840 SPECIAL SENSES. 



auditory nerves, the impression produced must of necessity be dependent exclusively 

 upon the character, regularity, and number of the sonorous vibrations. And, again, the 

 physical laws of sound, which are fixed and unchangeable, teach us that a membrane^ 

 like the membrana tympani, must return or reproduce sounds which are in unison or are 

 harmonious with its fundamental tone, much more perfectly than discordant or irregular 

 vibrations. In a loud confusion of noisy sounds, we can readily distinguish pure melody 

 or harmony, even when the vibrations of the latter are comparatively feeble. In follow- 

 ing with the ear any piece of music, reasoning from purely physical considerations, it 

 must at times occur that the tones are in exact unison or in harmony with the funda- 

 mental tone of the membrana tympani. Supposing the fundamental tone of the mem- 

 brane to be constant and invariable, such tones would be heard much more distinctly 

 than others, as a physical necessity. Such a difference in the appreciation of certain 

 notes in melody does not occur ; and the only reasonable explanation of this is that the 

 tension of the membrane is altered. It is shown by anatomical researches that the ten- 

 sion can be altered by muscular action, and, as the muscles are striated, we may suppose 

 that it may be modified rapidly. Physiological observations show that such modifica- 

 tions in tension do occur ; and there are on record unquestionable instances in which the 

 membrana tympani is tightened by a voluntary contraction of the tensor tympani muscle. 



Another important point to note in this connection is the following: Can it be 

 shown that the appreciation of the pitch of tones bears any relation to the degree of ten- 

 sion of the tympanic membrane ? We can answer this question unreservedly in the 

 affirmative. When the membrane is rendered tense, there is insensibility to low tones. 

 When the membrane is brought to the highest degree of tension by voluntary contrac- 

 tion of the tensor tympani, the limit of appreciation of high tones may be raised from 

 three thousand to five thousand vibrations. It is a fact in the physics of the membrana 

 tympani, that the vibrations are more intense the nearer the membrane approaches to a 

 vertical position. It has also been shown that the membrane has a strikingly vertical 

 position in musicians, and that the position is very oblique in persons with an imperfect 

 musical ear. This fact has a most important bearing upon the probable relation between 

 the membrana tympani and the correct appreciation of musical sounds. 



In view of all facts and considerations for and against the theory which we have 

 given of the action of the tympanic membrane in the appreciation of musical sounds, 

 does it not seem probable that there are, acting upon this membrane, muscles of auditory 

 accommodation, analogous in their operation to the muscle of visual accommodation ? 

 We have carefully studied this subject in all its bearings, and, if the reader follow closely 

 our process of reasoning, it must seem probable that the muscles of the middle ear are 

 muscles of auditory accommodation ; but it should be remembered that the action of the 

 membrane is not absolutely essential, and that musical tones, however conducted, must 

 of necessity be correctly appreciated, whenever and however they find their way to the 

 auditory nerves. 



Experiments have shown pretty conclusively that the tympanic membrane vibrates 

 more forcibly when relaxed than when it is tense. It is evident that the relaxed mem- 

 brane must undergo vibrations of greater amplitude than when it is under strong tension. 

 In certain cases of facial palsy, in which it is probable that the branch of the facial going 

 to the tensor tympani was affected, the ear became painfully sensitive to powerful impres- 

 sions of sound. This probably has no relation to pitch, and most sounds that are pain- 

 fully loud are comparatively grave. The tension of the membrane may be modified as a 

 means of protection of the ear, but the facts belonging to cases of facial palsy are all 

 that we have bearing upon this point. Artillerists are in danger of rupture of the mem- 

 brana tympani from sudden concussions. To guard against this injury, it is recom- 

 mended to stop the ear, draw the shoulder up against the ear most in danger, and parti- 

 cularly to inflate the middle ear after Valsalva's method. " This method consists in 

 making a powerful expiration, with the mouth and nostrils closed." 



