662 THE SENSES 



the double hearing of a single sound with one ear, owing to the sound coming 

 to the ear through media of unequal conducting power. The first kind of 

 double hearing is very rare; instances of it, however, have been recorded. 

 The second kind, which depends on the unequal conducting power of two 

 media through which the same sound is transmitted to the ear, may easily 

 be experienced. 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 con- 

 veyed to the ear through the medium of the atmosphere, the other through 

 the conducting-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, however induced, 

 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 is capable of giving rise to 

 movements in the body and to sensations in other organs of sense. In both 

 cases it is probable that the laws of reflex action, through the medium of the 

 brain, 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 throughout 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. 



