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enables us to say whether the auditory meatus is blocked by 

 wax, e.g. -beyond the tympanic membrane. 



A membrane like a drum-head has a note of its own, which it gives 

 out when struck, and it vibrates more readily to this note than to 

 any other. It would evidently be a serious disadvantage if the 

 tympanic membrane, whose office it is to receive all kinds of vibra- 

 tions, and respond to all, had a marked fundamental tone which 

 would be continually obtruding itself among other notes. The 

 difficulty is obviated by the damping action of the ossicles and the 

 liquids of the labyrinth on the movements of the membrane, which 

 in addition is not stretched, but lies slackly in its bony frame, so 

 that when the handle of the malleus is detached from it, it retains 

 its shape and position. 



The tensor tympani, when it contracts, pulls inwards the handle 

 of the malleus, and thus increases the tension of the tympanic 

 membrane. The precise object of this is obscure. It has been 

 suggested that damping of the movements of the auditory ossicles 

 is thus secured. Another theory is that the increased tension of 

 the membrane renders it more capable of responding to higher 

 tones, and that the muscle thus acts as a kind of accommodating 

 mechanism. But Hensen has observed that the tensor only con- 

 tracts at the beginning of a sound, and relaxes again when the 

 sound is continued ; and this is difficult to reconcile with either of 

 these hypotheses. The muscle is normally excited reflexly through 

 the vibrations of the membrana tympani, but some individuals 

 have the power of throwing it into voluntary contraction, which is 

 accompanied by a feeling of pressure in the ear and a harsh sound. 

 The function of the stapedius is unknown. Its contraction would 

 tend to press the posterior end of the foot-plate of the stapes deeper 

 into the foramen ovale, and cause the anterior end to move in the 

 opposite direction ; but it is not easy to see how this would affect 

 the action of the auditory mechanism. 



The tensor tympani is supplied by the fifth nerve through a branch 

 from the otic ganglion ; the stapedius is supplied by the seventh. 

 Paralysis of the fifth nerve may be accompanied with difficulty 

 of hearing, especially for faint sounds. When the seventh nerve 

 is paralyzed, increased sensitiveness to loud sounds has been 

 observed. 



We have already recognised the organ of Corti, particularly 

 the hair-cells, as a sensory epithelium which constitutes the 

 terminal apparatus of the cochlear nerve. The adequate stimulus 

 of the auditory receptors is the periodic changes of pressure in 

 the endolymph. But there are various opinions as to how these 

 vibrations are transmitted to the hair-cells, and as to how the 

 vibrations of the hair-cells are translated into nerve impulses in 

 the auditory fibres. The pillars of Corti, the basilar membrane, 

 and the membrana tectoria, have in turn been regarded as the 

 structures immediately set into vibration by the changes in the 

 endolymph. The case for the tectorial membrane is perhaps 

 the most plausible, for its position renders it most capable of 

 acting on the hairs. Others have supposed that the hairs of the 

 hair-cells are directly affected by the endolymph. Some, 



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