. A text-book of comparative physiology for students and practitioners of comparative (veterinary) medicine. Physiology, Comparative. 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 par
. A text-book of comparative physiology for students and practitioners of comparative (veterinary) medicine. Physiology, Comparative. 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; , fenestra ovalis; , 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).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of
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