SENSE OF HEARING THE SEMICIRCULAR CANALS. 793 



impression, reinforced by the resonance of the superimposed otoconise, and 

 exciting a single and distinct impulse and perception of sound. These bodies 

 act like the sounding-board of the piano. The sonorous vibration having 

 thus completed its office, the specific excitation of the sense of hearing must, 

 like that of the visual vibration, cease, or be suppressed. This occurs, in 

 part, in the ampulla, but mostly in the semicircular canals. 



' The vibrations of the endolymph, reaching the ampullse, are partially 

 broken and weakened at their openings: those entering the ampullee again 

 expand, losing thereby their impetus, and either die away against the mem- 

 branous walls, or come in contact with the vibrations of the perilymph or 

 their exterior. The two can scarcely be in perfect consonance of expansion 

 or condensation, and interference ensues, by which they are suppressed. In 

 this mode all the feebler vibrations are terminated. Those of greater force 

 enter simultaneously the two opposite openings of the semicircular canals. 

 The orifices and commencement of each canal differ as to size and form, and 

 consequently each entering wave of undulatory vibration is modified, thus 

 losing their consonance of expansion, and when they meet interference and 

 suppression result. Reflection of sonorous vibrations is completely provided 

 against. 



" Parallel conditions exist in the cochlea. Its two canals the superior 

 scala vestibuli, and the inferior scala tympaui are filled with lymph fluid, 

 continuous and identical with that of the vestibule. The first, the scala 

 vestibuli, is the principal seat of hearing. On its lamina spiralis is expanded 

 a sentient nervous structure, the recipient of the sonorous vibrations excita- 

 tive of the sense of hearing. It is the homotype of the retina of the eye. 

 The scala tympaui furnishes space for spreading out the filaments of the 

 nerve, but the terminal extremities pass through the membranous spiral 

 lamina, to be incorporated with the sentient organ of hearing in the superior 

 canal or scala vestibuli. The filaments of the inferior canal or scala tym- 

 paui are mere conductors of the nervous excitement of the auditive sentient 

 membrane. The scala tympani, similar to the semicircular canals, has no 

 direct connection with the production of hearing. It is the homotype of the 

 semicircular canals, and performs the same office. 



' The sonorous vibrations, starting from the stapes and fenestra ovalis, 

 rushing into the adjacent opening of the scala vestibuli, excite, by their im- 

 pulse, the auditory membrane or retina, spread over its lamina spiralis, and 

 reach its termination, where it opens into the scala tympani. Feeble vibra- 

 tions may subside spontaneously by exhaustion from their extension. The 

 stronger pass on into the scala tympani, where they fade away, or are sup- 

 pressed by the interference of vibrations entering the inferior canal by the 

 fenestra rotunda from the tympanum. The condition for perfect hearing, 

 for the distinct perception and appreciation of the finest tones and notes, so 

 that each vibration shall make but one, single, distinct impression, and then, 

 be suppressed, is thus amply fulfilled.' 



"Analogous provisions are perceived to exist in the tympanum, to pre- 

 serve in that cavity the perfect wave systems of undulations, indispensable 

 to the perfection of the sense of hearing. Vibrations existing in air, con- 

 tained in a cavity with plane walls, would continue to be reflected from side 

 to side, producing confused sounds or noise. The air in the tympanum is 

 thrown into vibrations by impulses of the niembrani tympaui. They are 

 communicated, pure, and in perfect accord, to the membrane of the fenestra 

 rotunda. This curious and beautiful result is effected in the following man- 

 ner: on one side the tympanum communicates, by numerous openings, with 

 the mastoid cells communicating with one another. All the vibrations im- 

 pinging on this side are suppressed in the mastoid cells. Those that reach 



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