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 Part II. 222 A N A T O M Y. ties on the infide thereof, oppofite to the infertion of the The two pterygoidxi externi bring the lower jaw forward, in order to fet the lower incifores before the upmaffeter. per; in which adtion they are antagonids to the podePTERYGOID.EUS MINOPv five EXTERNUS. rior portion of the temporales, and the great portion of the When one of them arts, it carries the This is an oblong flefhy mufcle, much fmaller than mafleters. obliquely forward, or turns it toward the other the other, and fxtuated almoft horizontally between the chin fides. This oblique motion is performed alternately by outfide of the apophyfis pterygoides, and the condyloid thefe arting fingly. apophyfis of the lower jaw, the fubjeft being confidered Thetwotwomufcles digadrici ferve to deprefs the lower jaw, and in an eredt pofture. to open the mouth. It is fixed by one extremity to the outfide and edge of of thefe mufcles is very confiderable, as the outer ala of the pterygoid apophyfis, filling the fof- mayThebe force by laying the elbow on a table, and leanfula which is at the bafis of this apophyfis, near the bafis ing withftiewn the chin on the hand, while we endeavour at of the temporal apophyfis, of the fphenoidal bone. the fame time to deprefs the lower jaw; for as in that From thence it runs backward, and a little outward, cafe this jaw cannot defeend, the digadrici, by their ininto the void fpace between the two apophyfes of the low- fertions in the apophyfis madoidasa, raile the upper jaw, er jay, and is inferted anteriorly in the condyloid apo- bending the head backward on the condyles of the phyfis, at a fmall fofiula immediately under the inner by lower jaw. angle of the condyle. It is alio fixed to {he capfular ligament of the joint. Sect. XVII. Tkc TAvsclks which move the D I G A S T R I C U S. Os Hyoides. This is a fraall long mufcle, fimated laterally between the whole bafis of the jaw and the throat. It is flefiry M Y L O-H Y O I D iE U S. at both extremities, and tendinous in the middle, as if it confifted of two fmall mufcles joined endwife by a ten- This is a broad, thin, penniform mufcle, fitpated don, and from thence it is called digaflricus in Greek, tranfverfely between the internal lateral parts of the baand biventer in Latin. jaw, and lying on the anterior portions It is fixed by one fkfhy extremity in the fulcus of the fis of the lower digadric mufcles. maftoid apophyfis. From thence it runs forward, in- of Ittheis two made up of two equal flefliy portions, one lying clining towards the os hyoides, where the firft flelhy bo- on the right fide, the other on the leit, both in the fame dy ends in a roijnd tendon, which is connedted to the la- plane, and joined middle tendon, which is interal part and root of the cornua of that bone by a kind ferted anteriorly intothea fmall of the bafis of the os hyof aponeurotic ligament, and not by a vagina or pulley. oides, and from thencemiddle Here the tendon is inciirvated, and prefently ends in ing gradually in its courfe.runs diredtly forward, diminilhthe other fiefny body, which is fixed immediately above Each portion is fixed, by flelhy fibres, to the internal the internal labium of the bafis of the chin near the fym- lateral part of the lower jaw, between the oblique prophyfis, in a fmall nnecjual depreffion. This infertion minent line and the bafis, under the firft four dentes mois broader than that of tfie other extremity. lares and caninus. The anterior and greateft part of other fibres of each portion run obliquely from beUses of the Mufcles nuhich move the Lonskr Janu. the fore backward, to the middle tendon, in which they are The two temporales adting together, raife the lower regularly fixed, the anterior fibres being the flrorteft, and law, prefs the teeth in that j%w againft the upper teeth, a fmall triangular void fpace being formed between them and pull it back when it has been carried fo far forward and the fymphyfis of the chin. as that the lower incifores get before the upper. They The pofterior fibres of each portion, which make aperform the laft motion by their moft poflerior portion, bout a fourth part of the whole, run likewife on each which paffes over the root of the zygomatic apophyfis, fide to,the bafis of the os hypides, and are inferted along and the other motions by the co-operation of all their the lower edge of its anterior or convex fide, and from inufcular radii. thence a little upward. . The two mafleters ferve to raife the lower jaw, and to pulh the lower teeth againll the upper, in which ufe GENIO-HYOID-E US. they co-operate with the temporales. They Jikewife bring this jaw forward by their external and larged por- This is a fmall and pretty long flelhy mufcle, fituated between the fymphyfis of the chin and the os byoides, tion ; draw it back by their middle portion; and move it laterally by their fuperior portions adling alternately. clofe by its fellow. By the co-operation of all the three portions, they prefs It is fixed, by its anterior extremity, to a rough, and the lower teeth againd the upper. fometimes prominent furface, on the inner or pofterior Both pterygoidsei interni ferve to raife the lower jaw, fide of the fymphyns of the lower jaw, a little above the to bring the lower teeth near the upper, and to move the chin. From thence it runs backward, and is .inferted jaw laterally, as in grinding the food. anteriorly in the upper edge of the bafis ef the os hyoides.