Il6 HEARING. 



spiralis of which the vibration-rate agrees with those of 

 vibrations existing in the liquid. 



Sensations of sound are divided, according to the cha- 

 racter (form) of the air-vibrations which occasion them, 

 into non-musical sounds or noises, and musical sounds 

 or " tones." Of tones, simple and compound are distin- 

 guished. The former are heard when the air molecules 

 at the external surface of the membrana tympani are in 

 simple pendular vibration ; the latter when the vibrations 

 are compounded of vibrations of different rates of fre- 

 quency, of which the relations to each other may be 1:2, 

 1:3, 1:4, 1:5, and so on in the same order. The tones 

 which correspond to each of these simple constituent 

 vibrations, are called " partial tones." According to the 

 number, relative strength and relative frequency of the 

 simple tones into which they can be resolved, compound 

 tones differ in timbre or quality. Tones whether simple 

 or compound differ also in pitch : the pitch of a simple 

 tone is expressed by the number of its vibrations in a 

 second : that of a compound tone by the vibration-number 

 of its predominant partial-tone. 



The middle ear. The cavity of the tympanum across 

 which sonorous vibrations are transmitted by the ear 

 bones, communicates with the pharynx by the Eustachian 

 tube : the pharyngeal end of this tube is, however, usually 

 closed and can only be opened by bringing into action 

 the muscles of deglutition. The external wall of the 

 cavity is formed by the membrana tympani, which, by the 

 arrangement of the radiating and annular fibres which 

 compose its two layers, by the conical form of its internal 

 surface, and by the mode of attachment of the handle of 

 the malleus, is fitted for its function the passive reception of 

 the motions communicated to it by the vibrating air parti- 

 cles at its surface. In the communication of the move- 

 ments of the membrana tympani to the foramen ovale, the 

 malleus and incus act as one piece ; for, so long as the mem- 



