MASTICATION. 203 



Muscle. Attachments. 



Mylo-hyoid Body of the hyoid bone Mylo-hyoid ridge on the 



internal surface of the inferior maxilla. 



Genio-hyoid Body of the hyoid bone Inferior genial tubercle 



on the inner surface of the inferior maxilla near 

 the symphysis. 



Platysma myoides Clavicle, acromion, and fascia Anterior half of 



the body of the inferior maxilla near the in- 

 ferior border. 



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



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 inferior maxilla. 



External pterygoid Pterygoid ridge of the sphenoid, the surface be- 

 tween it and the pterygoid process, external 

 pterygoid plate, and the 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 fibro-cartilage. 



Action of the Muscles which depress the Lower Jaw. The most important of these 

 muscles have for their fixed point of action the hyoid bone, which, under these circum- 

 stances, is fixed by the muscles which extend from it to the upper part of the chest. 

 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 tend to depress the jaw. The attachments of the raylo- 

 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. 



It has been a disputed question whether the upper jaw does or does not participate 

 in the act of opening the mouth. That depression of the lower jaw is the main action 

 in ordinary mastication is sufficiently evident; but it is possible, by fixing the lower jaw, 

 to perform the acts of mastication laboriously and imperfectly it is true by movements 

 of the upper jaw. In ordinary mastication, however, the upper jaw undergoes a slight 

 movement of elevation in opening the mouth ; and this becomes somewhat exaggerated 

 when the mouth is opened to the fullest possible extent. 



Action of the Muscles which elevate the Lower Jaw and move it laterally and antero- 

 posteriorly. The temporal, masseter, and internal pterygoid muscles are chiefly con- 

 cerned in the simple act of closing the jaws. As this is almost the only movement of 

 mastication in many of the carnivora, in this class of animals these muscles are most 

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

 action ; and their immense power, even in the human subject, is explained by the number 

 of their fibres, by the attachments of many of these fibres to the strong aponeuroses by 

 which they are covered, and 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 alter- 

 nate 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-maxillary articulation. The articulation 

 of the lower jaw is of such a nature that, in its lateral movements, the condyles themselves 



