710 THE SENSES 



nose and mouth be closed and the cheeks blown out, a sense of pressure is 

 produced in both ears the moment we swallow. This is due, doubtless, to 

 the bulging out of the tympanic membrane by the compressed air, which at 

 that moment enters the Eustachian tube. The principal office of the Eusta- 

 chian tube has relation to the prevention of the effects of increased tension of 

 the membrana tympani. Its existence and openness will provide for the 

 maintenance of the equilibrium between the air within the tympanum and 

 the external air, so as to prevent the inordinate tension of the membrana 

 tympani which would be produced by too great or too little pressure on either 

 side. While discharging this office it serves as an outlet for mucus. If the 

 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 sensory 

 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 con- 

 cerned 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 vibration 

 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 interpreted 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 



