THE SENSE OF HEARING. 645 



vibration which can produce a distinct sound varies in different* 

 individuals from 14 to 18; the highest rate varies from 35,000 to 

 40,000 per second. Between these two extremes lies the range of 

 audibility, which embraces about n octaves. Vibrations less than 

 14 per second can not be perceived as a continuous sound; vibrations 

 beyond 40,000 also fail to be so perceived. In the ascent of the 

 music scale from the lowest to the highest regions there is a gradual 

 increase in the vibration frequency. 



The quality of a sound depends on the form of the vibration. It 

 is this feature which gives rise to those differences in sensations which 

 permit one to distinguish one instrument from another when both 

 are emitting the same note. The form of the sound-wave in any 

 given instance is the resultant of a combination of a fundamental 

 vibration and certain secondary vibrations of subdivisions of the 

 vibrating body. These secondary vibrations give rise to what is 

 known as overtones. By their union with and modification of the 

 fundamental vibration there is produced a special form of vibration 

 which gives rise not to a simple but a composite sensation. It is 

 for this reason that the same note of the piano, the violin, and the 

 human voice varies in quality. 



The Function of the Pinna and External Auditory Canal. 

 In those animals possessing movable ears the pinna plays an im- 

 portant part in the collection of sound-waves. In man it is doubtful 

 if it plays a part at all necessary for hearing. Nevertheless an indi- 

 vidual with defective hearing may have the perception of sound 

 increased by placing the pinna at an angle of 90 degrees to the side 

 of the head or by placing the hand behind it. The external auditory 

 canal transmits the sonorous vibrations to the tympanic membrane* 

 From the obliquity of this canal it has been supposed that the vibra- 

 tions, after passing the concha, undergo a series of reflections on their 

 way to the tympanic membrane, which, owing to its inclination, would 

 be struck by them in a much more effective manner. 



The Function of the Tympanic Membrane. The function of 

 the tympanic membrane is the reception of the atmospheric vibrations 

 which are transmitted to it. This it does by vibrating in unison with 

 them. The vibrations which the membrane exhibits correspond in 

 amplitude, in frequency, and in form to those of the atmosphere. 

 That this membrane actually reproduces all vibrations within the 

 range of audibility has been experimentally demonstrated. The 

 membrane not being fixed, as far as its tension is concerned, does 

 not possess a fixed fundamental note, like a stationary fixed mem- 

 brane, 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 or frequency of the atmos- 

 pheric vibrations. In the absence of vibration the membrane is in 



