PHYSICS OF SOUND. 737 



direct and important relations to the physiology of hearing, while many are 

 of purely anatomical interest. Such facts as bear directly upon physiology 

 will be considered fully in connection with the uses of the internal ear. 



PHYSICS OF SOUND. 



The sketch just given of the general anatomical arrangement of the 

 auditory apparatus conveys a general idea of the uses of the different parts of 

 the ear. The waves of sound must be transmitted to the terminal extremi- 

 ties of the auditory nerve in the labyrinth. These waves are collected by the 

 pinna, are conducted to the membrana tympani through the external auditory 

 meatus, produce vibrations of the membrana tympani, are conducted by the 

 chain of ossicles to the openings in the labyrinth and are communicated 

 through the fluids of the labyrinth to the ultimate nervous filaments. The 

 free passage of air through the external meatus and the communications 

 of the cavity of the tympanum with the mastoid cells, and by the Eustachian 

 tube, with the pharynx, are necessary to the proper vibration of the mem- 

 brana tympani ; the integrity of the ossicles and of their ligaments and mus- 

 cles is essential to the proper conduction of sound to the labyrinth ; the 

 presence of liquid in the labyrinth is a condition essential to the conduction 

 of the waves to the filaments of distribution of the auditory nerves ; and 

 finally, from the labyrinth, the nerves pass through the internal auditory mea- 

 tus, to the auditory centre in the brain, where the auditory impressions are 

 appreciated. 



Most of the points in acoustics which are essential to the comprehension 

 of the physiology of audition are definitely settled. The theories of the prop- 

 agation of sound involve wave-action, concerning which there is no dispute 

 among physicists. 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 distin- 

 guished 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 farther, 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 pro- 

 duction of others which correspond to certain of the components of the pre- 

 dominating note. For example, if there be added to a single note, the third, 



