HEARING. 



565 



or to occlusion of the Eustachian tuhe from excess of secretion, 

 cicatrices, etc., in consequence of which the relations of atmos- 

 pheric pressure become altered, the membrana tympani being 

 indrawn, and the whole series of conditions on which the nor- 

 mal transmission of vibrations depends disturbed, with the 

 natural result, partial deafness. The hardness of hearing ex- 

 perienced during a severe cold in the head (catarrh, etc.) is 

 owing in great part to the occlusion of the Eustachiaji tube, 

 which may be either partial or complete. 



By filling one or both of the ears external to the mem- 

 brana tympani with cotton-wool, one may satisfy himself how 

 essential for hearing is the vibratory mechanism, which is, of 

 course, under such circumstances inactive or nearly so ; hence 

 the deafness. 



When the middle ear is not functionally active,, it is still 

 possible, so long as the auditory nerve is normal, to hear vibra- 

 tions of a body (as a tuning-fork) held against the head; 

 though, as would be expected, discrimination as to pitch is 

 very imperfect. 



Auditory impulsea originate within the inner ear — tl^t is 

 to say, in the vestibule and possibly the semicircular canals, 

 but especially in the cochlea. It is to be remembered that the 



Fig. 411. — Diagram intended to illustrate relative position of various parts of ear (after 

 Huxley). E. M, external auditory meatus; Ty. M. tympanic membrane: Ty, tym- 

 panum; MaU, malleus; Inc, incus; Stp, stapes; F.o, fenestra ovalis; F.r, fenes- 

 tra rotunda; Eu, Enstacliian tube; M. L, membranous labyrintb, only one of the 

 semicircular canals and its ampulla bemg represented; Sea. Y, Sea, T, Sea M 

 scalse of cochlea, represented as straight (uncoiled). 



