THE SENSE OF HEARING. 815 



the remaining portion would be drawn out of it. Its action probably reduces 

 the pressure in the cavity of the perilymph, and thus is antagonistic to that 

 of the tensor tympani (PI. 2, Fig. 2, A, B). 



Vibrations of the Tympanic Membrane. It is a general physical law 

 that every elastic body can be made to vibrate more easily at one definite rate 

 than at any other. The musical tone represented by this rate of vibration is 

 known as the prime or fundamental tone of the body. Membranes have funda- 

 mental tones (see p. 827), whose pitch is determined by their area, thickness, 

 and tension, but they differ from rods and strings in being less strictly confined 

 to a single fundamental tone in their vibration. The tympanic membrane is 

 quite peculiar in that it can hardly be said to have a definite fundamental tone. 

 It would obviously be a great imperfection in an organ of hearing were cer- 

 tain sounds intensified by it out of proportion to others, as would be the case 

 if the tympanic membrane had a marked fundamental tone of its own. This 

 is prevented in the case of the membrana tympani probably both by reason of 

 the peculiar form of its surface and its structure, and also because its oscilla- 

 tions are damped by the pressure of the malleus held in position by the other 

 mechanisms of the tympanum. When the tympanic membrane is perforated 

 or is wholly removed, without destructive inflammatory changes in the middle 

 ear, sounds are still heard, though usually with diminished loudness. A 

 musician who had suffered this accident was no longer able to play his violin, 

 probably because sounds of different pitch ceased to be perceived in their true 

 relations of loudness. We may thus conclude that the function of the tym- 

 panic membrane is not only to guard against injury to the delicate mem- 

 branes of the fenestre and the internal ear, but also to transmit to the ossicles 

 sonorous vibrations with their true proportion of intensity. The membranes 

 covering the round and oval windows of the internal ear have no means of 

 damping sympathetic vibrations (see p. 829), and, should complex air-waves 

 strike directly upon them, they would, probably, by sympathetic resonance, 

 respond more powerfully to tones of certain pitch than to any others. 



The sensation of sound may be excited by conduction through the bones 

 of the skull as well as in the ordinary way. Thus, a tuning-fork set vibrating 

 and held between the teeth or on the forehead is heard perfectly, and more 

 loudly when the ears are closed than when open. The vibrations thus con- 

 ducted probably partly affect the internal ear directly, and partly indirectly by 

 setting in oscillation the tympanic membrane. When a sounding tuning-fork 

 is held between the teeth until the sound dies away, it may still be heard if 

 held in front of the ear, though the contrary statement is frequently erroneously 

 made. When the sound of the fork held between the teeth has failed, it may 

 again be heard by stopping the ears. 



The Internal Ear, or Labyrinth. The internal ear is the site of the true 

 organ of hearing. The membranous labyrinth (PL 2 7 Fig. 4 ; Fig. 278) is a com- 

 plicated system of membranous tubes and sacs, in which terminate at particular 

 points the filaments of the auditory nerve ; it is contained within a chamber, 

 the bony labyrinth, hollowed out in the petrous bone. The cavity of the bony 



