MASTICATION. 193 



of the lower jaw. It is also important in the lateral movements of the jaw, 

 in which one of the condyles remains in the glenoid cavity and the other is 

 projected, so that the bone undergoes a slight rotation. 



Muscles of Mastication. To the lower jaw are attached certain muscles 

 by which it is depressed, and others by which it is elevated, projected for- 

 ward, drawn backward and moved from side to side. The following are the 

 principal muscles concerned in the production of these varied movements : 



MUSCLES OF MASTICATION". 

 Muscles which depress the lower jaw. 



MUSCLE. ATTACHMENTS. 



Digastric Mastoid process of the temporal bone Lower 



border of the inferior maxilla near the symphy- 

 sis, with its central tendon held to the side of 

 the body of the hyoid bone. 



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 infe- 

 rior 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, tuberosity of the palate and the 

 superior maxillary bone Inner surface of the 

 neck of the condyle of the inferior maxilla and 

 the interarticular fibro-cartilage. 



Action of the Muscles which depress the Lower Jaw. The most impor- 

 tant 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 

 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 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 possible extent. 



