SUBJECTIVE SENSATIONS OF HEARING. 411 



turbed, enter, with augmented force, the lymph fluid of the scala tym- 

 pani, and meet the corresponding undulations coming from the scala 



vestibuli, from which both systems are suppressed by interference. 

 # * #"# * * # 



"The small space through which the vibrations pass, and the rapid- 

 ity of their movements in fluids, cause the impressions they make on 

 the nervous sentient organ, and the perceptions they excite, to appear 

 as an instantaneous act. The mind has cognizance of them, however 

 diversified they may be, as a unity of sounds simultaneously instant 

 in action; whence it forms the compound idea of perfect harmony. 



" An analogous phenomenon occurs in vision. When a body, com- 

 posed of different forms and colors, is presented to the eye as a bou- 

 quet of flowers, a landscape, or picture each different form, color, 

 tint, and shading, are perceived blended, but perfect and distinct, 

 forming the image of a single object. Yet thousands of ether vibra- 

 tions are traversing the eye, and are exciting each a separate, dis- 

 tinct impression, without confusion, on the retina, and as many distinct 

 and separate perceptions, from which a corresponding compound idea 

 of a single object is formed by the mind." F. G. S.] 



The otoliihs, or otoconia, when present, are supposed to intensify 

 the sonorous undulations, at or near the fine extremities of the audi- 

 tory nerves of the vestibule, saccule, and ampullae of the semicircular 

 canals. 



The sense of hearing varies much, as regards acuteness, in different 

 persons; so also does the power of distinguishing differences in pitch. 

 The power of judging of the direction of sounds, would seem to be 

 almost wholly the result of habit. The different intensity of a given 

 sound, as heard by the two ears, may assist in the determination of a 

 knowledge of its direction ; but, to a certain extent, one ear will suffice 

 for this purpose. The notion of the distance of sounding bodies is 

 also acquired by habit, the mind chiefly judging from the relative loud- 

 ness or faintness of a known kind of sound ; but, in this respect, there 

 is great liability to deception, and hearing is not so accurate a guide 

 as sight. Like the other senses, hearing can be much improved by 

 education ; in the blind, it is so highly developed, that they are mainly, 

 if not entirely guided by it, in walking. The Indian by listening on 

 the ground, can detect the distant footfall of his enemy or prey. 



The auditory nerves can be excited by various internal mechanical 

 causes, operating generally through certain movements of the blood 

 and bloodvessels ; for example, in dilatation of the vessels from con- 

 gestion of the head, in extravasation of blood, in morbid conditions 

 of the circulation in the brain and internal ear, dependent on extreme 

 debility, in narcotic poisoning, and in great bodily collapse, as before 

 the commencement of fainting. Obstructions in the tympanum or 

 Eustachian tube may also produce abnormal noises in the head. It is 

 not yet determined whether electricity ca'n excite the auditory sense, 

 unless indirectly, by disturbance of the tympanic apparatus. 



Sonorous undulations conducted through the tympanum are re- 



