THE MOVEMENTS OF THE OSSICLES. 1159 



is increased thirty-fold (1-5x20), when it acts on the plate of the 

 stapes. 



Before leaving the subject of the transmission of pressures corresponding 

 to sound-waves, from the air to the labyrinth, we may consider the effect 

 which the dimensions of the conducting apparatus have on the transmission. 

 The first who drew attention to this subject was Weber, 1 and it has been 

 further investigated by von Helmholtz. 2 By dividing the velocity of 

 sound in air by the number of vibrations per second, we obtain the wave- 

 length. It is thus ascertained that the wave-length of sounds of the lowest 

 pitch recognised by the human ear is about 40 ft., that the lowest tone of 

 the organ has a wave-length of about 37 ft., that the wave-length of the 

 middle c of the piano is above 4 ft., that the highest tone of the piccolo stop 

 of the organ has a wave-length of 3 - 5 in., while the wave-length of the 

 highest tone audible to the ear is at least - 5 in. Thus it is evident that the 

 total length of the solid and fluid masses of the ear is only a small fraction 

 of the wave-length of ordinary perceptible sounds. Hence we must regard 

 the ossicles and the intra-labyrinthine fluid, not as forming an independent 

 vibrating mass, but as one body which has practically no proper vibration 

 period. If, for example, it were possible to throw the ossicles into periodic 

 vibrations, the tone resulting thereby would be so high in pitch as to be 

 beyond the limits of hearing. The movement of the chain of ossicles, there- 

 fore, forms only a small part of a sound-wave. 



The magnitude of the movements of the ossicles. — The move- 

 ments of the bones of the ear may be studied experimentally in 

 the dead animal by various methods. Politzer 3 opened the cavity 

 of the tympanum, and fixed bits of fine glass thread to the various 

 ossicles ; the free ends of the threads were then adjusted to a surface of 

 a recording drum. When a sound was allowed to fall on the membrana 

 tympani, the glass threads oscillated and traced curves on the smoked 

 paper covering the drum. The movement of greatest amplitude was 

 found to be at the tip of the manubrium, 0*76 mm. ; the movement at 

 the tip of the long process of the incus was 021 mm.; while the greatest 

 amplitude at the stapes was "0714 mm. A similar series of measurements 

 were made by Mach and Kessel 4 and by Buck 5 in 1870. In the following 

 year the matter was re-investigated by Burnett. 6 He opened the 

 tympanum, illuminated the interior by a strong beam of light, sprinkled 

 the ossicles with powdered starch, so as to secure brightly luminous 

 points, fixed a tube into the external auditory meatus so as to convey 

 vibrations from an organ-pipe to the outer surface of the membrana 

 tympani, and then observed the excursions of the shining particles when 

 the organ-pipe was sounded. The chain of bones vibrated in excursions 

 bearing a fixed relationship to each other. Particles on the fenestra 

 rotunda were seen to vibrate synchronously with oscillating particles on 

 the stapes. Further, the excursions at the fenestra rotunda were equal 

 to those of the stapes, and sometimes they were equal to those at the 

 point of the manubrium. The amplitude of movement at the base of the 

 stapes and round window varied from - 001 to 0032 mm., considerably 

 less than the measurement made by Politzer. Burnett also made the 

 interesting observation that variations in the labyrinthine pressure 

 modified the amplitude of movement at the stapes, an increase of 



1 Weber, Vcrhandl. d. sacks. Gesr.Jlsch. d. Wissensch., 1851. 



2 " Mechanism of the Ossicles of the Ear," p. 9. 3 Arch. f. Ohrenh., 1861, Bd. i. 

 4 Loc. eit. ■• Arch. Ophth. and Otol., New York, vol. i. p. 603. 

 6 "Treatise on the Ear," p. 95. 



