206 ANATOMY OF THE HEAD. 



these muscles may find a fixed point. The deglutition of 

 liquids is not very different. The biievis and other muscles 

 of the tongue draw it downward, whereby a tendency 

 towards a vacuum is formed : to counteract this, the water 

 is by atmospheric pressure forced into the mouth ; when the 

 entire agency which operated in the case of a solid is called 

 into action, the raised velum palati closing the opening to 

 the nostrils, and thus preventing the liquid from returning 

 by that passage. 



MUSCLES OF MASTICATION. 



Stylo maxillaris {Fig 14 . 8). Orujin. From the styloid 

 process of the occiput. Insertion. Into the angle of the 

 lower jaw. Action. To assist in gaping, and widely opening 

 the mouth. 



JVIasseter externus {Plate II. 8). Origin. From the 

 inferior of the zygomatic arch and spine. Insertion. Into 

 the external surface and inferior border of the lower jaw. 

 Action. To powerfully close the mouth. 



Temporalis {Plate II. 10). Origin. From the superior 

 sides of the cranium and interior of the zygomatic arch. 

 Insertion. Into the coronary process and border of the 

 lower jaw. This muscle cannot be fairly separated from 

 the masseter externus. Action. To firmly close the mouth. 



Masseter internus. Origin. From the superior maxil- 

 lary and sphenoid bone. Insertion. Into the inner surface 

 and inferior border of the lower jaw. Action. To firmly 

 close the mouth, 



Pterygoideus. Origin. From the pterygoid process, 

 and from the sphenoid bone. Insertion. Into the lower 

 jaw below the joint. Action. To rotate the jaw, or give to 

 it that circular motion which is observed when the horse is 

 masticating. 



Mylo-hyoideus. Origin. From the alveolar edges of 

 the lower jaw. Insertion. By a central tendon into the 

 spur process of the os hyoides. Action. To draw the bone 

 forwards, and to raise the tongue. 



DiGASTRicus. Origin. From the substance of the stylo- 

 maxillaris. Insertion. To the inner side of the lower jaw, 

 as forward as the third molar tooth. Action. To open the 

 mouth, aided by the stylo-maxillaris and the weight of the 



