THE EUSTACHIAN TUBE. 367 



tion ; the tensor tympani contracts to such an extent as to bring the 

 membrane to a standard of tension, and, since this muscle is of the vol- 

 untary kind, the mind judges of fhe degree of force which is required to 

 produce that result just as, when we lift from the ground bodies of dif- 

 ferent weights, we judge with a certain precision of the degree of force 

 necessary to be put forth. The condition of contraction of the tensor 

 tympani therefore enables the mind to measure the intensity of the sound- 

 ing waves. 



But this muscular contraction is strictly a reflex act, and is therefore 

 preceded, as all such acts are, by an impression. That impression is 

 made, as we shall presently find, primarily on the auditory nerve. But 

 since these reflected acts are not sensory, the mind has no knowledge of 

 the effect impressed in this respect upon the auditory nerve, and only be- 

 comes sensible of it in an indirect way, through the contractions which 

 have ensued in the tensor tympani muscle. 



In this view of the case, the use of the Eustachian tube becomes ob- 

 vious ; it is to form a ready passage for the air inwardly or tlseoftheEus- 

 outwardly, so that no condensation or rarefaction shall occur tachian tube - 

 within the tympanic cavity ; for such rarefactions and condensations 

 would disturb the action of the contracting muscle, and make it yield a 

 false estimate. Besides this, the Eustachian tube, as has long been 

 known, affords an outlet for mucus. 



In the explanation here presented, the function of the ossicles is rather 

 for the purpose of tension than of conduction, though it is Function of the 

 not denied that sounds may pass readily along them. They ossicles - 

 are to be regarded as aiding in the perfection of auditory perceptions, but 

 yet not as being absolutely essential to the appreciation of sounds, or 

 even of their finer modifications. Whatever affects the facility of vibra- 

 tion of the membrana tympani, such as its thickening, or stiffening, or 

 unusual dryness, will render the hearing dull, but the membrane itself 

 may be perforated, or even undergo extensive lesions, without any appar- 

 ently corresponding effect. But if the stapes be injured or be removed, 

 deafness is at once the result. 



There is nothing remarkable in the precision with which the contrac- 

 tions of the two muscles which stretch the membrane of the Precision in es- 

 drum are made. The same precision is illustrated in the timatingthe 



,, , , i i f i i -i rm contraction of 



case of the muscles which adjust the vocal cords. The state the auditory 

 of these may be determined to the - llr ^ 00 part of an inch. scies. 



It might perhaps be inquired, Why should not the function of determ- 

 ining the intensity of sounds, as well as their wave length, be imputed 

 directly to the auditory nerve ? It is with the ear as with the eye, the 

 mechanism for determining wave length can only act with uniformity 

 when the agent to be measured is reduced to a standard intensity. A 



