CHAPTEE VIII. 



PHYSICS OF SOUND. 



Laws of sonorous vibrations Keflection and refraction of sound Xoise and 

 musical sounds Intensity, pitch, and quality of musical sounds Mechan- 

 ism of the siren Musical scale Harmonics, or overtones Resonators of 

 Helmholtz Resultant tones Summation tones Harmony Discord 

 Beats a cause of discord Tones by influence (consonance). 



THE sketch that we have given of the general anatomical 

 arrangement of the auditory apparatus conveys an idea of 

 the uses of the different parts of the ear. The waves of 

 sound must be transmitted to the terminal extremities of the 

 auditory nerve in the labyrinth. These waves are collected 

 by the pinna, are conducted to the membrana tympani 

 through the external auditory meat as, produce vibrations of 

 the membrana tympani, are conducted by the chain of ossi- 

 cles 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 mea- 

 tus 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 lig- 

 aments and muscles is essential to the proper conduction of 

 sound to the labyrinth ; the presence of liquid in the laby- 

 rinth 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 inter- 

 nal auditory meatus to the brain, where the auditory impres- 

 sions are appreciated. 



