AUDITORY PERCEPTIONS. 911 



as to form cavities opening backward, a sound coming from in front 

 will be heard as though coming from behind. The semicircular canals 

 probably also possess the function of determining the direction of sound, 

 as a sound coming from a certain direction must always strike one canal 

 (or the same one of both sides) more strongly than the others. For 

 example, the left horizontal canal is most strongly excited by a hori- 

 zontal sound-impulse coming from the .left side. Other investigators 

 ascribe to the tympanic membrane the function of localizing the sound, 

 inasmuch as certain portions of the membrane are often affected alone. 



As to the distance of the sound, the strength of the sound-vibrations 

 serves as a guide, an estimate of this having been formed as a result of 

 experience in the case of familiar sounds, but error in this connection is 

 not rare. 



A certain time always elapses before a tone is heard by the ear, 

 especially if the tone is faint (from i to 2 seconds). Likewise, the 

 auditory sensation persists for some time after the sound has ceased. 



Among subjective auditory sensations there may be distinguished : 



After-vibrations, especially of loud and persistent musical sounds. Roaring 

 in the ears, which often is caused by abnormalities in the circulation of the blood 

 (hyperemia or anemia) in the ear, depends upon mechanical irritation of the 

 auditory nerve-fibers (by the blood-current) . Abnormal pressure in the labyrinth 

 may also cause subjective noises. There are also undoubted subjective sensa- 

 tions of a purely nervous character in the entire nervous apparatus. Ringing 

 in the ears is ascribed partly to tetanic contraction of the tensor tympani muscle, 

 and partly to circulatory abnormalities. Also, many poisons, such as quinin, 

 and others, cause subjective noises. Entotic perceptions, which are due to processes 

 within the ear itself, consist in hearing the pulse-beat in the neighboring arteries, 

 and rushing noises in the blood-current, which are especially loud when there 

 is increased resonance in the ear, as from occlusion of the external canal or of the 

 tympanic cavity, or a collection of fluid in the latter; further, when the action 

 of the heart is increased, or in association with hyperesthesia of the auditory 

 nerve. Entotic sounds are produced also by crunching and crackling noises in 

 the articulations of the lower jaw, by muscular traction on the Eustachian tube, 

 and by the entrance of air into the tube or when the drum is moved inward or 

 outward. Other instances of subjective auditory sensations are referred to on 

 p. 701: Pathological. 



The ear exhibits the phenomena of fatigue; and this confines itself to that 

 tone or group of tones to which the ear is exposed, while its sensitiveness to 

 other tones is not demonstrably diminished. In the course of a few seconds, 

 however, complete recovery takes place. 



The auditory phenomena resulting from applications of the galvanic current 

 are discussed on p. 701. 



The following auditory after-sensations can be distinguished: (i) Those that 

 correspond to positive after-images, and may be designated echoes or resonances, 

 that is the after-sensation is so intimately related to the original sound that 

 they appear to be continuous. (2) There are also auditory after-sensations 

 attended with a pause between the end of the objective and the beginning of the 

 subjective tone. A splashing sound has been heard as a peculiar after-sensation 

 for a minute after a tone has been listened to for some time. (3) A third variety 

 of after-sensation may be compared with negative after-images. As such may 

 be designated the sense of striking stillness noted by Landois after interruption 

 of a long-continued, loud sound. 



Some persons associate the perception of tones with the appearance of sub- 

 jective sensations of color or of light (colored hearing), for example, the tone of 

 the trumpet with the sensation of yellow. Photisms of this kind are more rarely 

 observed when the nerves of taste, smell, and sensation are stimulated. There 

 are persons in whom every form of sensory impression necessarily calls forth 

 another subjective one. It is more frequent to find a sympathetic irritation 

 of sensory nerves in connection with loud, sharp sounds. In this category be- 

 longs the cold chill that many feel when they hear the squeaking of a slate-pencil, 

 or any similar shrill tone. 



