SENSE OF HEARING. 



623 



These tones are believed to form a composite wave, and as 

 such to strike the membrana tympani : and, according to the 

 " piano theory," this wave is analyzed into its component tones 

 by the basilar membrane, each of whose fibers is caused to vibrate 

 by a partial tone ; while according to the " telephone theory" 

 this analysis takes place in the brain. 



Resonators. That musical sounds possess this compound char- 

 acter Helmholtz demonstrated by means of resonators (Fig. 401), 

 which consist of metallic globes of various 

 sizes having two openings of unequal diam- 

 eter. If one of these is held with its 

 small opening to the ear, and the large 

 one is held toward a source of sound, the 

 resonator will resound when a tone is 

 emitted which corresponds to the vibration- 

 rate of its contained air, and to no other, 

 and by using a series of these the various 

 overtones may be identified. 



To sum up the properties of sounds and 



their causes, we may say that the amplitude of a wave determines 

 its intensity ; its vibration-frequency, its pitch ; its form, its quality. 



Semicircular Canals, Utricle, and Saccule. The utricle and 

 saccule have been regarded by some authorities as having the 

 function of responding to irregular vibrations, and as being 

 connected, therefore, with the perception of noises, while the 

 perception of musical sounds depends upon the cochlea; but 

 the consensus of opinion now is that, together with the semicir- 

 cular canals, they are connected with the important function of 

 the preservation of the equilibrium of the body, the utricle and 

 saccule with static equilibrium i. e., when the body is in a 

 state of rest ; the semicircular canals with dynamic equilibrium 

 i. e., when the body is in motion. This subject is discussed in 

 connection with the cerebellum (p. 493). 



FIG. 401. Kesonator. 



