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5 4 THE C O M M U N I C A T I O N OF SOUND
C H A P T E R X.
Of the feveral Ways in which the TREMOR of SONOROUS
BODIES is communicated, in the different CLASSES of ANIMALS.
to the Nerves fpread out on the Bottom of the
E A R .
HAVING dcfcribed the ftruflure of the ear in the amphibious animals and
in fiflies, and that of the mammaha and birds being fuppofed to be underftood,
we flrall proceed to confider the way in which the tremor excited by
fonorous bodies is communicated in the different claiTes of animals to the nerves
fpread out on the bottom of the ear.
It is very generally fuppofed, that in the mammalia and in birds, belide the
impreffion which may be communicated to the nerves of the ear by the tremor
of the whole bones of the head a diftind impreffion may be conveyed to
i h em in three different ways.
In the fitti place, and chiefly, by the chain of bones, cartilages, and ligaments,
regulated by their mufcles, which connedls the membrane of the drum to the
membrane of the oval hole.
In the next place, as one fcala, or one half of our cochlea, begins at the
membrane of the round itJit^«l>iclLÌs not diredly connefted to the membrane
of the drum, and as men and other aniììmls i iav« feemed to diilinguiih founds,
with confiderablc acutenefs, after the chain of fmall bones was deilroyed by difeafe
or experiment, there is reafon to believe that we receive a fécond and
pretty diilincl impreffion by the medium of the air contained in the cavity of
the tympanum.
This air is fuppofed to be agitated, and to communicate its tremor m two
different ways, to wit, by receiving motion from the membrane of the drum ;
and beCdes this, by a tremor of the external air communicated to it through
the Euftachian tube, which, authors pretend, is admirably fuited to this purpofe
by its fituation, ihape, and the elaftic materials of which it is compofcd.
Nay, the generally accurate Valfalva relates a cafc, which has been confidered as
an undoubted proof that found is conveyed by this channel.
Yet
- The found of an ordinary watoh applied to the fore-tceth of the upper jaw is heard diftiuaiy i but Its found ia not fo
tvell perceived on applying it in like manner to the teeth of the under jaw r fo that the tremor feem, to be eonveyed to
the portio molliî, not by the branches of the fifth pair of nerves, or porrlo dura of iJie fevcnth pair, as ha» been fuppofed,
but by the bones of the head.
THE N E R V Ë S OF THE EAR; 5 5
Yet the following fimple experiment which I made twenty years ago, is fuf -
ficient to convince us that no diiUniit impreffion is tranfmitted through the Euftachian
tube.
I placed an alarm-clock, which rings very loud, upon one culhion, and I
ftood upon another. Then having ftopped my meatus auditorii externi, I fct
the clock a-ringing, without being able to hear its found. In like manner,
when 1 held my watch near to the external ear, I heard its found diftinftly;
but when I held it within my mouth, between my tongue and the roof of my
mouth, without allowing it to touch either, and then ftopped my external ears,.
T did not hear its ilrokes.
T h e primary ufe, therefore, of the Euftachian tube is to furnilli air to the
cavity of the tympanum, through which the tremor of the membrane of the
drum may, in the firft place, be communicated to the membrane of the round
hole : but, in the next place, as air is conveyed to the cavity of the tympanum
in amphibious animals where the round hole and cochlea are wanting, we muft
fappofc that the tremor of the air in the tympanum fcrves to influence the foot
of the ftapes and membrane of the oval hole; or that two different impreffions
are made on the membrane of the oval hole.
W e ought farther to confider, that in the amphibia the tremor of the chain
of fmall bones will be much lefs interrupted by air than it would have been by
a watery liquor filling the cavity of the tympanum.
From the membranes of the oval and round holes the impreffion of found is
undoubtedly conveyed to the portio mollis by the medium of a watery liquor,
which fills the cavities of the veftible, femicircular canals, and cochlea, in all
animals.
When we now compare the mammalia and birds with the amphibia and
fifties, we undcrftand the reafon of fome principal differences of ftrudure.
T h e whale, though he feems at firft fight amphibious, has the fame ftruSure
of heart and lungs as man ; and is therefore obhged to brcatlie air frequently and
regularly, and to live chiefly upon the furface of the ocean. Hcnce his ear is
calculated to receive found f rom die air b y an external meatus.
But in the amphibia, "where a part only of the niafs of blood pafles through
the lungs, and which therefore poftifs the power of breathing arbitrarily, or of
plunging under water, and ccafing from breathing for a length of time f . the
• ear is conf t ruaed fo as to receive the firft impreffion cither from the air or from
the water; but by means of an Euftachian tube, air is introduced into the cav
i ty of the tympanum when they breathe ; and through it the impreffion is conveyed
from the atmofphere, to which their ears are generally expofed, to the
bottom of the car with more force than it would have been by the medium of a
watery fluid fccrcted into the cavity of the tympanum.
In fiflies, living in and breathing in water, not only the impreffion of found
on the furface of the ear is tranfmitted by the water, but is of neceffity conveyed
b y
t 1 l.avc obfcTvcd a turtle keep under water for three quarters cf an hour j and I have found that it requires five or fix
hours to drown a frog.