THE FUNCTION OF THE INTERNAL EAR 625 



tube were permanently open, the sound of one's own voice would probably 

 be greatly intensified, a condition which would of course interfere with the 

 perception of other sounds. At any rate, it is certain that sonorous vibra- 

 tions can be propagated up the tube to the tympanum by means of a catheter 

 inserted into the pharyngeal orifice of the Eustachian tube. 



The Function of the Internal Ear. The fluids of the labyrinth re- 

 ceive the sonorous vibrations at the fenestra ovalis and, we must assume, 

 conduct the same throughout the cavity. In all forms of organs of hearing 

 even to the simplest, liquid is the medium through which the auditory sen- 

 sory epithelium is stimulated. We have already seen that in the mammalian 

 ear there is a special mechanical arrangement to intensify the vibrations of 

 the fluid in the cochlear canal. 



The utriculus, sacculus, and semicircular canals are probably not concerned 

 with auditory function, but with the sense of equilibrium; hence they will 

 be discussed separately a little later. 



The cochlea is the special organ of hearing. When it is set in vibrations 

 the movement stimulates the sensory hair cells on the basement membrane, 

 producing a sensory impulse which is transmitted along the paths to the brain 

 and there produces an auditory sensation. If the stimulus results from a 

 disturbance of an explosive or non-harmonic nature, the sensation is inter- 

 preted as a noise. If the disturbance is rhythmic or harmonic and repeated 

 in sequence within certain limits of rate, then a tone is perceived. 



The intensity of sound, the energy of the disturbance, affects the basilar 

 membrane by producing motion of varying amplitude. This stimulates the 

 hair cells with greater or less intensity, which can be detected by the sensorium 

 as loudness. Loudness of the sound sensation is interpreted as intensity of 

 sound wave. 



The interpretation of pitch is accomplished by the ear through a wide 

 range of rates of vibration that produce sensations of tone. The average 

 person can perceive musical tones over a range of vibration of from sixty-four 

 double vibrations per second for the lower notes, to four thousand and ninety- 

 six for the higher notes. These limits may be extended to thirty per second 

 and forty thousand per second, respectively, but only a small number of 

 tones can be perceived outside of the narrower limits given above. This 

 extraordinary range of tone is conceivable only on the supposition of local- 

 ization of the stimulus in some part of the organ. Most physiologists look 

 to the basilar membrane and the organ of Corti for the localization. 



Suppose a simple tuning fork to be vibrating with a frequency of sixty- 

 four per second, then these waves will be conducted through the auditory 

 apparatus until they fall on the basilar membrane, and will set it in vibration 

 at the same rate. The exact type of the vibration is at present a matter of 

 inference. The piano theory of Helmholtz is probably the most satisfactory. 

 It assumes that the basilar membrane vibrates as would a number of strings 

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