Volltext: Elements of Physiology

fection of hearing is produced by rendering the membrana tympani 
tense, and convex towards the interior, by the effort of inspiration; a 
fact first noticed by Dr. Wollaston. (Philos. Transact. 1820.) The 
imperfection of hearing, produced by the last-mentioned method, con¬ 
tinues even after the mouth is opened, in consequence of the previous 
effort at inspiration having induced collapse of the walls of the Eusta¬ 
chian tubes, which prevents the restoration of equilibrium of pressure 
between the air within the tympanum and that without; hence we have 
the opportunity of observing that even our own voice is heard with 
less intensity when the tension of the membrana tympani is great. 
If the pressure of the external air or atmosphere be very great, while, 
on account of collapse of the walls of the Eustachian tubes, the air in 
the interior of the tympanum fails to exert an equal counter-pressure, 
the membrana tympani will of course be forced inwards, and imperfect 
deafness be produced. It is thus, in my opinion, that we must explain 
the singular observation of M. Colladon, that in the diving-bell both the 
voices of his companions and his own voice sounded faintly. The fact 
cannot be owing, as has been supposed by some writers, to the bad 
conducting power of the condensed air, since air conducts sound better 
in proportion to its density. 
The effect of the increased tension of the membrana tympani is not 
to render both grave and acute sounds equally'fainter than before. On 
the contrary, it was observed by Dr. Wollaston, that, when he had 
rendered his membrana tympani tense by exhausting the cavity of the 
tympanum, he was deaf to grave sounds only. By striking the table 
with the ends of his finger, he produced a deep dull note; striking it 
with his nail, he gave rise to a sharp sound. When he had exhausted 
the tympanum by the effort of inspiration, he could hear the latter sound 
only; the former deeper note-was inaudible. 
These facts admit of a practical application in pathology. It is not 
very rare to meet with persons who are deaf to the more grave sounds 
only, while they still hear distinctly acute sounds, even though they be 
not loud. One of my colleagues, who is deaf, hears acute better than 
grave sounds. In such cases it is very probable that the membrana 
tympani is in a state of too great tension. In the present state of obscu¬ 
rity of the diagnosis of diseases of the ear, this hint may be of some use. 
Such unnatural tension of the membrana tympani may, of course, be 
produced by several different causes. It may arise from occlusion of the 
Eustachian tube; in which case the air in the tyinpanurçi may either be 
expanded by the heat of the body, and so force outwards the membrana 
tympani, or it may be partially absorbed, when the membrane would 
be pressed inwards by the air without. Another cause may be a con¬ 
tracted state of the tensor tympani muscle. In my colleague thq 
Eustachian tube is free, for he can force air into the tympanum. 
When the Eustachian tube is closed, and the tension of the membrana 
tympani is the consequence of the expansion or partial-absorption of 
the air in the cavity of the tympanum, the operation of puncturing the 
membrane, or the mastoid process, may be easily conceived to be bene¬ 
ficial; but when the too great tension of the membrana tympani, and- 
consequent deafness, are owing to a contracted state of the tensor tyni- 
64 .


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