. Human physiology. produceevery intermediate movement between elevation and depression. Theraisers or levator muscles of the jaw extend from the cranium andupper jaw to the lower. They are four in number on each side,—thetemporal and masseter,which are entirelyconcerned in the func-tion; t\xeexternal ptery-goid^ which, whilst itraises the jaw, carriesit at the same timeforward, and to oneside; and the internalpterygoid^ which, ac-cording as it unites itsaction with the tem-poral or with the ex-ternal pterygoid, is anelevator of the jaw or a lateral motor. The depressors may be dividedinto imm
. Human physiology. produceevery intermediate movement between elevation and depression. Theraisers or levator muscles of the jaw extend from the cranium andupper jaw to the lower. They are four in number on each side,—thetemporal and masseter,which are entirelyconcerned in the func-tion; t\xeexternal ptery-goid^ which, whilst itraises the jaw, carriesit at the same timeforward, and to oneside; and the internalpterygoid^ which, ac-cording as it unites itsaction with the tem-poral or with the ex-ternal pterygoid, is anelevator of the jaw or a lateral motor. The depressors may be dividedinto immediate and mediate, according as they are, or are not, attachedto the lower jaw itself. There are only three of the former class:1, the digastricus^ the anterior fasciculus of which, or that which passesfrom the os hyoides to the lower jaw, depresses the latter; 2, the genio-hyoidens; and 8, the mylo-hyoidens^ all of which concur in the formationof the floor of the mouth. The indirect or mediate depressors are all. Skull of the Polar Bear. 76 DIGESTION-. those, that are situate between the trunk and the lower jaw, withoutbeing directly attached to the latter;—as the thyro-liyoideus^ the sterno-thyroideus, and the omo-hyoideus; the names of which indicate theirorigin and insertion. These, in the aggregate, form a muscular chain,which, when it makes the trunk its fixed point, depresses the lowerjaw. The arrangement of the elevators and depressors is such, thatthe former predominate over the latter; and hence during sleep thejaws continue applied to each other, and the mouth is consequentlyclosed. The human organs of mastication hold an intermediate place betweenthose of the carnivorous and herbivorous animal. In the carnivorousanimal, which has to seize hold of, and retain its prey between itsteeth, the jaws have considerable strength; and the movement of ele-vation is all that is practicable; or, at least, that can be effected to anyextent. This is dependent upon organizatio
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