THE SENSE OF EQUILIBRIUM 713 



unequal refraction is the double hearing of a single sound owing to the 

 sound coming to the ear through media of unequal conducting power. 

 The phenomenon depends on the unequal conducting power of two media 

 through which one and the same sound is transmitted to the ear. If a 

 small bell be sounded in water while the ears are closed by plugs, and a 

 solid conductor be interposed between the water and one ear, two sounds 

 will be heard differing in intensity and tone, one being conveyed to the 

 ear through the medium of the atmosphere, the other through the con- 

 ducting-rod. 



Subjective Sensations. Subjective sounds are the result of a state of irri- 

 tation or excitement of the auditory nerve produced by other causes than 

 sonorous impulses. A state of excitement of this nerve or its tracts 

 gives rise to the sensation of sound, hence the ringing and buzzing in the 

 ears heard by persons of irritable and exhausted nervous system, and by 

 patients with cerebral disease, or disease of the auditory nerve itself; hence 

 also the noise in the ears heard for some time after a long journey in a rat- 

 tling, noisy vehicle. Ritter found that electric currents also excite sounds in 

 the ears. From the above truly subjective sound we must distinguish those 

 dependent, not on a state of the auditory nerve itself merely, but on sonorous 

 vibrations excited in the auditory apparatus. Such are the buzzing sounds 

 attendant on vascular congestion of the head and ear or on aneurysmal dilata- 

 tion of the vessels. Frequently even the simple pulsatory circulation of the 

 blood in the ear is heard. To the sounds of this class belong also the buzz 

 or hum heard during the contraction of the palatine muscles in the act of 

 yawning, during the forcing of air into the tympanum so as to make tense the 

 membrana tympani. 



Irritation or excitement of the auditory nerve may give rise to move- 

 ments in the body and to sensations seemingly from other organs of sense. 

 In both cases it is probable that associated centers of the brain stem and 

 connected areas of the cortex come into play. An intense and sudden noise 

 excites in every person closure of the eyelids, and in nervous individuals 

 a start of the whole body or an unpleasant sensation through the body 

 like that produced by an electric shock. 



THE SENSE OF EQUILIBRIUM. 



Although the utriculus, sacculus, and semicircular canals form the major 

 part of the labyrinth and are closely associated with the cochlea in develop- 

 ment, there is increasing evidence that these structures are not concerned 

 with hearing, but rather with a sense of equilibrium. This view has been 

 strengthened by recent investigations into the anatomical relations of the 

 different elements in the auditory nerve, figure 435. 



