Table 3-4.—Muscles of Mastication Muscle Origin Masseter Zygomatic arch Temporalis Temporal bone Insertion Mandible   (external surface) Coronoid process at the anterior border of the ramus Description Closes jaw; flat, thick muscle Closes jaw; fan-shaped Medial pterygoid Lateral pterygoid Sphenoid,  palatine,  and maxillary bones Sphenoid  bone Inner (medial) surface of Closes jaw; parallels the ramus masseter muscle Anterior surface of man- Opens jaw; allows dibular  condyle grinding action side to side, and protrudes the mandible reception. We receive food in the mouth, reducing it in size,  and  mixing  it  with  saliva  for  the  digestion process. CHEEKS LIPS The  lips  are  covered  externally  by  skin  and internally by the same mucous membranes that line the oral  cavity. They form the anterior border of the mouth. The area of the external lips where the red mucous membrane ends and normal outside skin of the face begins is known as the vermilion border. Figure 3-17 illustrates the anatomy of the lips. The cheeks are the side walls of the mouth. They are made up of layers of skin, a moist inner lining called mucosa, fat tissue, and certain muscles. The buccinator muscle of the cheeks prevents food from escaping the chewing action of the teeth. The lips are very sensitive and act as sensory receptors, allowing food and liquids to be placed in the Figure 3-17.—Anatomy of the lips. 3-10