ANATOMY. 
may be attended with dangerous fymptoms. The fur face 
of the lower jaw is hard and firm, except at the l'pongy 
lock ts. where, however, it is ftronger than the upper jaw. 
Its internal JubJlance is cellular, without any folid partition 
between the cancelli in its middle. At the bale, especially 
of the chin, where this bone is molt expofed to injuries, 
the folid Tides of it are thick, compact, and hard. 
The lower jaw generally receives the root of fixteen 
teeth into its fockets, by gomfihojis ; and its condyloid pro- 
eeiTes, covered with a cartilage, are articulated with the 
temporal bones, in a manner that is not in general rightly 
defcribed: for, not only the fore-part of the cavity be¬ 
tween the zygomatic, auditory, and vaginal, precedes, but 
alfo the adjoining tubercle at the root of the zygomatic 
procefs of each os temporum, is covered with a fmooth 
cartilage for this articulation. Here alio an intermediate 
moveable cartilage is placed ; which being thin in the 
middle, and thick at the edges, is concave on both Tides ; 
and is connefted fo firmly by ligaments to each condyle, 
as to follow the motions of the condyle; and To loofely to 
the temporal bone, as readily to change its filiation from 
the cavity to the tubercle, and to return again; while the 
common ligament of the articulation affords fpace enough 
for fuch a change of place backwards and forwards : but, 
like other ligaments of the joints by ginglimus, is ftrong 
and fliort at the Tides, to confine the lateral motions. When, 
therefore, the teeth of both jaws coincide, the condyles 
are lodged fecurely in the temporal cavities ; but their 
motions to either Tide muft be confined both by the firm- 
refs of the ligaments and the riling brims which are on 
each Tide of the cavities. When the jaw is brought directly 
forwards, the condyle and intermediate cartilages defcend 
*nd advance forwards upon the tubercles. In this fitua- 
tion, the lateral motions are a little more free than in the 
former one, from the want of riling brims to (top the 
condyles. When the fore-teeth of the lower jaw are moved 
forwards and to a fide, the condyle of the oppofite fide is 
either advanced from the cavity to the tubercle, while the 
condyle of the fame fide remains in the cavity; or, if both 
condyles are on the tubercles, when the jaw is moved ob¬ 
liquely to a fide, the condyle of the fide to which the mo¬ 
tion is made Aides back from the tubercle to the cavity. 
When the mouth is opened by the defcent of the lower 
jaw, the fore-part of it, where the deprefiing mufcles are 
fixed, is drawn backwards, as well as downwards, while 
refifiance is made to the angles moving backwards by the 
mafleter and internal pterygoid mufcles, and at the fame 
time, the external pterygoid draws the condyles and their 
moveable cartilages forwards ; and therefore, when the 
mouth is opened, the condyles are carried forwards upon 
the tubercles, and the axis of motion of the bone is a little 
above its angles. But in this fituation there is lefs refin¬ 
ance, than in any other, to the condyles luxating forwards; 
a difeafe which feldom happens, except when people are 
gaping too wide; and therefore the common practice of 
nurfes, who fupport the jaw of infants when they are yawn¬ 
ing, is reafonable. In chewing, there is a fuccelfion of the 
motions above defcribed. 
The teeth are the hard white bodies placed in the fockets 
of both jaws. Their number is generally fixteen above 
and as many below ; though fome people have more, others 
lefs. The broad thick part of each tooth, which appears 
without the focket, is the bade or body. The fmaller pre¬ 
cedes, funk into the maxillae, are the roots or fangs. At 
the place where the bafe ends and the roots begin, there, 
is generally a fmall circular deprefiion, which fome call 
the neck or collar. Without the gums the teeth are co¬ 
vered with no membrane, and they are faid to have no 
proper peridfieum within the fockets; but that is fupplied 
by the reflected membrane of the gums, which after a good 
injection may be evidently Teen in a young fubjedl, with 
the velfels from it penetrating into the fitbftance of the 
teeth; and it may be difcovered in any tooth recently 
pulled, by macerating it in water. The adhefion of this 
membrane to thefe roots is firengthened by the fmall fur¬ 
rows obfervable on them. Each tooth is compofed of its 
cortex or enamel, and an internal bony fubllance. The 
cortex has no cavity or place for marrow ; and is fo folid 
and hard, that favvs or files can with difficulty make an im- 
prellion on it. It is thickeft upon the bafe, and gradually, 
as the roots turn fmaller, becomes thinner, but not pro¬ 
portionally to the difference of the fize of the bafe and 
roots. The fibres of this enamel are all perpendicular to 
the internal fubftance; and are ftraight on the bafe, but 
at the tides are arched with a convex part towards the 
roots ; which makes the teeth refill the compreffion of any 
hard body between the jaws with lefs danger of breaking 
thefe fij/res than if they had been fituated tranfverfely. 
The bony part of the teeth has its fibres running ftraight, 
according to the length of the teeth. When it is expofed 
to the air, by the breaking or falling off of the hard cor¬ 
tex, it foon corrupts. And thence carious teeth are often 
entirely hollow within, when a very fmall hole appears on¬ 
ly externally. 
The teeth have canals formed in their middle, wherein 
their nerves and blood-veffels are lodged. The velfels are 
eafily traced as long as they are in the large canal, but can 
fcarcely be obferved in their diftribution from that to the 
fubftance of the teeth of adults. Ruyfch however affirms, 
that after injection he could trace the arteries into the 
hardeft part of the teeth And Leuwenhoek fufpedled the 
fibres of the cortex to be veffels. This plentiful fupply 
of velfels muft expofe the teeth to the fame diforders that 
attack other vafcular parts; and fuch teeth as have the 
greateft number of veffels muft have the moft numerous 
chances of being l'eized with thefe difeafes. 
Every root of each tootli has a diftindl canal, with vef¬ 
fels and n.erves in it. Thefe canals in the teeth with more 
than one root, come nearer each other as,they approach 
the bafe of the tooth ; and at laft are only feparated by ve¬ 
ry thin plates, which, being generally incomplete, allow a 
communication of all the canals ; and frequently one com¬ 
mon cavity only appears within the bafe, in which a pulpy 
fubftance compofed of nerves and veffels is lodged. The 
condition therefore of the nerves here bears a ftrong ana¬ 
logy to that of the cutaneous nerves which ferve for the 
fenfation of touching. The entry of the canals for thefe 
velfels is a fmall hole placed a little to a fide of the extreme 
point of each root; fometimes, efpecially in old people, 
this hole is entirely clofed up, and confequently the nerves 
and blood-veffels are deftroyed. 
Children are feldom born with teeth ; but at two years 
of age they have twenty ; and their number does not in- 
creafe till they are about feven years old ; when the teeth 
that firft made their way through the gums are thruft out 
by others that have been formed deeper in the jaw, and 
fome more of the teeth begin to difcover themfelves far¬ 
ther back in the mouth. About fourteen years of age, 
fome more of the firft crop are fhed, and the number is in- 
creafed. This (bedding of the teeth is of great ufe : for, 
if the firft had remained, they would have ftood at a great 
diftance one from another; becaufe the teeth are too hard 
in their outer cruft to increafe fo fall as the jaws do. 
Whereas, both the fecond layer and the teeth that come 
out late, meeting, while they are foft, with a confiderable 
refiftance to their growth in length, from thole fituated upon 
them, neceffarily come out broad, and fit to make that clofe 
guard to the mouth which they now form. 
The teeth are joined to the fockets by gomphofis, and the 
gums contribute to fix them there; as is evident by the 
teeth falling out when the gums are any way deftroyed or 
made too fpongy, as in the fcurvy or lalivation: whence 
fome authors clafs this articulation with th c, [yjfarcojis. 
Though the teeth fo far agree in their ftrudlure, yet, be¬ 
caufe of fome things wherein they differ, they are gene¬ 
rally divided into three dalles, viz. incifores, canini, and 
molares. The inciforcs are the four fore-teeth in each jaw, 
receiving their name front their office of cutting our ali¬ 
ment ; for which they are excellently adapted, being each 
formed into a fliarp-cbtting edge at their bale, by their 
fore- 
