THE INTERNAL EAR OR LABYRINTH 779 



to have been derived. This localization, however, involves not only 

 a judgment regarding the intensity of the sounds as individually 

 perceived by the two ears, but also an analysis of the position of the 

 head and of the conjugate deviation of the eyes. Naturally, a median 

 localization of the sound necessitates an equally intense activation of 

 the two receptors and a lateral localization, an unequal activation. 

 In the latter case, our judgments as to right and left, are surprisingly 

 accurate, although we are frequently in error as to whether the sound 

 has arisen in front or behind us, above or below us. Consequently, 

 our ears act in the manner of the two eyes during binocular vision, 

 our judgments regarding the special relationship of objects being 

 derived from the two visual fields. It seems doubtful, however, that 

 our judgments regarding the direction and distance of sounds are 

 much less exact than those pertaining to our visual impressions. Thus, 

 a ventriloquist plays upon the judgment of other persons by altering 

 the quality of his vocal sounds in such a way that they imitate the 

 peculiarities of those sounds which he desires to impart to his hearers. 

 He thus makes 'use of perfectly normal mental concepts of sounds to 

 produce an erroneous impression. 



Conduction of Sound Waves by the Cranial Bones. It has been 

 pointed out that the organ of Corti is activated by the vibrations in the 

 neighboring endolymph and perilymph, and that the latter are ordi- 

 narily the result of the oscillation of the ossicles in consequence of 

 sound waves. But, conditions may also arise in space which allow of a 

 direct transfer of these waves to the bones of the cranium and in turn 

 evoke a vibration of the lymph in the internal ear. Thirdly, it is pos- 

 sible to produce these vibrations by bringing a resonant body, such as a 

 tuning fork, in direct contact with one of the cranial bones. If placed 

 upon the region of the interparietal suture, the localization will be 

 median in character, for the reason that both ears are now affected in 

 an equal measure. If one of the ears is then protected by placing the 

 tip of a finger into the auditory meatus, the sound is immediately 

 diverted into this ear, and, if both ears are shielded in this way, again 

 into the midline of the cranium. 1 In explaining this phenomenon, 

 it must be remembered that the oscillations of the lymph resulting in 

 consequence of this direct transmission of the sound waves, are also 

 transferred to the ossicles and to the eardrum. If the ear is now held 

 shut, the initial energy of the vibration in lymph is prevented from 

 being spent in this way, and hence, must be able to act with greater 

 intensity upon this particular receptor. In all these cases, however, 

 the projection is intracranial, as against the extracranial localization 

 noted whenever the sound waves are permitted to enter in the normal 

 way through the auditory meatus. 



Subjective sensations of sounds, such as ringing in the ears, most 

 commonly arise in consequence of a local or general hypersensitiveness 

 of the nervous system. This condition leads to spastic contractions 



1 Weber, Archiv fur Ohrenheilkunde, xviii, 1882, 130. 



