THE SENSE OF HEARING. 253 



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 funda- 

 mental vibration with certain secondary vibrations of subdivisions 

 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. Nevertheless an individual with dull hearing may 

 have the perception of sound increased by placing the pinna at an 

 angle of 45 degrees to the side of the head. The external auditory 

 canal transmits the sonorous vibrations to the tympanic membrane. 

 Owing to the obliquity of this canal it has been supposed that the 

 waves, concentrated at the concha, undergo a series of reflections 

 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 vibration^ 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 membrane not being fixed, so far as its tension 

 is concerned, does not possess a fixed fundamental note, like a sta- 

 tionary fixed membrane, and is, therefore, just as well adapted for 

 the reception of one set of vibrations as for 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 music scale, the 

 tension of the tympanic membrane is gradually increased until its 

 maximum tension is reached at the upper limit of the range of 

 audibility. By this change in tension certain tones become per- 

 ceptible and distinct, while others become indistinct and inaudible. 



Function of the Tensor Tympani Muscle. The function of 

 this muscle is, as its name indicates, to increase the tension of the 



