€o* ANATOMY. 
with each of thefe. Thefe four little bones appear to 
have been unknown to the Greek anatomifts. 
The labyrinth is divided into three parts; the anterior, 
middle, and pofterior. The middle portion is termed vcf- 
tibulum, the anterior cochlea, and the pofterior labyrinth in 
particular ; which comprehends tire three femicircular ca¬ 
nals. The cochlea lies forward and inward toward the 
extremity of the pars petrofa; the femicircular canals 
backward and outward toward the bafis of the procefs; 
and the veftibulum between the other two. 
The foft parts of the internal ear are chiefly the mem- 
brana tympani, the periofteum of the tympanum, and of 
the ofiicula auditus, labyrinth, and of all its cavities, the 
membrana mafloidea interna, the mufcles of the oflictrla, 
the parts which complete the formation of the Euftachian 
tube, the arteries, veins, and nerves. 
The periofteum of the tympanum, or barrel of the ear, 
produces that of the fmall bones; and it may be made vi¬ 
able by means of anatomical injections, which difcover 
capillary veftels very diftinCllv ramified on the furface of 
the ofiicula. It is likewife continued over the two fcnef- 
trar, and enters the Euftachian tube, where it is loft in the 
inner membrane of that duCt. 
As the fenfe of hearing perceives the elaftic tremors or 
impulfions of the air, fo we obferve the fenlitive organ of 
the ear to be compofedin a different manner from that of 
any of the other f’enfes. The fonorous waves of the elaf- 
tic air, being driven into the cartilaginous funnel of our 
ear, are repelled and collected together, by alternate re¬ 
flections front its elaftic tides, into the cavity of the con¬ 
cha, whence they proceed through the auditory paffage ; 
Where their force is increafed by being contracted into fo 
narrow a compafs. They are likewife increafed by new 
founds from the percuflion of the elaftic cartilages- and 
hard bones, which mix imperceptibly with the primitive 
founds. The bottom or end of the auditory paffage is 
terminated internally by the membrana tympani ; which Dr. 
Haller fays is not naturally perforated, as far as he has 
been able to difcover; and that the tranfmiffion of tobac- 
co-fmoke from the mouth through the ear is fabulous. 
Authors- in general agree with him now in the former of 
thefe opinions; but that an accidental opening has fome- 
times happened,, and that the fmoke of tobacco has been 
blown through it during life, is certain. Upon the fur- 
face of this membrane, and more efpecialty upon its co¬ 
nical cavity pointing inward, the fonorous waves ftrikc, 
after they have received their laft reflections from the au¬ 
ditory paffage, by which the elaftic fabric of this mem¬ 
brane is forced into ofciliation. The membrane is ftretch- 
ed over the tympanum, containing the little bones to which 
the fmall mufcles of the, internal ear are fixed. By means 
of the tenfor of the malleus, the membrane of the tym¬ 
panum is the better difpofed to hear weak founds; and 
the ether mufcle ferves to moderate thofe that are too vio¬ 
lent, by drawing the malleus from the incus; by which 
means the propagation of the fonorous tremors is inter¬ 
rupted. If the membrane of the tympanum be broken, 
or the bones of hearing diflocated, the perfon becomes at 
firft hard of hearing, and afterwards perfectly deaf. There 
have been inftances, however, where the membrana tym¬ 
pani has been partly deftroyed, and yet the perfon has re¬ 
tained the fenfe of hearing, though lefs diftinCtly than be¬ 
fore. The malleus returns the tremors inrpreffed upon 
the membrane of the tympanum to the incus, which fends 
it inwards to the next bones. The Jlapes, aptly enough 
fo called from its figure, lies inclined, and is covered by 
its own mufcle, which feems to draw the ftapes, that it 
may lie higher up, under the back-part of the feneftra 
ovalis, and pafs out of it before. Thus the nervous pulp 
of the veftibulum is preffed by the bafts of the ftapes, 
and by the air of the tympanum; and the Euftachian tube, 
by the aCtion of tfie circumjacent mufcles, may be com- 
preffed and 1 doled, and probably a little relaxed and open¬ 
ed again, by the circumflex mufcle of the moveable pa¬ 
late. By this canal the infpired air enters into the tympa¬ 
num, to be changed or renewed; and it alfo ferves to 
convey the mucus that covers the little bones and the tym¬ 
panum. Nor is it at all improbable that the air enters by 
this tube, to fupport the tympanum when it is preffed in¬ 
ward by the more violent founds; for founds themfelves, 
received into the mouth, are this way conveyed to the 
organ of hearing. In infpiration, the air preffes the mem¬ 
brane of tIre tympanum outward ; and thence proceeds 
that clalhing or whifpering noife, by which the hearing is 
obfeured, in yawning; for then the air entering more 
abundantly through the cavity of the tube, to the tympa¬ 
num, refills the tremors of the external air. 
From what has been faid, it appears that the elaftic 
waves or tremors of the air come through tire outer ear 
and auditory paffage, to the membrane of the tympanum; 
which being injured, and not repaired, the hearing is in a 
great meafure deftroyed. This feems to be ftretched, for 
bearing weak founds, by the mufcles of the malleus. 
From this membrane the found is conveyed through the 
fmall bones to the veftibulum; for, thefe bones being de-- 
ftroyed, the hearing is again abolifhed. The bony fides 
of the veftibulum, by their tremulation, agitate the fmall 
quantity of aqueous fluid furrounding tire nervous pulp, 
ft feems to be ftruck by the nervous pulp fufpended in the 
veftibulum, and that tremor feems to be continued through 
the continuous pulp of the cochlea and femicircular canals. 
Of more than this we are not certain ; but, by undoubted 
experiments, tremors, and even elaftic founds, communi¬ 
cate themfelves by the internal Euftachian tube, and tlrro 5 
all the bones of the fkull, fo as to inrprefs their force upon 
the auditory nerve. 
The diftinCtion of founds proceeds from the celerity of 
the tremors excited in the hearing nerve, according as they 
fucceed each other more fwiftly or (lowly. It is not ne- 
ceffary that the mind ftiould number them ; it is fufficient- 
that (lie perceive their numbers to be different, and that' 
this difference excites a variation in the thoughts and ideas 
thence arifing. Does the harmony or agreeablenefs of 
founds arife from the number of parts founding together 
in unifon ? and does the mind number the degrees of con- 
fonance, fo as to pleafe herfelf in the majority- of them ? 
Thefe ar.e queftions denied by the mod expert muficians, 
who make it appear that there is an agreeablenefs, and 
that very confiderable, in founds approaching the leaft to 
a confonancC, and w hich are. in a proportion very difficult 
to determine. Why do founds often become too (harp fer¬ 
tile ear ? Becaufe our auditory nerves feem to be fo drain¬ 
ed upon the fpiral plates, as to be in danger of breaking. 
Thus, drinking-glaffes are broken by (harp founds; and 
the hearing is fometimes almoft loft for awhile by the vio- 
lently-ihrill whiftlings of the inhabitants of the Canaries. 
0 & THE MOUTH. 
The cheeks and lips form the (ides and entry of the ca¬ 
vity of the mouth. The fubftance which forms the red 
border of the lips is very different from the reft of the 
(kin, being a collection of very fine, long, villous, papil¬ 
lae, clofely connected together, and covered by a fine mem¬ 
brane, which feems to be both a continuation of the epi¬ 
dermis, and of that pellicle which covers the glandular 
membrane of the cavity of the mouth. This fubftance 
is extremely fenlible, and very painful when the outer 
membrane is by any accident deftroyed. The fubftance of 
the gums is alio of a very lingular ftruCture, refembling, 
in Come meafure, tire texture of a hat, fuppofed to be ve» 
ry compaCt and elaftic. It is not immediately fixed to the 
bones of the jaws, but by the intervention of the periof¬ 
teum, with which it is perfectly united; and it is covered 
by a fine ftrong even membrane, which (ticks very clofe to 
the fubftance of the gums; and feems to be a continuation 
of that thin membrane which goes to the lips and cheeks, 
and of that which gees to.the tongue. 
The palate is that arch or cavity of the mouth furround- 
ed anteriorly by the alveolary edge and teeth of the upper 
jaw, and reaching from thence to the great opening of 
the 
