 able to say correctly that he has passed by a fence, and whether it be of solid 

 board or of open picket. If any one with closed eyes holds a book at half-arm's 

 length in front of the ear, a different sensation will be experienced according 

 as the book is turned flat or edgewise to the face ;' the feeling is one of " shut- 

 in-ness " or " open-ness," respectively. The air is in ceaseless agitation, and 

 its waves, striking against various objects, must be reflected to the ear with an 

 intensity dependent on the position and the physical character of the reflecting 

 media. We may assert that the tympanic membrane is the peripheral organ 

 of a pressure-sense by which we become more or less accurately aware of the 

 nature and position of surrounding objects, irrespective of the sensations of 

 sight and hearing. Whether that group of sensations depends on the excite- 



1 Mach: Physikalischen Notizen Lotos, Aug., 1873; V. Kries und Auerbach: Du Bois-Rey- 

 mond's Archiv fur Physiologic, 1877, p. 297 ; Helmholtz : Sensations of Tone, translated by Ellis. 



