748 SPECIAL SENSES. 



illustrated by strings merely for the sake of simplicity ; but they have a more 

 or less perfect application to all bodies capable of producing musical tones, 

 except that some are thrown into vibration with more difficulty than others. 

 A thin membrane, like a piece of bladder or thin rubber, stretched over a 

 circular orifice, such as the mouth of a wide bottle, may readily be tuned to 

 a certain note. When arranged in this way, the membrane can be made to 

 sound its fundamental note by influence. In addition, the membrane, like a 

 string, will divide itself so as to sound the harmonics of the fundamental, 

 and it will likewise be thrown into vibration by the 5th, 3d etc., of its funda- 

 mental, thus obeying the laws of vibrations of strings, although the har- 

 monic sounds are produced with greater difficulty. 



The account just given of some of the laws of sonorous vibrations and 

 their relations to musical effects and combinations, although by no means 

 complete, may seem rather extended for a work on physiology ; but it should 

 be borne in mind that the mechanism of the appreciation of musical sounds 

 includes the entire physiology of audition. This subject can not be compre- 

 hended without a general knowledge of the physics of sound and of some of 

 the laws of harmony ; for not only is there a perception of single notes by 

 the auditory apparatus, but the most intricate combinations of sounds in 

 harmony are all appreciated together and at one and the same instant, as will 

 be seen in studying the action and uses of different parts concerned in audi- 

 tion. Many of the laws of musical combinations are directly applicable to 

 the physiology of hearing. 



USES OF DIFFERENT PARTS OF THE AUDITORY APPARATUS. 



The uses of the pavilion and of the external auditory meatus are suffi- 

 ciently apparent. The pavilion serves to collect the waves of sound, and 

 probably it inclines them toward the external meatus as they come from vari- 

 ous directions. Although this action is simple, it has a certain degree of 

 importance, and the various curves of the concavity of the pavilion tend more 

 or less to concentrate sonorous vibrations. Such has long been the opinion 

 of physiologists, and this seems to be carried out by experiments in which 

 the concavities of the external ear have been obliterated by wax. There 

 probably is no resonance or vibration of much importance until the waves of 

 sound strike the membrana tympani. The same remarks may be made with 

 regard to the external auditory meatus. It is not known precisely how the 

 obliquity and the curves of this canal affect the waves of sound, but it is 

 probable that the deviation from a straight course protects, to a certain 

 extent, the tympanic membrane from impressions that might otherwise be 

 too violent. 



Structure of the Membrana Tympani. The general arrangement of the 

 membrana tympani has already been described in connection with the topo- 

 graphical anatomy of the auditory apparatus. The membrane is elastic, 

 about the thickness of ordinary gold-beater's skin, and is subject to various 

 degrees of tension by the action of the muscles of the middle ear and under 



