780 THE SENSE OF HEARING 



of the tensor tympani. Humming or rushing noises most generally 

 have their origin in circulatory disturbances (hemic murmers). A 

 common entotic phenomenon is the audibility of the heart beat when 

 the left side of the head is placed upon a pillow. This position increases 

 the resonance of the left internal ear in a greater degree than that of 

 the right. 



The Limits of Hearing. Auditory Fatigue. While inheritance and 

 training play an important part in determining our range of the appre- 

 ciation of sounds, it is usually stated that the human ear cannot be 

 activated by musical tones possessing a lesser vibratory rate than 24 to 

 30 in a second. Some persons, however, are capable of perceiving 

 sounds of only 16 vibrations to the second. Below this limit mere 

 sensations of pressure are produced, although some of these low sounds 

 may give rise to high overtones which are clearly recognizable. The 

 upper limit of audibility of musical sounds is generally placed at 

 40,000 double vibrations in a second. Beyond this point, the notes 

 give rise to unpleasant sensations rather than to true sounds and can- 

 not be used in music. At about 60,000 they become inaudible. A 

 convenient way in which the range of hearing may be tested is to 

 strike steel rods of varying vibrating frequency (Konig). 



It is commonly accepted that rhythmically repeated or long con- 

 tinued sounds eventually give rise to a condition of auditory fatigue. 

 In many cases, however, this fatigue is only apparent and is due rather 

 to inattention. Thus, the ticking of a watch may become inaudible 

 to us, because other matters temporarily occupy our attention. In- 

 tense sounds produce a peculiar deafening effect, rather than a true 

 fatigue. 



The Perception of Noises. Noises form a physical as well as a 

 physiological entity, because they lack the rhythmic and harmonic 

 character of musical sounds. In spite of this fact, however, they 

 possess a definite pitch, quality and intensity. Helmholtz has advo- 

 cated the view that they are mediated by a special receptor formed 

 by the sensory epithelium of the utricle and saccule. Exner, 1 on 

 the other hand, states that they are also received by the organ of 

 Corti, and that they activate a large number of resonators, in contra- 

 distinction to the musical sounds which affect only particular ones. 

 Being a believer in the Helmholtz resonance theory, Exner holds that 

 they stimulate the radial fibers of the basilar membrane. 



1 Pfluger's Archiv, xiii, 1876, 228. 



