72 THE NERVOUS SYSTEM 



more above the zygomatic arch. At a lower level, the 

 temporal fascia is so strong and dense that the contractions 

 of the muscle cannot be felt. The contractions of the 

 masseter can be felt in the posterior part of the side of the 

 face, below the zygomatic arch. 



The Pterygoid Muscles arise in the region of the 

 pterygoid process of the sphenoid, and pass backwards and 

 laterally to be inserted into the mandible. They therefore 

 protrude the mandible and move it from side to side. They 

 are deeply situated under cover of the mandibular ramus, 

 and their contractions cannot be examined satisfactorily. 

 When the p'.erygoid muscles of one side act in unison, that 

 half of the mandible is drawn towards the median plane, and 

 the opposite half is consequently thrust in a lateral direction. 

 No other muscles assist the pterygoids in producing side to 

 side movements. 



The Mylo - hyoid and the Anterior Belly of the 

 Digastric act as depressors of the mandible. The former 

 extends from the mylo-hyoid line on the inner surface of the 

 body of the mandible to the body of the hyoid bone. The 

 anterior fibres of the two mylo hyoids are inserted into a 

 median raphe, which extends from the symphysis menti to the 

 middle of the hyoid bone, and together they form a muscular 

 floor for the mouth. In addition to their action as depressors 

 of the mandible, the mylo-hyoids act as elevators of the hyoid 

 bone, when the mandible is fixed, e.g. in swallowing (p. 229), 

 and they steady the hyoid during the movements of the 

 tongue. 



In the median plane, between the chin and the hyoid bone, 

 the mylo-hyoid is covered only by the skin and fasciae and 

 its contractions can be palpated in this situation. They are 

 felt most readily when the tongue is pressed against the hard 

 palate. Unfortunately, the genio-hyoid (p. 106) is rendered 

 tense by the same action and, in consequence, paralysis of 

 the mylo-hyoid is very difficult to determine. 



The anterior belly of the digastric is probably more efficient 



