168 DIGESTION 



Muscles that elevate the lower jaw and move it laterally and antero-posteriorly 



MUSCLE ATTACHMENTS 



Temporal .... Temporal fossa coronoid process of the inferior maxilla. 



Masseter .... Malar process of the superior maxilla, lower border and internal 



surface of the zygomatic arch surface of the ramus of the 

 inferior maxilla. 



Internal pterygoid . . . Pterygoid fossa inner side of the ramus, and angle of the in- 

 ferior maxilla. 



External pterygoid . . Pterygoid ridge of the sphenoid, the surface between it and the 



pterygoid process, external pterygoid plate, tuberosity of the 

 palate and the superior maxillary bone inner surface of 

 the neck of the condyle of the inferior maxilla and the inter- 

 articular fibre-cartilage. 



Action of the Muscles that depress the Lower Jaw. The most 

 important of these muscles have for their fixed point of action the 

 hyoid bone, which is fixed by the muscles extending from it to the 

 upper part of the thorax. The central tendon of the digastric, as it 

 perforates the stylo-hyoid, is connected with the hyoid bone by a loop 

 of fibrous tissue; and acting from this bone as the fixed point, the 

 anterior belly must of necessity depress the jaw. The attachments of 

 the mylo-hyoid and the genio-hyoid render their action in depressing 

 the jaw sufficiently evident, which is also the case with the platysma 

 myoides, acting from its attachments to the upper part of the thorax. 

 In ordinary mastication the upper jaw undergoes a slight movement of 

 elevation, and this becomes somewhat exaggerated when the mouth is 

 opened to the fullest extent. 



Action of the Muscles that elevate the Lower Jaw and move it later- 

 ally and antero-posteriorly. The temporal, masseter and internal 

 pterygoid muscles are chiefly concerned in the simple act of closing the 

 jaws. Their anatomy alone gives a sufficiently clear idea of their mode 

 of action ; and their great power is explained by the number of their 

 fibres, by the attachments of many of these fibres to the strong aponeu- 

 roses by which they are covered, and by the fact that the distance 

 from their origin to their insertion is very short. 



The attachments of the internal and external pterygoids are such 

 that by their alternate action on either side, the jaw may be moved 

 laterally, as their points of origin are situated in front of and internal to 

 the temporo-m axillary articulation. The articulation of the lower jaw is 

 such that in its lateral movements the condyles themselves can not be 

 sufficiently displaced from side to side ; but with the condyle on one 

 side fixed or moved slightly backward, the other may be brought forward 

 against the articular eminence, producing a slight movement of rotation. 



The above explanation of the lateral movements of the jaw presup- 

 poses the possibility of movements in an antero-posterior direction 



