148 Mufcks cftbe Head, &V. [Bookltf, 



The pterygoideus externus takes its origin from the 

 outer fide of the pterygoid procefs of the fphenoid 

 bone, from part of the tuberofity of the os maxillare 

 adjoining to it, and from the root of the temporal pro- 

 cefs of the fphenoid bone. It is inferted into the neck 

 of the condyloid procefs of the lower jaw, and pulls 

 it forwards and to the oppcfite fide, or when both the 

 external pterygoid mufcles at, the fore teeth of the 

 under jaw are pufhed forwards beyond thofe of the 

 upper jaw. 



On the fide of the neck, towards its fore part, are 

 two mufcles. The external of thefe is a mufcle of the 

 fkin, and is called platifma myoides. It arifes by a 

 number of (lender fibres from the cellular fubftance, 

 which covers the upper parts of the deltoid and pec- 

 toral mufcles; in their afcent they all unite to form a 

 thin miifcle, adhering to the fkin, and which is inferted 

 into the lower jaw. It draws the fkin of the cheek 

 downwards. 



The fterno-cleido-mafloideus has two origins, one 

 from the fternum, the other. from the clavicle, which, 

 uniting, form one mufcle, which runs oliquely upwards 

 and outwards, and is inferted into the maftoid procefs 

 of the temporal bone. When it contracts, it turns the 

 head to one fide, and bends it forwards ; or when its 

 fellow ads with it, they draw the head directly for- 

 wards. 



Six pair of mufcles are fituated between the os 

 hyoides and the lower jaw. 



The mufcle, which forms the external layer, is called 

 the digaftricus. It rifes near the maftoid procefs, runs 

 downwards and forwards to the os hyoides, and thence 

 proceeds to the bone of the chin, into which it is in- 

 ferted. When it>ad:s, it pulls the lower jaw, down- 

 wards and backwards, and therefore opens the mouth, 

 6 When 



