THE SENSES. 195 



hearing, endures somewhat longer than the period during which the un- 

 dulations are passing through the ear. If, however, the impressions of 

 the same sound be very long continued, or constantly repeated for a long 

 time, then the sensation produced may continue for a very long time, 

 more than twelve or twenty-four hours even, after the original cause of 

 the sound has ceased. 



Binaural Sensations. Corresponding to the double vision of the 

 same object with the two eyes, is the double hearing with the two ears; 

 and analogous to the double vision with one eye, dependent on unequal 

 refraction, is 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 are recorded, 

 however, by Sauvages and Itard. 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 the 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 conduct- 

 ing-rod. 



Subjective Sensations of Sound. Subjective sounds are the result 

 of a state of irritation 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 rattling noisy vehicle. Hitter found that 

 electricity also excites a sound in the ears. From the above truly subjec- 

 tive sound we must distinguish those dependent, not 011 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 aneurismal dilatation 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; and in the act of blowing the nose, as well as dur- 

 ing the forcible depression of the lower jaw. 



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, came into play. An intense and sudden noise ex- 

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



