124 HUMAN PHYSIOLOGY 



soaked by the saliva, forms a pulp so that the food is soft, 

 lubricated, and suitable for deglutition. 



Biting and chewing are caused by the movement of the 

 upper and lower teeth against each other. 



The lower jaw is raised (turning around a horizontal axis pass- 

 ing through both limbs of the jaw) by the masseter, temporal and 

 internal pterygoid muscles of both sides of the head. It is lowered 

 by the digastric, mylohyoid and geniohyoid. During the lower- 

 ing of the jaw, the hyoid bone must be fixed; this is performed 

 by the omo-, sterno-, and thyro-hyoid and the sternothyroid. 

 The lower jaw is moved forward in a horizontal direction by the 

 two external pterygoids; to the left, by the right external ptery- 

 goid; and to the right, by the left external pterygoid. The morsel 

 is held between the teeth, during chewing, by the buccinator from 

 the outside and by the tongue from the inside. 



The tongue is pulled forward and downward by the genio- 

 glossus, while it is pulled downward and backward by the hyo- 

 glossus, and upward and backward by the palato- and stylo- 

 glossus. 



The tongue is composed of vertical, longitudinal, and diagonal 

 fibres. By the combination of their contractions, it can assume 

 many varying positions. 



Sucking serves to take up liquid food. By the sucking, 

 a negative pressure is produced in the mouth, and the fluid 

 to be taken up is thus sucked in. 



Sucking is produced in one of two ways: 



(a) By inspiration after separating the nasal passage from 

 the pharyngeal cavity by elevating the soft palate. 



($) After the mouth cavity is separated from the pharyn- 

 geal cavity by pressing the posterior part of the tongue 

 against the palate, the lower jaw is depressed and the 

 tongue is pulled backward and downward (pure mouth-suc- 

 tion). 



The negative pressure in the mouth during pure mouth-suction 

 may, by repeated sucking, be made as low as 700 mm Hg below 

 atmospheric pressure. 



2. Chemistry of digestion in the mouth. Digestion in 

 the mouth only affects starch. This is split up by the ptyalin 

 of the saliva, the animal diastase, into sugar. 



The animal diastase differs from that of the plants (the sugar- 



