794 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS 



the opposite side are swallowed up and lost in the Eustachian tube. All 

 resonance and reflection of vibrations are suppressed, and the wave-systems 

 of sonorous vibrations traverse the tympanum undisturbed, enter, with aug- 

 mented force, the lymph fluid of the scala tympani, and meet the corre- 

 sponding undulations coming from the scala vestibuli, from which both sys- 

 tems are suppressed by interference. 



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

 pound idea of perfect harmony. 



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

 of different forms and colors, is presented to the eye as a bouquet 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 vibrations are traversing the eye, and are 

 exciting each a separate, distinct 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."] 



648. We have now to consider the functions of the accessory parts, the 

 External Ear, and the Meatus. The Cartilage of the external ear may 

 propagate sonorous vibrations in two ways ; by reflection, and by conduc- 

 tion. In reflection, the concha is the most important part, since it directs 

 the reflected undulations towards the tragus, whence they are thrown into 

 the auditory passage. The other inequalities of the external ear cannot pro- 

 mote hearing by reflection ; and the purpose of the extension of its cartilage 

 is evidently to receive the sonorous vibrations from the air, and to conduct 

 them to its source of attachment. In this point of view the inequalities be- 

 come of importance; for those elevations and depressions upon which the 

 undulations fall perpendicularly, will be affected by them in the most intense 

 degree ; and in consequence of the varied form and position of these in- 

 equalities, sonorous undulations, in whatever direction they may come, must 

 fall advantageously upon some of them. The functions of the Meatus ap- 

 pear to be threefold. The sonorous undulations entering from the atmos- 

 phere are propagated directly, without dispersion, to the membraua tym- 

 paui ; the sonorous undulations received on the external ear are conveyed 

 along the walls of the meatus to the membrana tympani ; whilst the air 

 which it contains, like all insulated masses of air, increases the intensity of 

 sounds by resonance. That in ordinary hearing, the direct transmission of 

 atmospheric vibrations to the membrana tympaui is the principal means of 

 exciting the reciprocal vibrations of the latter, is sufficiently evident; the 

 undulations which directly enter the passage, will pass straight on to the 

 membrane ; while those that enter obliquely will be reflected from side to 

 side, and at last will fall obliquely on the membrane, thus perhaps con- 

 tributing to the notion of direction. The power of the lining of the meatus 

 to conduct sound from the external ear, is made evident by the fact that, 

 when both ears are closely stopped, the sound of a pipe having its lower ex- 

 tremity covered by a membrane, is heard more distinctly when it is applied 

 to the cartilage of the external ear itself, than when it is placed in contact 

 with the surface of the head. The resonant action of the air in the tube is 

 easily demonstrated, by lengthening the passage by the introduction of another 



