PHYSICS OF SOUND 721 



rotunda (2, Fig. 188), closed in the natural state by a membrane, the 

 secondary membrana tympani, lies between the lower portion of the 

 cochlea and the cavity of the tympanum. 



What is called the membranous labyrinth is contained within the 

 bony parts just described. Some of the anatomical points connected 

 with its structure and the distribution and connections of the auditory 

 nerve have direct and important relations to the physiology of hearing, 

 while many are of purely anatomical interest. Such facts as bear 

 directly on physiology will be considered in connection with the uses 

 of the internal ear. 



PHYSICS OF SOUND 



Most of the points in acoustics that are essential to the comprehen- 

 sion of the physiology of audition are definitely settled. The theories 

 of the propagation of sound involve wave-action, concerning which there 

 is no question. For the conduction of sound a ponderable medium is 

 essential ; and it is not necessary, as in the case of the undulatory 

 theory of light, to assume the existence of an imponderable ether. The 

 human ear, though perhaps not so acute as the auditory apparatus of 

 some of the inferior animals, not only appreciates irregular waves, such 

 as produce noise as distinguished from sounds called musical, but is 

 capable of distinguishing regular waves, as in simple musical sounds 

 and harmonious combinations. 



In music certain successions of regular sounds are agreeable to the 

 ear and constitute what is called melody. Again, there is appreciation, 

 not only of the intensity of sounds, both noisy and musical, but of pitch 

 and different qualities, particularly in music. Still further, musical notes 

 may be resolved into certain invariable component parts, such as the 

 octave, the third, fifth etc. These components of what were formerly 

 supposed to be simple sounds which may be isolated by artificial 

 means, to be described farther on are called tones; while the sounds 

 themselves, produced by the union of the different tones, are called 

 notes, which may themselves be combined to form chords. 



The quality of musical sounds may be modified by the simultaneous 

 production of others which correspond to certain of the components of 

 the predominating note. For example, if there is added to a single 

 note, the third, fifth and octave, the result is a major chord, the sound 

 of which is different from that of a single note or of a note with its 

 octave. If the third is diminished by a semitone, there is a different 

 quality, which is peculiar to minor chords. In this way a great variety 

 of musical sounds may be made on a single instrument, as the piano ; 

 and by the harmonious combinations of the notes of different instru- 



