SUMMARY OF THE MECHANISM OF AUDITION. 351 



to vibrate individually, and that it is not certain that they are tuned in accord with dbiV-r- 

 ent tones. Hensen makes, upon this point, the following statement : 



" It is now my conviction, that by the hypothesis * more and more corroborated ' that 

 the fibres of Corti constitute the organ of the labyrinth tuned to the appreciation of 

 tones, our comprehension and the investigation of the internal ear have taken a false 

 direction. 



" I assert, next, that the rods of Corti cannot play the important part in the appreci- 

 tion of tones, which has been attributed to them in the hypothesis of Helmholtz." 



It is pretty evident that, although the theory of Helmholtz is undoubtedly the only 

 one affording any reasonable explanation of the appreciation of tones, it lacks positive 

 anatomical confirmation. And, farthermore, we do not even know the anatomical con- 

 nections between the rods of Corti and the filaments of the auditory nerves. 



In view of the considerations just given, we have simply recited the theory of Du 

 Verney, Le Cat, and Helmholtz, as one which may or may not be sustained hereafter by 

 more exact researches ; but at present it must be acknowledged that there is no more 

 satisfactory explanation of the mechanism of the final appreciation of musical tones. 



Summary of the Mechanism of Audition. 



The waves of sound are simply collected by the pavilion of the ear and are conveyed, 

 through the external meatus, to the membrana tympani. The membrana tympani, a 

 delicate, rounded, concave membrane, receives these waves and is thrown into vibration. 



The arrangement of the bones and muscles of the middle ear admits of variations in 

 the tension of the membrana tympani. By increasing the tension of this membrane, the 

 ear may be rendered insensible to grave sounds, while high-pitched sounds become more 

 intense ; and, in cases of voluntary tension, the limit of perception of high tones may be 

 greatly extended. The membrana tympani obeys the laws of consonance and vibrates 

 strongly under the influence of sounds in unison or in harmony with its fundamental 

 tone, returning, in this way, not only the pitch, but the quality of tones and combina- 

 tions of tones in harmony. Destruction of the membrane does not necessarily of itself 

 destroy hearing, or even the appreciation of tones, for the impressions may be conduct- 

 ed to the cochlea by the chain of ossicles. 



The arrangement of the ossicles and muscles of the middle ear is such that contrac- 

 tion of the tensor tympani renders the articulations firm, tightens the little ligaments, 

 and presses the stapes against the liquid of the labyrinth, so that the chain resembles, in 

 its action, a solid and continuous bony rod. By this arrangement, the sonorous vibra- 

 tions are conducted to the labyrinth with very little loss of intensity. 



The cavity of the tympanum is filled with air, communicates with the mastoid cells, 

 and with the pharynx by means of the Eustachian tube; and, by this means, the press- 

 ure of air in its interior is regulated. The labyrinth, consisting of the vestibule, semi- 

 circular canals, and cochlea, is filled with liquid, and the different cavities communicate 

 with each other. The vibrations, repeated by the membrana tympani, are conveyed by 

 the chain of bones to the liquid of the labyrinth, and by it to the terminal filaments of 

 the auditory nerves. 



The vestibule and semicircular canals seem to possess much less importance in the 

 appreciation of sound than the cochlea. In the cochlea, throughout the entire extent of 

 the spiral canal, is the organ of Corti, presenting, among other structures, about 8,700 

 rods, varying in length, called the rods of Corti. But little is known of the anatomical 

 relations between the auditory nerves and the organ of Corti; still, it is thought, as a 

 matter of pure theory, that the rods of Corti are tuned in unison with dim-rent tones, 

 that they repeat the tones conveyed to the cochlea, and that we are thus enabled to dis- 

 tinguish the different tones in music. 



We have no very definite knowledge of the functions of the cells of the organ of Cor- 



