THE SENSE OF HEARING. 109 



number of vibrations capable of producing a sound has been fixed at 1 6 per 

 second ; the highest pitched musical note capable of being heard has been 

 shown to be due to 38,000 vibrations per second. In the ascent of the 

 musical scale there is, therefore, a gradual increase in the number of vibra- 

 tions and a gradual increase in the pitch of the sounds. Between the two 

 extreme limits lies the range of audibility, which embraces eleven octaves, 

 of which seven are employed in the musical scale. 



The quality of sound depends upon a combination of the fundamental 

 vibration with certain secondary vibrations of sub divisions of the vibrating 

 body. These so-called over-tones vary in intensity and pitch, and by 

 modifying the form of the primary wave produce that which is termed 

 the quality of sound. 



Function of the Pinna and External Auditory Canal. In those 

 animals possessing movable ears, the pinna plays an important part in the 

 collection of sound-waves. In man, in whom the capability of moving the 

 pinna has been lost, it is doubtful if it is at all necessary for hearing. Never- 

 theless an individual with dull hearing may have the perception of sound 

 increased by placing the pinna at an angle of 45 to the side of the head. 

 The external auditory canal transmits the sonorous vibrations to the tym- 

 panic membrane. Owing to the obliquity of this canal it has been sup- 

 posed that the waves, concentrated at the concha, undergo a series of re- 

 flections on their way to the tympanic membrane, and, owing to the 

 position of this membrane, strike it almost perpendicularly. 



Function of the Tympanic Membrane. The function of the tym- 

 panic membrane appears to be the reception of sound vibrations by being 

 thrown by them into reciprocal vibrations which correspond in intensity and 

 amplitude. That this membrane actually reproduces all vibrations within 

 the range of audibility has been experimentally demonstrated. The mem- 

 brane not being fixed, as far as its tension is concerned, does not possess a 

 fixed fundamental note, like a stationary fixed membrane, and is therefore 

 just as well adapted for the reception of one set of vibrations as another. 

 This is made possible by variations in its tension in accordance with the 

 pitch of the sounds. In the absence of all sound the membrane is in a 

 condition of relaxation ; with the advent of sound-waves possessing a 

 gradual increase of pitch, as in the ascent of the musical scale, the tension 

 of the tympanic membrane is gradually increased until its maximum ten- 

 sion is reached at the upper limit of the range of audibility. By this 

 change in tension certain tones become perceptible and distinct, while 

 others become indistinct and inaudible. 



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