294 HUMAN PHYSIOLOGY 



crest of one corresponds to the trough of the other, beats 

 are heard, i.e. periodic increase and decrease in the auditory 

 sensation. Beats occurring more frequently than 32 per 

 second cause an auditory sensation called beat-tone. 



These heat-tones are a purely subjective -phenomenon ; they can- 

 not, like other tones of a mixed sound, be demonstrated by the reso- 

 nator, for they do not affect the resonator. Hence the production 

 of auditory sensations by such tones cannot be explained by Helm- 

 holtz's theory of resonants. 



The sensations of noises are produced by irregular vibra- 

 tions in which now one, now another portion of the basilar 

 membrane is set in vibration. 



In the sense of hearing, as in sight, there are certain 

 phenomena produced by the rise and fall of the auditory 

 sensation, as also the phenomenon of fatigue. 



Two sounds following each other are still heard as separate 

 sounds if the interval between them is not less than o. I 

 second. 



The judgment of the direction and distance from which a 

 sound comes is very imperfect. Both ears serve in judging 

 of the direction of the sound, it coming from the direction 

 towards which the ear most stimulated is turned. 



2. THE SENSE ORGANS FOR PERCEIVING THE 

 POSITION AND MOVEMENTS OF THE HEAD 



The posterior part of the bony labyrinth is composed of 

 the three semicircular canals, which are bony canals bent in 

 the form of a C. They originate and end at the vestibule. 

 Each canal has at both ends a dilation (ampulla). The 

 planes of the superior-anterior canal lie in the vertical longi- 

 tudinal plane ; that of the inferior posterior in the vertical 

 transverse ; that of the lateral in the horizontal ' plane. 

 Hence the three planes of the semicircular canals are per- 

 pendicular to each other. The bony canals surround the 

 membranous labyrinth (see page 291). 



The thin walls of the membranous labyrinth are thickened 

 in the utriculus and in the sacculus (maculae acusticaj utriculi 

 et sacculi) and in the ampulla; (crista; acusticae). 



