424 M E M O I R S ?/ //6^ 
necefTary both for the more eafy explication of the fenfe of hear- 
ing It {di\ and to account for the diieafes incident to that organ: 
In the firft pLce he difcovered a membrane of a very fine and cu- 
rious texture within the cavity of the lympanum or drum of the 
ear, which with regard to the place it poflefles, he calls the 
interior membrane of the "Tym'panum-, to diftinguilh it from 
that which clofes up the extremity of the Meatus auditor ius, and 
which he calls the exterior membrane of the T^ymfanum-. This 
membrane in each ear, if narrowly viewed, appears to be fur- 
nifiied with a vaft number of capillary veflels, proceeding from 
the carotid artery and jugular vein on each fide ; hence it 
happens, that thele fmall veflels being greatly diftended with 
blood, the membrane appears almoft entirely red, when viewed 
in the fun, but eipecially in a raicrofcopej thefe fmall veflels 
prevent the fuperior and lateral parts of the membrane, which 
lupport them from collapfing and being folded up, as undoubt- 
edly they would be, unlels they were thus fufpended ^ for thefe 
parts do not immediately adhere to the interior I'uperficies of the 
cavity wherein they are contained : This membrane, which is a 
production of that thin one that covers the interior parts of the 
aquredud, fliuts up the paflage that ieads to the Cellulce of the 
Apopbyfu Nafioides-^ and therefore prevents the free communi- 
cation betwixt the air included therein, and that which enters 
the tympanum J befides that, one of its fine produflions ilops up 
the aperture of the Stapes^ and the other the Foramen commonly 
called Roruridumy and extending further, is fpread over all the 
interior luperficies of that linall hollow fpace, which reaches 
from the Foramen Rotundum to the extremity of the femioval 
du6l of the Cochlea^ and to the Rimula or little chink in the 
bafis of the Concha -^ fo that the interior membrane of the Tym- 
panum^ by this its produ(fl:ion, communicates with that part 
of the nervous membrane, which is fpread over the interior 
'Parietes of the Concha^ and alio with the extremity of the 
nervous Ipiral Lamina or plate, which lies within the Icmi-oval 
fpiral duct of the Cochlea^ befides, this membrane, beneath 
that part of it, which regards the external membrane of the 
Tympamm^ leaves a large void fpace which receives the ex- 
ternal air conveyed to it by the aquitdu61:, and is fo \o\^^^ up, 
and complicated as to form three cavities j the firft of which 
poflefles that fpace that lies between the external Apophyfis of the 
Incus, and the paflage which leads to the CelluU of the Apophy- 
fis Mafloides, as wa"s obferved above ^ the lecond, which is be- 
tween the firil and third cavities, and lefs than either, lies exadlly 
under 
