5-14 ANATOM Y. 
fore-fide turning inwards there, while they are Hoped down 
and hollowed behind ; fo that they have the form of wedg¬ 
es, and therefore their power of aftiiig mult be confidera- 
bly increafed. Seeing, in the action of the incifores, a 
perpendicular compreflion is only necelfary, without any 
lateral motion, they are not fo firmly fixed in their lockets 
as the other teeth are, each having only one Ihort root; 
but that is broader from before backwards than to either 
lide, to have the greatefl ftrength where it is expol'ed to 
the ftrdngeft force applied to it. The incilores of the up¬ 
per jaw, efpecially the two middle ones, are generally 
broader and longer than thole of the under jaw. Theca- 
tiini, from the rel’emblance to dog’s talks, are one on each 
lkle of tlie incifores in each jaw. The two in the upper 
jaw are called eye-teeth, from the communication of nerves 
which is laid to be between them and the eyes. The two 
in the lower jaw are named angular or wiki teeth , becaufe 
they fupport the angles of the mouth. The canini are 
broader, longer, and lironger, than the incifores are; only 
that the edge riles into a point at the middle. Each of 
them has generally but one long root, though fometimes 
they have two. The roots are crooked towards the end. 
The canini of the upper jaw are larger, longer, and with 
more crooked roots, than thole of the under jaw. The 
.form of their bale is fit both for piercing and cutting, and 
the long crooked root of each makes it fecurc in the fock- 
ct. The dentes mclarcs, or grinders, which have this name 
becaufe they grind our food, are generally five in the lides 
of each jaw ; in all twenty. Their bafes are broader, more 
icabrous, and with a thinner cortical fubtlance, than the 
otlief, teeth. They have alfo more roots; and, as thefe 
roots generally divaricate from each other, the partitions 
of the fockets between them bear a large fliare of the great 
p refill re they fuffer, and hinder it from acting on their 
points. The bale of the firfi grinder has an edge pointed 
in tlie middle, on its outfide refembling the canini; from 
which it Hopes inwards till it riles again into a point. It 
lias generally but one root, which fometimes is long and 
crooked at its point. The fecond dens molaris has two 
points on its bafe, riling nearly equally on its out and in- 
fide. It has two roots, either feparate or run together, 
but Hiorter than the root of the firft. Thefe two anterior 
grinders are much fmaller than the three that are placed 
farther back in the mouth. The third and fourth are ve¬ 
ry broad in their bafes, with four or five points Handing 
out; and they have three or more roots. The fifth, com¬ 
monly called dens fapientiev, from its coming through the 
gums later than the other grinders, has four points on its 
bafe, which is not fo large as the bale of tlie third and 
fourth, and its roots are lefs numerous. The incilores of 
the upper jaw being broader titan thofe of the lower jaw, 
make the fuperior grinders to be placed fo much farther 
back than the lower ones, that, when they are brought 
together, by fimtting the mouth, the points of the grind¬ 
ers of one jaw enter into the deprellions of the oppoiite 
grinders, and they are all equally applied to each other, 
notwithflanding the inequality of their furface. The nu¬ 
merous roots of tlie dentes molares prevent their loofening 
by the lateral p re flu re they fuffer in grinding ; and as the 
fockets in the upper jaw are more fpongy, and tlie teeth are 
more liable, by their fituation, to fall out, the grinders 
there have more numerous and more feparated roots than 
in the lower jaw. 
The os hyoidcs is fituated horizontally between the root 
of the tongue and the larynx. It is properly enough na¬ 
med hyoidcs, from the refemblance it bears to the Greek 
letter v ; and may, for a clearer demonflration of its flruc- 
ture, be diflinguilhed into its body, cornua , and appendices. 
The body is the middle broad part, convex before and hol¬ 
low behind. The convex fore-part is divided into two by 
a ridge, into the middle of which the mylo-hyoidei, and 
into the Hides the Hylo-hyoidei, mufcles are inferted. Above 
the ridge, the bone is horizontal; but pitted in the mid¬ 
dle by the infertion of the two genio-hyoidei mufcles, and 
a little hollowed more laterally by the bafio-gloffi. Below 
the ridge, it is convex ; bfft a little flatted in the middle 
by the flei no-hyoidei, and pitted more externally by the 
coraco-hyoidei. The concavity behind faces backwards 
and downwards to receive the thyroid cartilage, when the 
larynx and the os hyoides are pulled towards each other 
by the aftion of the flerno-hyoidei and hyo-thyroidei muf¬ 
cles ; and, to its upper edge, the ligamentous membranes 
ot the epiglottis, tongue, and thyroid cartilage, are fixed. 
Tlie cornua of the os hyoides are flretched backwards 
from each fide of its body, where often a fmall furrow 
points out tlie former reparation ; for, in young fubje-fts, 
the body and cornua are not one continued fubflance, as 
they come afterwards to be in adults. Thefe cornua are 
not always flraight, nor of an equal length ; their two plain 
furfaces Hand obliquely Hoping from above outwards and 
downwards. Into the external, the cerato-glofiis is in¬ 
ferted above, and the thyro-hyoideus mufcle below; and 
and, to the one behind, the ligamentous membrane of the 
tongue and larynx adheres. Each of the cornua becomes 
gradually fmaller as it is extended from the bafe; but ends 
in a round tubercle, from which a moveable cartilage Hands 
out, which is connected to the upper procefs of the carti- 
lago thyroidea. 
Where the body of the os hyoides joins on each fide with 
its cornua, a fmall Hyliform procefs, called appendix, riles 
upwards and backwards, into which the mufeuli Hylo- 
hyoidei alteri, and part of the hyo-glofli mufcles, are fix¬ 
ed. 'Thefubjlance of the os hyoides is cellular; but co¬ 
vered with a firm external plate, which is of fufficient 
ftrength to bear the aftions of fo many mufcles as are in¬ 
ferted into it. It is not articulated with any bone of the 
body, except by means of the mufcles and ligaments al¬ 
ready mentioned. Tlie vfe of os hyoides, is to ferve as a' 
folid lever for the mufcles to aft with, in railing and de¬ 
prefling- the tongue and larynx, or in enlarging and dimi- 
nilhing the capacity of the,fauces. 
BONES of the TRUNK. 
The trunk confiHs of tlie fpinc, pelvis, and thorax. The 
fpinc is the long piles of bones extended from the condyles 
of tlie occiput to the end of the rump. It fomewhat re- 
fembles two unequal pyramids joined in a common bafe. 
It is not, however, flraight; for, its upper part being drawn 
backwards by flrong mufcles, it gradually advances for¬ 
wards, to fupport the celbphagus, vedels of the head, &c. 
Then it turns backwards, to make room enough for the 
heart and lungs. It is next bent forwards, to fupport the 
vifeera of tlie abdomen. It afterwards turns backwards, 
for tlie enlargement of the pelvis. And, laflly, it is re- 
flefted forwards, for fuftaining the lowefl great gut. Tlie 
fpine is commonly divided into true and faJJlvertcbrce-, the 
former conflituting the long upper pyramid, which has its 
bafe below; while the falfe vertebrae make the Hiorter 
lower pyramid, whole bafe is above. 
The true vertebra: are the twenty-four upper bones of 
the fpine, on which the feveral motions of the trunk of 
our bodies are performed ; from which ufe they have jufl- 
ly derived their name. Each of thefe vertebrae is com- 
pol'ed of its body, and procefles. The body is the thick 
fpongy fore-part, which is convex before, concave back¬ 
wards, horizontal and plain in mofl of them above and 
below. Numerous fmall holes, efpecially on the fore and 
back part of their furface, give paflage to their vedels, 
and allow the ligaments to enter their fubflance. The 
edges of the body of each vertebra are covered, efpecially 
at the, fore-part, with a ring of bone firmer and more fo¬ 
lid than the fubflance of the body any where elfe. Thefe 
rings feem to be joined to the vertebrae in the form of epi- 
pliyfes, but are alleged by fome authors to be the liga¬ 
ments oilified. They are of great ufe in preventing the 
fpongy bodies from being broken in the motions of the 
trunk. Between the bodies of eacli two adjoining verte¬ 
brae, a fubflance between the nature of ligament and car¬ 
tilage is interpofed ; which feems to confifl of concentrical 
curved fibres, when it is cut horizontally; but, when it is 
3 divided 
