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 ANATOMY. Part f. 166 roots, and as thefe root's generally divaricate from each cept by means of the mufcles and ligaments menother, the partitions of the fockets between them bear tioned. a large (hare of the great preffure they fuffer, and hin- The ufe of the os hyoides, is to ferve as a folid lever der it to adt on their points. for die mufcles to aff with, in -railing or depreffing the The numerous roots of the dentes molares prevent tongue and larynx, or in enlarging and diminifhing the their loofening by the lateral preiTure they fuffer in grind- capacity of the fauces. ing ; and as the fockets in the upper jaw are more fpongy, and the teeth are more liable, by their fmiation, to OF THE TRUNK. fall out, the grinders there have more numerous and more deparated roots than in the lower jaw. The Trunk confifts of the fpine, pelvis, and, According to the divifion made of the ikeleton, we thorax. fhould now proceed to the defer!ption of the trunk of the The Spine is the long pile of bones extended from body. But muft -firft confider a bone, which cannot well the condyles of the occiput to the end of the rump. It be faid to belong to either the head or the trunk; nor is fomewhat refembles two unequal pyramids joined in a it Immediately joined to any other, and therefore is very common bafe. It is not, however, ftreight; for its upper part being drawn backwards by ftrong mufcles, it feldom preferved with Ikeletons. The Os Hyoides, which is fituated horizontally gradually advances forwards, to fupport the cefophagus, between the root of the tongue and the larynx. It is veffels of the head, &c. Then it turns backwards, to properly enough named hyoides, from the refemblance it make place enough for the heart and lungs. It is next bears to the Greek letter v, and may, for a clearer de- bended forwards, to fupport the vifcera,of the abdomen. monftration of its ftruffure, be diftinguiftied into its body, It afterwards turns backwards, for the enlargement of cornua, and appendices. the pelvis. And, laftly, it is reflefted forwards, for fuThe body is the middle broad part, convex before, ftaining the lowed great gut. and hollow behind.——The convex fore-part is divided The fpine is commonly divided into true and falfe into two, by a ridge, into the middle of which the mylo- vertebrae; the former conftituting the long upper .pyrahyoidei, and into the fides the •ftylo-hyoidei, mufcles mid, which has its bafe bafe below, while the falfe verare inferred. Above the ridge, the bone is horizontal, tebra; make the ftiorter lower pyramid, whofe bafe is but pitted in the middle by the infertion of the two ge- above. nio-hyoidei mufcles, and a little hollowed more laterally The True Vertebrae are the twenty-four upper by the bafiogloffi.—■—Below the ridge, it is convex, but bones of the fpine, on which the feveral motions of the a little flatted in the middle by the fterno-hyoidei, and trunk of our bodies are performed; from which ufe they pitted more externally by the coraco-hyoidei -The have juftly got their name. concavity behind faces backwards and downwards to re- Each of thefe vertebras is compofed of its body and ceive the jhyroid cartilage, when the larynx and the os proceffes. hyoidcs are pulled towards each other by the aftion of The body is the thick fpongy fore-part, which is conthe fterno-hyoidei and hyothyroidei mufcles; and to its vex before, concave backwards, horizontal and plain in upper edge, the ligamentous membranes of the epiglotis, mod of them above and below. Numerous fmall holes, efpecially on the fore and back-part of their furtongue, and thyroid cartilage, are fixed; The cornua of the os hyoides are ftretched backwards face, give paffage to their veffels, and allow the ligaments from each fide of its body, where often a fmall furrow to enter their fubdance. The edges of the body of points out the former feparation. Thefe cornua are each vertebra are covered, efpecially at the fore-part, not always ftreight, nor of equal length ; their two with a ring of bone firmer and more folid than the fubplain furfaces (land obliquely flopping from above, out- dance of the body any where elfe. Thefe rings are of wards and downwards. Into the external, the cerato great ufe in preventing the fpongy bodies from being gloffus is inferted above, and the thyro-hyoideus mufcle broken in the motions of the trunk. below; and to the one behind, the ligamentous mem- Between the bodies of each two adjoining vertebra, brane of the tongue and larynx adheres. Each of the a fubdance between the nature of ligament and, cartilage cornua becomes gradually fmaller, as it is extended from is interpofed; which feems to confid of concentrical the bafe; but ends in a round tubercle, from which a curve fibres, when it is cut horizontally; but when it is moveable cartilage ftands out, which is connefted to the divided perpendicularly, the fibres appear oblique and decuffating each other. The outer part of the interverupper procefs of the cartilage thyroidea. Where the body of the os hyoides joins on each fide tebral ligaments is the mod folid and hard ; and they grawith its cornua, a fmall ftyliform procefs, called appen- dually become fofter till they are almod in the form of a dix, rifes upwards and backwards, into which the muf- glairy liquor in the centre ; and therefore thefe fubculi flylo-hyoidei alteri, and part of the hyo-glofli muf- dances were not improperly called muons ligaments by cles are fixed. the ancients. Tire external fibrous part of each is capaThe fubftance of the os hyoides is cellular, but cover- bleof being greatly extended, and of being compreffedinto ed with a firm external plate, which is of fufficient a very fmall fpace, whild the middle fluid part is incomftrength to bear the affions of fo many mufcles as are in- preflible, or nearly fo ; and the parts mf this ligament ferted into it. between the circumference and centre approach in their It is not articulated with any bone of the body, ex- properties to either, in proportion to their more folid