THE ORGANS OF SENSE. H5 



spiral lamina. The peripheral processes of these cells emerge from the 

 spiral lamina to be distributed as fibrils over the hair cells of the 

 organ of Corti. 



THE MECHANISM OF HEARING. 



When sound waves fall upon the tympanic membrane they cause 

 the latter to vibrate. The membrane has no periodicity of , its. own, 

 partly because of its peculiarities of structure, and partly because its 

 vibrations are damped by the attached handle of the malleus. With 

 each inward movement of the membrane the handle of the malleus and 

 the long process of the incus also move inwards, the latter carrying the 

 stapes with it. The malleus and incus together form a lever, the 

 fulcrum of which is the axis of movement described above, the handle 

 of the malleus being the power arm and the long process of the incus 

 the load arm. The length of 'the handle of the malleus is to that of 

 the process of the incus as 3 to 2, and it therefore follows that the 

 movement of the tympanic membrane is diminished in amplitude in 

 the proportion of 3 to 2, while at the same time it is increased in force 

 by one half, or in the proportion of 2 to 3. Further, as the membrana 

 tympani is twenty times the size of the membrane in the fenestra oyaUs 

 on which the vibrations are directed, it follows that the pressure of a 

 sound wave on the membrana tympani is increased to thirty times in 

 its passage across the middle ear (3/2 x 20 = 30). 



The vibrations of the membrana tympani, transmitted by the 

 chain of ossicles to the fenestra ovalis, set up corresponding vibrations 

 in the perilymph; these travel up the scala vestibuli and down 

 the scala tympani. The wave in the perilymph is communicated 

 through the delicate membrane of Reissner to the endolymph of the 

 canal of the cochlea, and thus the stimulus is conveyed to the organ of 

 Corti. The increase of pressure in the canal of the cochlea is passed 

 on to the scala tympani, causing the membrane which closes the fenestra 

 rotunda to bulge towards the middle ear. 



The adequate stimulus for the organ of Corti is the wave set up in 

 the endolymph as the result of sound waves in the air. Sound waves 

 consist in an alternate condensation and rarefaction of the gases of 

 the atmosphere, and they travel through the air at a rate of about 

 350 metres a second. They give rise to two kinds of sensation, one 

 that of noise, when the sound waves follow each other irregularly, the 

 other that of a musical note, when the waves follow one another with 

 a certain rhythm. 



Musical sounds vary in pitch, in intensity, and in timbre or quality. 

 (1) Pitch depends on the rapidity of the vibrations constituting the 



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