SENSE OF HEARING. 653 



tendinous connections undisturbed, these bones are held in such close 

 connection with each other that they vibrate as a single solid body. 



The vibratory movement of the ossicles of the ear has no immediate 

 dependence upon the action of the muscles attached to them, but results 

 from the shocks received by the tympanic membrane. The influence of 

 the muscles is to increase or diminish the tension of this membrane, 

 and thus to influence the mode of transmission of the sound. 



The action of the internal muscle of the malleus, or tensor tympani, 

 is beyond doubt, as its name indicates, to increase the tension of the 

 membrana tympani. It has long been known that, on opening the canal 

 in which this muscle is lodged, as well as the cavity of the tympanum, by 

 drawing upon its tendon within the canal, the membrana tympani may 

 be manifestly rendered more tense ; and according to Helmholtz, all the 

 ligaments holding the ossicles in place are at the same time put upon 

 the stretch. 



The effect produced upon the act of hearing by increased tension of the 

 membrana tympani has been interpreted in a different sense by different 

 observers. Savart, 1 who first studied systematically the vibrations 

 induced in stretched membranes by the proximity of sounding bodies, 

 estimated the extent of these vibrations by the agitation of particles of 

 fine sand spinkled on the surface of the membranes ; and he found the 

 vibrations more difficult of production, other things being equal, when 

 the tension of the membrane was increased. He applied the same mode 

 of experimentation to the membrani tympani in the ear of man and 

 animals, and found not only that sand, sprinkled on its surface, would 

 be thrown into agitation by holding near it a sounding body, but that 

 also, as in the former case, these appearances were less easy of produc- 

 tion when the membrane was rendered more tense by traction upon the 

 tensor tympani muscle. He concluded from that, that during life the 

 ear is more susceptible to sounds of a given intensity when the mem- 

 brana tympani is relaxed, and less so when it is put upon the stretch ; 

 the tensor tympani, accordingly, exerting a protective action by lessen- 

 ing the apparent intensity of very loud sounds. 



This view has been adopted by many eminent authors, owing in great 

 measure to the valuable experiments of Savart. But this observer was 

 not aware of an important fact which has been established by subse- 

 quent investigations, namely, that stretched membranes, like cords, can- 

 not respond indiscriminately to sounds of every grade of tone, but only 

 to a certain number of these tones, which are separated from each other 

 by definite intervals ;* and they will respond to a different set of tones 

 only after their tension has been increased or diminished. In order, 

 therefore, that a membrane may be easily thrown into induced vibra- 

 tion, its tension must correspond in a certain ratio with the tone of the 

 sound produced. 



1 Journal de Physiologic. Paris, 1825, tome iv. p. 2^5. 



2 Daguiu, Traite elementaire de Physique. Paris, 1867, tome i. p. 596. 



